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The quantity is a combination of a floating point number and a unit. It is used throughout all of FreeCAD to handle parameters and all other kind of input/output.


In a CAD or CAE system it is very important to keep track of the unit of a value. Lots of trouble can arise when mixing up units or calculating results in different systems of units. One famous disaster is the crash of the Mars Climate Orbiter due to a unit mix-up. Even in the same system of units the units come in lots of different flavours always tailored to the field of use. Simple examples are e.g. velocity in km/h (cars), m/s (robotics) or mm/minute (milling). A CAD system has to keep track of units reliably. Also it has to do calculations with them and check on the right unit for special parameters.

For that reason the FreeCAD Quantity framework was created. It includes all the code and objects to deal with units, unit calculations, user input, conversion to other systems of units and the pretty output of units and values. In the long run no parameter in FreeCAD should be just a number.

Supported units

The FreeCAD input parser supports a bunch of units and systems of units. We do use the greek letter for micro but also accept 'u' as a replacement.

  • Length
    • "nm"
    • "µm"
    • "mm"
    • "cm"
    • "dm"
    • "m"
    • "km"
    • "in"
    • "ft"
    • "thou"
    • "mil"
    • "yd"
    • "mi"

Todo: all the rest...

The detailed specifications you will find in the code:

Internal representation

All physical units can be expressed as a combination of the seven SI-Units:


An easy way to express a unit is an integer array of size 7 (number of base units) that defines what the unit is. The signature of the 7 base units are:

  • LENGTH: [1,0,0,0,0,0,0]
  • MASS: [0,1,0,0,0,0,0]
  • TIME: [0,0,1,0,0,0,0]
  • ELECTRIC CURRENT: [0,0,0,1,0,0,0]
  • AMOUNT OF SUBSTANCE: [0,0,0,0,0,1,0]
  • LUMINOUS INTENSITY: [0,0,0,0,0,0,1]

Using these seven units we are then able to express all derived units defined in Guide for the Use of the International System of Units (SI) and create new ones as needed such as for instance:

  • MASS DENSITY: [-3,1,0,0,0,0,0]
  • AREA: [0,2,0,0,0,0,0]

Since angle is physically dimensionless, but nevertheless important to a CAD system we add one more virtual unit for Angle. This makes a vector of 8 in the FreeCAD unit signature.

Units calculator

Often you are in need of converting values from one system of units to another. For example you have old parameter tables with wired units. In these cases FreeCAD offers a conversion tool called Units-Calculator which helps in translating units.

Its description in detail is here: Std_UnitsCalculator


The InputField is a QLineEdit derived Qt widget to handle all kinds of user interaction with quantities and parameters. It features the following properties:

  • parsing arbitrary value/unit input
  • checking on the right unit (if given) and give the user feedback
  • special context menu for operations on quantities/values
  • history management (save the last used values)
  • save often needed values as shortcut in context menu
  • selecting values with mouse wheel and arrow keys (tbd)
  • selecting with middle mouse button and mouse move (tbd)
  • Python integration for usage in Python only dialogs (tbd)

The UnitsCalculator uses the InputField already.

Main documentation: InputField


Python scripting

The Unit and Quantity system in FreeCAD is (as nearly everything) fully accessibly via Python.


The Unit class represents the fingerprint of any physical unit. As described in the Basics section a vector of eight numbers is used to represent this fingerprint. The Unit class allows the handling and calculation based on this information.

from Units import Unit

# creating a unit with certain signature
Unit(0,1)      # Mass     (kg)
Unit(1)        # Length   (mm)
Unit(-1,1,-2)  # Pressure (kg/mm*s^2)

# using predefined constants

# parsing unit out of a string
Unit('kg/(m*s^2)')    # Pressure
Unit('Pa')            # the same as combined unit Pascale
Unit('J')             # Joul (work,energy) mm^2*kg/(s^2)

# you can use units from all supported systems of units
Unit('psi')           # Imperial pressure
Unit('lb')            # Mass
Unit('ft^2')          # Area

