From FreeCAD Documentation
Revision as of 20:08, 5 September 2023 by Roy 043 (talk | contribs) (Vector functions: changed "," to ";".)


It is possible to define properties using mathematical expressions. In the GUI, spin boxes or input fields that are bound to properties contain a blue icon . Clicking on the icon or typing the equal sign = brings up the expression editor for that particular property.

A FreeCAD expression is a mathematical expression using the standard mathematical operators, functions and predefined constants as described below. In addition, the expression may reference object properties, and also use conditionals. Numbers in an expression may have an optional unit attached to them.

Numbers may use either a comma , or a decimal point . to separate whole digits from decimals. When the decimal marker is used, it must be followed by at least one digit. Thus, the expressions 1.+2. and 1,+2, are invalid, but 1.0 + 2.0 and 1,0 + 2,0 are valid.

Operators and functions are unit-aware, and require valid combinations of units, if supplied. For example, 2mm + 4mm is a valid expression, while 2mm + 4 is not. This also applies to references to object properties that have units, such as Length properties. Thus Pad001.Length + 1 is invalid since it adds a pure number to a property with length units, it requires Pad001.Length + 1mm.

Some unit related errors can seem unintuitive, with expressions either being rejected or producing results that do not match the units of the property being set. Here are some examples:

1/2mm is not interpreted as half a millimeter but as 1/(2mm), resulting in: 0.5 mm^-1.

sqrt(2)mm is not valid because the function call is not a number. This has to be entered as sqrt(2) * 1mm.

Function arguments

Multiple arguments to a function may be separated by either a semicolon ; or a comma followed by a space , . In the latter case, the comma is converted to a semicolon after entry. When a semicolon is used, no trailing space is necessary.

Arguments may include references to cells in a spreadsheet. A cell reference consists of the cell's uppercase row letter followed by its column number, for example A1. A cell may also be referenced by using the cell's alias instead, for example Spreadsheet.MyPartWidth.

Referencing objects

You can reference an object by its DataName or by its DataLabel. In the case of a DataLabel, it must be enclosed in double << and >> symbols, such as <<Label>>.

You can reference any property of an object. For example, to reference a Cylinder's height, you may use Cylinder.Height or <<Long_name_of_cylinder>>.Height. To reference the object itself use the _self pseudo property. For example, you may use Cylinder._self or <<Label_of_cylinder>>._self.

To reference list objects, use <<object_label>>.list[list_index] or object_name.list[list_index]. If you want for example to reference a constraint in a sketch, use <<MySketch>>.Constraints[16]. If you are in the same sketch you may omit its name and just use Constraints[16].
Note: The index starts with 0, therefore Constraint17 has to be referenced as Constraints[16].

For more information about referencing objects, see Reference to CAD_data.


Supported constants

The following constants are supported:

Constant Description
e Euler's number
pi Pi


Supported operators

The following operators are supported:

Operator Description
+ Addition
- Subtraction
* Multiplication
/ Floating point Division
% Remainder
^ Exponentiation


Supported functions

General mathematical functions

The following mathematical functions are supported:

Trigonometric functions

Trigonometric functions use degree as their default unit. For radian measure, add rad following the first value in an expression. So e.g. cos(45) is the same as cos(pi rad / 4). Expressions in degrees can use either deg or °, e.g. 360deg - atan2(3; 4) or 360° - atan2(3; 4). If an expression is without units and needs to be converted to degrees or radians for compatibility, multiply by 1 deg, 1 ° or 1 rad as appropriate, e.g. (360 - X) * 1deg; (360 - X) * 1°; (0.5 + pi / 2) * 1rad.

Function Description Input range
acos(x) Arc cosine -1 <= x <= 1
asin(x) Arc sine -1 <= x <= 1
atan(x) Arc tangent, return value in the range -90° < value < 90° all
atan2(y; x) Arc tangent of y/x accounting for quadrant, return value in the range -180° < value <= 180° all, the invalid input x = y = 0 returns 0
cos(x) Cosine all
cosh(x) Hyperbolic cosine all
sin(x) Sine all
sinh(x) Hyperbolic sine all
tan(x) Tangent all, except x = n*90 with n = odd integer
tanh(x) Hyperbolic tangent all
hypot(x; y) Pythagorean addition (hypotenuse), e.g. hypot(4; 3) = 5 x and y >= 0
cath(x; y) Given hypotenuse, and one side, returns other side of triangle, e.g. cath(5; 3) = 4 x >= y >= 0

