Revit: Troubleshooting: Formulas for Dimensions

@AniphaeS

Published in

Architecture

· 26 Feb 2020

Understand errors in units and numerical constants in formulas that are used to calculate dimensions.

Formulas and Units

Formulas can operate on the following types of data:

Numbers with units
Boolean values (yes/no)
Strings (text)

When using formulas to calculate dimensions, an error in units typically results when you do not follow the rules required by an operation.

Examples

For addition, both inputs and the result must have the same numerical unit type.
Multiplication can support any numerical unit type for the inputs (not necessarily the same type). However, the result must be the product of the input units. For example: length * area = volume.
sin( ) requires the input to be an angle, and the output to be a number with no unit type.
if( ) requires the first argument to be Boolean. The other arguments and the result can be of any data type (numeric, Boolean, text), but they must all be the same data type.

Revit is working with the base physical dimension (Time, Length, Mass, Temperature, and so on). Dimensional analysis is independent of the unit used to represent or measure the base physical dimension (seconds, meters, kg, degrees Celsius, and so on).

Formulas and Constants

Another source of dimensional errors is the interpretation of numerical constants in formulas.

When a formula includes a parameter name or a numerical constant with explicit units (such as 5 kg), Revit knows the units of the expression with certainty. However, the software also allows you to enter a number without a unit (such as 5). In this case, Revit guesses the units of the number to make the units in the formula agree. This issue is the reason for the common trick of adding *1 or /1 to a formula.

The best practice is to specify units for each constant in the formula.

Examples

You want the result of a formula to be a length. You enter 3 * 3. When you edit the formula again, you can see that Revit has guessed that one of those numbers is a length by the notation 3' * 3.
You want the result of a formula to be a length. When you enter 3 * 3 kg/3, you get an inconsistent units error. However, 3' * 3 kg/3 gives the desired result because Revit guesses that the last constant is intended to be kg. This formula also mixes units (imperial and metric), so when you edit the formula, you can see that Revit changed it to project units: 3' * 6.614 lbm/3 lbm.

Units for Constants

The following units can be used in formulas for constants. This list is not exhaustive, but it includes the most common unit types.

Tip: to use degrees for angle or temperature, first enter radians (rad) or kelvin (K) and then re-edit the formula. Revit replaces those units with the degree symbol if it is set as your project unit for angle or temperature.

US Customary Units

  
Unit                      Symbols 
Linear feet               ' or LF 
Inches                    " 
Square feet               SF or ft2 or ft^2 
Cubic feet                CF or ft3 or ft^3 
Seconds (time)            s 
Hours                     h 
Pounds mass               lbm 
Pounds force              lbf 
Kips                      kip 
Radians                   rad 
Fahrenheit degrees        °F 
Decimal degrees           ° 
Kips per square inch      ksi or kip/in2 
Pounds per square inch    psi or lbf/in2 or psig or psia

SI Units (International System of Units)

  
Unit                  Symbols 
Millimeters           mm 
Centimeters           cm 
Decimeters            dm 
Meters                m 
Kilograms             kg 
Celsius degrees       °C 
Kelvin                K 
Newtons               N 
Kilonewtons           kN 
Meganewtons           MN 
Pascals               Pa 
Megapascals           MPa

Parent topic: Troubleshooting: Document and Present the Design