# comparing units
Unit(0,1) == Unit(FreeCAD.Units.Mass)

# getting type of unit
Unit('kg/(m*s^2)').Type == 'Pressure'

# calculating
Unit('kg') * Unit('m^-1*s^-2') == Unit('kg/(m*s^2)')

The unit is mainly used to describe a certain type of unit for a parameter. Therefore a special property type in FreeCAD can pass a unit to check and ensure the right unit. A unit and a float value is called quantity.


from Units import Unit,Quantity

# to create a quantity you need a value (float) and a unit
Quantity(1.0,Unit(0,1))               # Mass      1.0 kg
Quantity(1.0,Unit(1))                 # Length    1.0 mm
Quantity(1.0,Unit(-1,1,-2))           # Pressure  1.0 kg/mm*s^2
Quantity(1.0,FreeCAD.Units.Pressure)  # Pressure  1.0 kg/mm*s^2

# you can directly give a signature
Quantity(1.0,0,1)      # Mass      1.0 kg
Quantity(1.0,1)        # Length    1.0 mm
Quantity(1.0,-1,1,-2)  # Pressure  1.0 kg/mm*s^2

# parsing quantities out of a string
Quantity('1.0 kg/(m*s^2)')    # Pressure
Quantity('1.0 Pa')            # the same as combined Unit Pascale
Quantity('1.0 J')             # Joul (Work,Energy) mm^2*kg/(s^2)

# You can use a point or comma as float delimiter
Quantity('1,0 m')    
Quantity('1.0 m')   

# you can use units from all supported systems of units
Quantity('1.0 psi')           # Imperial pressure
Quantity('1.0 lb')            # Mass
Quantity('1.0 ft^2')    

# the quantity parser can do calculations too
Quantity('360/5 deg')           # splitting circle 
Quantity('1/16 in')             # fractions
Quantity('5.3*6.3 m^2')         # calculating an area
Quantity('1/(log(2.3)/sin(pi)*3.4)+1.8e-3 m')
Quantity('1ft 3in')             # imperial style

# and for sure calculation and comparison
Quantity('1 Pa')* Quantity(2.0) == Quantity('2 Pa')
Quantity('1 m')* Quantity('2 m') == Quantity('2 m^2')
Quantity('1 m')* Quantity('2 ft') + Quantity('2 mm^2')
Quantity('1 m') > Quantity('2 ft')

# accessing the components
Quantity('1 m').Value     # get the number (allways internal system (mm/kg/s))
Quantity('1 m').Unit      # get the unit
Quantity('1 m') == Quantity( Quantity('1 m').Value , Quantity('1 m').Unit )

# translating the value into other units than the internal system (mm/kg/s)
Quantity('1 km/h').getValueAs('m/s')               # translate value
Quantity('1 m').getValueAs(2.45,1)                 # translation value and unit signature
Quantity('1 kPa').getValueAs(FreeCAD.Units.Pascal) # predefined standard units 
Quantity('1 MPa').getValueAs(Quantity('N/m^2'))    # a quantity

User facing values

Normally in scripts you can use Quantity for all kinds of calculations and checking, but there comes the time you have to output information to the user. You could use getValueAs() to force a certain unit, but normally the user sets his preferred unit-schema in the preferences. This unit-schema does all the translations to the representation the user likes to see. At the moment there are three schemes implemented:

  • 1: Internal (mm/kg/s)
  • 2: MKS (m/kg/s)
  • 3: US customary (in/lb)

There can be easily additional schemas implemented in the future...

The Quantity class has two options to use the actual schema translation:

from Units import Unit,Quantity

# Use the translated string:
Quantity('1m').UserString             # '1000 mm' in 1; '1 m' in 2; and '1.09361 yr' in 3

This does the job if you only need a string. But sometimes you need more control, e.g. if you want to have a dialog button which dials up and down. Then you need more information about the translation output. Therefore the getUserPreferred() method of quantity is used:

Quantity('22 m').getUserPreferred()  # gets a tuple:('22 m', 1000.0, 'm')
Quantity('2  m').getUserPreferred()  # Tuple: ('2000 mm', 1.0, 'mm')

Here you get more information using a tuple (three items). You get the string as before, plus the factor of the value and the raw string with only the unit chosen by the translation schema. With this information you can implement a much richer user interaction.