Exponential and logarithmic functions

Function Description Input range
exp(x) Exponential function all
log(x) Natural logarithm x > 0
log10(x) Common logarithm x > 0
pow(x; y) Exponentiation all
sqrt(x) Square root x >= 0
cbrt(x) introduced in version 0.21 Cubic root all

Rounding, truncation and remainder functions

Function Description Input range
abs(x) Absolute value all
ceil(x) Ceiling function, smallest integer value greater than or equal to x all
floor(x) Floor function, largest integer value less than or equal to x all
mod(x; y) Remainder after dividing x by y, sign of result is that of the dividend. all, except y = 0
round(x) Rounding to the nearest integer all
trunc(x) Truncation to the nearest integer in the direction of zero all


Statistical / aggregate functions

Aggregate functions take one or more arguments.

Individual arguments to aggregate functions may consist of ranges of cells. A range of cells is expressed as two cell references separated by a colon :, for example average(B1:B8) or sum(A1:A4; B1:B4). The cell references may also use cell aliases, for example average(StartTemp:EndTemp).

The following aggregate functions are supported:

Function Description Input range
average(a; b; c; ...) Average value of the arguments, same as sum(a; b; c; ...) / count(a; b; c; ...) all
count(a; b; c; ...) Count of the arguments, typically used for cell ranges all
max(a; b; c; ...) Maximum value of the arguments all
min(a; b; c; ...) Minimum value of the arguments all
stddev(a; b; c; ...) Standard deviation of the values of the arguments all
sum(a; b; c; ...) Sum of the values of the arguments, typically used for cell ranges all


String manipulation

String identification

Strings are identified in expressions by surrounding them with opening/closing double chevrons (as are labels).

In following example, "TEXT" is recognized as a string : <<TEXT>>

String concatenation

Strings can be concatenated using the '+' sign.

Following example <<MY>> + <<TEXT>> will be concatenated to "MYTEXT".

String conversion

Numerical values can be converted to strings with the str function:


String formatting

String formatting is supported using the (old) %-style Python way.

All %-specifiers as defined in Python documentation.

As an example, supposing you have a default 10mm-side cube named 'Box' (default FreeCAD naming), the following expression <<Cube length : %s>> % Box.Length will expand to "Cube length : 10.0 mm"

For more than one %-specifier use the following syntax: <<Cube length is %s and width is %s>> % tuple(Box.Length; Box.Width). Or use concatenation: <<Cube length is %s>> % Box.Length + << and width is %s>> % Box.Width. Both will expand to "Cube length is 10.0 mm and width is 10.0 mm".

A FreeCAD sample file using string formatting is available in the forum


Object creation functions

The following objects may be created in expressions using the following functions:

Type Function Description
Tuple tuple(a; b; ...) Example: tuple(2; 1; 2)
List list(a; b; ...) Example: list(2; 1; 2)
Vector vector(x; y; z) Create a vector using three unit-less or Length unit values.

Example: vector(2; 1; 3)

create(<<vector>>; x; y; z)
  a11; a12; a13; a14;
  a21; a22; a23; a24;
  a31; a32; a33; a34;
  a41; a42; a43; a44
Create a 4x4 matrix in row-major order:

A minimum of 1 argument can be supplied such as matrix(1) which creates an identity matrix.

Example: matrix(1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16)

create(<<matrix>>; a11; a12; ...; a44)
Rotation rotation(axis; angle) Create a Rotation by specifying its axis (Vector) and angle (Angle unit or unit-less), or three Euler angles α, β, γ.


  • rotation(vector(0; 1; 0); 45)
  • create(<<rotation>>; 30; 30; 30)
rotation(α; β; γ)
create(<<rotation>>; axis; angle)
create(<<rotation>>; α; β; γ)
Placement placement(base; rotation) Create a Placement with various parameters, including:
  • base: base location (Vector)
  • center: center location (Vector)
  • rotation: Rotation
  • axis: Rotation axis (Vector)
  • angle: Rotation angle (unit-less or Angle unit value)
  • matrix: Matrix


  • placement(vector(2; 1; 3); rotation(vector(0; 0; 1); 45))
  • create(<<placement>>; create(<<vector>>; 2; 1; 2); create(<<rotation>>; create(<<vector>>; 0; 1; 0); 45))
placement(base; rotation; center)
placement(base; axis; angle)
create(<<placement>>; ...)


Vector functions

Functions: introduced in version 0.22.

