Understanding Dimensional Errors in Formulas

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