The code of the schema translation can be found here:


Parser supported units

Although all physical units can be described with the seven SI units, most of the units used in technical areas are common combined units (like Pa = N/m^2 Pascal ). Therefore the units parser in FreeCAD supports lot of SI and Imperial combined units. These units are defined in src/Base/QuantityParser.l file and can be further expanded in the future.

"nm"   = Quantity(1.0e-6    ,Unit(1));           // nano meter
"µm"   = Quantity(1.0e-3    ,Unit(1));           // micro meter
"mm"   = Quantity(1.0       ,Unit(1));           // milli meter
"cm"   = Quantity(10.0      ,Unit(1));           // centi meter
"dm"   = Quantity(100.0     ,Unit(1));           // deci meter
"m"    = Quantity(1.0e3     ,Unit(1));           // meter
"km"   = Quantity(1.0e6     ,Unit(1));           // kilo meter
"l"    = Quantity(1000000.0 ,Unit(3));           // Liter      dm^3
"µg"   = Quantity(1.0e-9    ,Unit(0,1));         // micro gram
"mg"   = Quantity(1.0e-6    ,Unit(0,1));         // milli gram
"g"    = Quantity(1.0e-3    ,Unit(0,1));         // gram
"kg"   = Quantity(1.0       ,Unit(0,1));         // kilo gram
"t"    = Quantity(1000.0    ,Unit(0,1));         // ton
"s"    = Quantity(1.0       ,Unit(0,0,1));       // second                          (internal standard time)
"min"  = Quantity(60.0      ,Unit(0,0,1));       // minute
"h"    = Quantity(3600.0    ,Unit(0,0,1));       // hour  
"A"    = Quantity(1.0       ,Unit(0,0,0,1));     // Ampere          (internal standard electric current)
"mA"   = Quantity(0.001     ,Unit(0,0,0,1));     // milli Ampere         
"kA"   = Quantity(1000.0    ,Unit(0,0,0,1));     // kilo Ampere         
"MA"   = Quantity(1.0e6     ,Unit(0,0,0,1));     // Mega Ampere         
"K"    = Quantity(1.0       ,Unit(0,0,0,0,1));   // Kelvin (internal standard thermodynamic temperature)
"mK"   = Quantity(0.001     ,Unit(0,0,0,0,1));   // Kelvin         
"µK"   = Quantity(0.000001  ,Unit(0,0,0,0,1));   // Kelvin         
"mol"  = Quantity(1.0       ,Unit(0,0,0,0,0,1));   // Mole     (internal standard amount of substance)        
"cd"   = Quantity(1.0       ,Unit(0,0,0,0,0,0,1)); // Candela   (internal standard luminous intensity)        
"deg"  = Quantity(1.0           ,Unit(0,0,0,0,0,0,0,1));  // degree         (internal standard angle)
"rad"  = Quantity(180/M_PI      ,Unit(0,0,0,0,0,0,0,1));  // radian         
"gon"  = Quantity(360.0/400.0   ,Unit(0,0,0,0,0,0,0,1));  // gon         
"in"   = Quantity(25.4          ,Unit(1));       // inch
"\""   = Quantity(25.4          ,Unit(1));       // inch
"fo"   = Quantity(304.8         ,Unit(1));       // foot
"'"    = Quantity(304.8         ,Unit(1));       // foot
"th"   = Quantity(0.0254        ,Unit(1));       // thou
"yd"   = Quantity(914.4         ,Unit(1));       // yard

"lb"   = Quantity(0.45359237    ,Unit(0,1));    // pound
"oz"   = Quantity(0.0283495231  ,Unit(0,1));    // ounce
"st"   = Quantity(6.35029318    ,Unit(0,1));    // Stone
"cwt"  = Quantity(50.80234544   ,Unit(0,1));    // hundredweights