Function / Operator Description
v1 + v2 Add two vectors.
v1 - v2 Subtract two vectors.
v * s Uniformly scale a vector by s.
vangle(v1; v2) Angle between two vectors in degrees.
vcross(v1; v2) Cross product of two vectors .
v1 * v2 Dot product of two vectors .
vdot(v1; v2)
vlinedist(v1; v2; v3) Distance between vector v1 and a line through v2 in direction v3.
vlinesegdist(v1; v2; v3) Distance between vector v1 and the closest point on a line segment from v2 to v3.
vlineproj(v1; v2; v3) Project vector v1 on a line through v2 in direction v3.
vnormalize(v) Normalize a vector to a unit vector.
vplanedist(v1) Distance between vector v1 and a plane defined by a point v2 and a normal v3.
vplaneproj(v1) Project vector v1 on a plane defined by a point v2 and a normal v3.
vscale(v; sx; sy; sz) Non-uniformly scale a vector by sx in the X direction, sy in the Y direction, and sz in the Z direction.
vscalex(v; sx)
vscaley(v; sy)
vscalez(v; sz)


Matrix functions

Rotation and Placement can each be represented by a Matrix. The following functions all take in a Matrix, Rotation, or Placement as their first parameter denoted in the table below by m. The type of the returned object is the same as the object supplied in the first argument except when using mtranslate on a Rotation, in which case a Placement will be returned.

Function Description
minvert(m) Calculate the Inverse matrix.
mrotate(m; rotation) Rotate by either:
  • a Rotation
  • an axis (Vector) and an angle (Angle unit or unit-less)
  • three Euler angles α, β, γ
mrotate(m; axis; angle)
mrotate(m; α; β; γ)
mrotatex(m; angle) Rotate around the X axis.
mrotatey(m; angle) Rotate around the Y axis.
mrotatez(m; angle) Rotate around the Z axis.
mtranslate(m; vector) Translate by a vector (Vector) or X, Y, Z values. If a Rotation is translated, the returned object is a Placement.
mtranslate(m; x; y; z)
mscale(m; vector) Scale by a vector (Vector) or X, Y, Z values.
mscale(m; x; y; z)


Conditional expressions

Conditional expressions are of the form condition ? resultTrue : resultFalse. The condition is defined as an expression that evaluates to either 0 (false) or non-zero (true). Note that enclosing the conditional expression in parentheses is currently considered an error.

The following relational operators are defined:

Unit Description
== equal to
!= not equal to
> greater than
< less than
>= greater than or equal to
<= less than or equal to



Units can be used directly in expressions. The parser connects them to the previous value. So 2mm or 2 mm is valid while mm is invalid because there is no preceding value.

All values must have a unit. Therefore you must in general use a unit for values in spreadsheets.
In some cases it works even without a unit, for example if you have e.g. in spreadsheet cell B1 just the number 1.5 and refer to it for a pad height. This only works because the pad height predefines the unit mm that is used if no unit is given. It will nevertheless fail if you use for the pad height e.g. Sketch1.Constraints.Width - Spreadsheet.B1 because Sketch1.Constraints.Width has a unit and Spreadsheet.B1 has not.

Units with exponents can directly be entered. So e.g. mm^3 will be recognized as mm³ and m^3 will be recognized as m³.

If you have a variable whose name is that of a unit you must put the variable between << >> to prevent it from being recognized as a unit. For example if you have the dimension Sketch.Constraints.A it would be recognized as the unit ampere. Therefore you must write it in the expression as Sketch.Constraints.<<A>>.

The following units are recognized by the expression parser:

Amount of substance

Unit Description
mmol Millimole
mol Mole


Unit Description
° Degree; alternative to the unit deg
deg Degree; alternative to the unit °
rad Radian
gon Gradian
S Second of arc; alternative to the unit ″
Second of arc; alternative to the unit S
M Minute of arc; alternative to the unit ′
Minute of arc; alternative to the unit M


Unit Description
mA Milliampere
A Ampere
kA Kiloampere
MA Megaampere

Electric capacitance

Unit Description
pF Picofarad
nF Nanofarad
uF Microfarad; alternative to the unit µF
µF Microfarad; alternative to the unit uF
mF Millifarad
F Farad; 1 F = 1 s^4·A^2/m^2/kg

Electric charge

Unit Description
C Coulomb; 1 C = 1 A*s

Electric conductivity

Unit Description
uS Microsiemens; alternative to the unit µS
µS Microsiemens; alternative to the unit uS
mS Millisiemens
S Siemens; 1 S = 1 s^3·A^2/kg/m^2
kS KiloSiemens
MS MegaSiemens

Electric inductance

Unit Description
nH Nanohenry
uH Microhenry; alternative to the unit µH
µH Microhenry; alternative to the unit uH
mH Millihenry
H Henry; 1 H = 1 kg·m^2/s^2/A^2

Electric potential

Unit Description
mV Millivolt
V Volt
kV Kilovolt

Electric resistance

Unit Description
Ohm Ohm; 1 Ohm = 1 kg·m^2/s^3/A^2
kOhm Kiloohm
MOhm Megaohm


Unit Description
mJ Millijoule
J Joule
kJ Kilojoule
eV Electronvolt; 1 eV = 1.602176634e-19 J
keV Kiloelectronvolt
MeV Megaelectronvolt
kWh Kilowatt hour; 1 kWh = 3.6e6 J
Ws Watt second; alternative to the unit Joule
VAs Volt-ampere-second; alternative to the unit Joule
CV Coulomb-volt; alternative to the unit Joule
cal Calorie; 1 cal = 4.184 J
kcal Kilocalorie


Unit Description
mN Millinewton
N Newton
kN Kilonewton
MN Meganewton
lbf Pound of force


Unit Description
nm Nanometer
um Micrometer; alternative to the unit µm
µm Micrometer; alternative to the unit um
mm Millimeter
cm Centimeter
dm Decimeter
m Meter
km Kilometer
mil Thousandth of an inch; alternative to the unit thou
thou Thousandth of an inch; alternative to the unit mil
in Inch; alternative to the unit "
" Inch; alternative to the unit in
ft Foot; alternative to the unit '
' Foot; alternative to the unit ft
yd Yard
mi Mile

Luminous intensity

Unit Description
cd Candela

Magnetic flux

Unit Description
Wb Weber; 1 Wb = 1 kg*m^2/s^2/A

Magnetic flux density

Unit Description
G Gauss; 1 G = 1 e-4 T
T Tesla; 1 T = 1 kg/s^2/A


Unit Description
ug Microgram; alternative to the unit µg
µg Microgram; alternative to the unit ug
mg Milligram
g Gram
kg Kilogram
t Tonne
oz Ounce
lb Pound; alternative to the unit lbm
lbm Pound; alternative to the unit lb
st Stone
cwt Hundredweight


Unit Description
W Watt
kW Kilowatt


Unit Description
Pa Pascal
kPa Kilopascal
MPa Megapascal
GPa Gigapascal
uTorr Microtorr; alternative to the unit µTorr
µTorr Microtorr; alternative to the unit uTorr
mTorr Millitorr
Torr Torr; 1 Torr = 133.32 Pa
psi Pound-force per square inch; 1 psi = 6.895 kPa
ksi Kilopound-force per square inch


Unit Description
uK Microkelvin; alternative to the unit µK
µK Microkelvin; alternative to the unit uK
mK Millikelvin
K Kelvin


Unit Description
s Second
min Minute
h Hour
Hz (1/s) Hertz
kHz Kilohertz,
MHz Megahertz
GHz Gigahertz
THz Terahertz


Unit Description
ml Milliliter
l Liter
cft Cubicfoot

Special imperial units

Unit Description
mph Miles per hour
sqft Square foot

Unsupported units

The following commonly used units are not yet supported, for some an alternative is provided:

Unit Description Alternative
°C Celsius [°C] + 273.15 K
°F Fahrenheit; ([°F] + 459.67) × ​5/9
u Atomic mass unit; alternative to the unit Da 1.66053906660e-27 kg
Da Dalton; alternative to the unit u 1.66053906660e-27 kg
sr Steradian not directly
lm Lumen not directly
lx Lux not directly
px Pixel not directly


Invalid characters and names

The expression feature is very powerful but to achieve this power it has some limitations concerning some characters. To overcome this, FreeCAD offers to use labels and reference them instead of the object names. In labels you can use almost all special characters.

In cases where you cannot use a label, such as the name of a sketch's constraints, you must be aware what characters are not allowed.


For labels there are no invalid characters, however some characters need to be escaped:

Characters Description
', \, " Need to be escaped by adding \ in front of them.

For example, the label Sketch\002 must be referenced as <<Sketch\\002>>.


Names of objects like dimensions, sketches, etc. may not have the characters or character sequences listed below, otherwise the name is invalid:

Characters / Character sequences Description
+, -, *, /, ^, _, <, >, (, ), {, }, [, ], ., ,, = Characters that are math operators or part of mathematical constructs
A, kA, mA, MA, J, K, ' , ft , °, and many more! Characters and character sequences that are units (see the Units paragraph)
#, !, ?, §, $, %, &, :, ;, \, |, ~, , ¿, and many more! Characters used as placeholder or to trigger special operations
pi, e Mathematical constants
´, `, ' , " Characters used for accents
space A space defines the end of a name and can therefore not be used

For example, the following name is valid: <<Sketch>>.Constraints.T2üßµ@. While these are invalid names: <<Sketch>>.Constraints.test\result_2 (\r means "carriage return") or <<Sketch>>.Constraints.mol (mol is a unit).

Since shorter names (especially if they have only one or two characters) can easily result in invalid names, consider using longer names and/or establishing a suitable naming convention.

Cell aliases

See Spreadsheet SetAlias.


Reference to CAD data

It is possible to use data from the model itself in an expression. To reference a property If the property is a compound of fields, the individual fields can be accessed as

The following table shows some examples:

CAD data Call in expression Result
Length of a Part Box Box.Length Length with units (mm)
Volume of the Box Box.Shape.Volume Volume in mm³ without units
Shape type of the Box Box.Shape.ShapeType String: Solid
Label of the Box Box.Label String: Label
X-coordinate of center of mass of the Box Box.Shape.CenterOfMass.x X-coordinate in mm without units
X-coordinate of the Box placement Box.Placement.Base.x X-coordinate with units (mm)
X-component of the rotation axis of the Box placement Box.Placement.Rotation.Axis.x X-component of the unit vector in mm without units
Rotation angle of the Box placement Box.Placement.Rotation.Angle Rotation angle with units (deg)
Full Box object Box._self Object of type <Part::PartFeature>
Value of constraint in a sketch Constraints.Width Numeric value of the named constraint Width in the sketch, if the expression is used in the sketch itself.
Value of constraint in a sketch MySketch.Constraints.Width Numeric value of the named constraint Width in the sketch, if the expression is used outside of the sketch.
Value of a spreadsheet alias Spreadsheet.Depth Value of the alias Depth in the spreadsheet Spreadsheet
Value of a local property Length Value of the DataLength property in e.g a Pad object, if the expression is used in e.g DataLength2 in the same object.


Document-wide global variables

There is no concept of global variables in FreeCAD at the moment. Instead, arbitrary variables can be defined as cells in a spreadsheet using the Spreadsheet workbench, and then be given a name using the alias property for the cell (right-click on cell). Then they can be accessed from any expression just as any other object property.


Cross-document linking

It is possible (with limitations) to define a Property of an object in your current document (".FCstd" file) by using an Expression to reference a Property of an object contained in a different document (".FCstd" file). For example, a cell in a spreadsheet or the DataLength of a Part Cube, etc. in one document can be defined by an Expression that references the X Placement value or another Property of an object contained in a different document.

A document's name is used to reference it from other documents. When saving a document the first time, you choose a file name; this is usually different from the initial default "Unnamed1" (or its translated equivalent). To prevent links being lost when the master document is renamed upon saving, it is recommended that you first create the master document, create a spreadsheet inside it, and save it. Subsequently, you can still make changes to the file and its spreadsheet but you should not rename it.

Once the master document with the spreadsheet is created and saved (named), it is safe to create dependent documents. For example, assuming you name the master document master, the spreadsheet modelConstants, and give a cell an alias-name Length, you can then access the value as:


Note: that the master document must be loaded for the values in the master to be available to the dependent document.

Unfortunately, the integrated checker sometimes claims that a valid name doesn't exist. Continue typing anyway. When you have completed the full reference, the OK button will become active.

Of course, it's up to you to load the corresponding documents later when you want to change anything.


Known issues / remaining tasks

  • The dependency graph is based on the relationship between document objects, not properties. This means that you cannot provide data to an object and query that same object for results. For example, even though there are no cyclic dependencies when the properties themselves are considered, you may not have an object which gets its dimensions from a spreadsheet and then display the volume of that object in the same spreadsheet. As a work-around use multiple spreadsheets, one to drive your model and the other for reporting.
  • The expression parser does not handle parentheses well, and is unable to properly parse some expressions. For example: = (A1 > A2) ? 1 : 0 results in an error, while = A1 > A2 ? 1 : 0 is accepted. The expression = 5 + ((A1>A2) ? 1 : 0) cannot be entered in any form.
  • As stated above, unfortunately, the integrated checker sometimes claims that a valid name doesn't exist. Continue typing anyway. When you have completed the full reference, the OK button will become active.
  • FreeCAD does not yet have a built-in expression manager where all expressions in a document are listed, and can be created, deleted, queried, etc. But an addon is available: fcxref expression manager.
  • Open bugs/tickets for Expressions can be found in the FreeCAD Bugtracker Expressions category