The unit belongs to the number
A measurement is a quantity: a numeric value and a unit. 12 ft and 12 m describe different lengths even though the visible number is the same.
Keep units in the expression. This allows the calculator to check whether the operation makes sense and choose a result dimension.
| Expression | Meaning |
|---|---|
12 ft + 6 in |
Add two lengths after converting them to a common scale. |
12 ft * 8 ft |
Multiply lengths to get an area. |
96 ft2 / 8 ft |
Divide an area by a length to get a length. |
3 m + 2 m2 |
Incompatible dimensions; there is no meaningful direct sum. |
Entering a fraction
Fractions are especially useful with inches. The expression engine accepts 1/2 in, mixed numbers such as 5 1/2 in, and compound measurements such as 5 ft 6 in or 5'6". Keep the unit explicit. A plain mathematical division and a fraction within a measurement can look similar, so review the displayed expression before using it.
The slash spacing is meaningful. A fraction literal has no spaces around its internal slash. A spaced slash is the division operator; the keypad inserts spaced operators.
| Entry | How it is read |
|---|---|
1/2 in |
A half-inch measurement. |
5 1/2 in |
Five and a half inches. |
12/2 ft |
The fractional value 12/2, attached to feet: 6 ft. |
12 / 2 ft |
12 divided by a two-foot quantity: an inverse length, not 6 ft. |
(12 ft) / 2 |
A twelve-foot measurement divided by two: 6 ft. |
Use units and parentheses to make your intent explicit when dividing quantities.
You can always write an explicit grouped expression:
5 in + (1 in / 2)This is 5½ inches. The group makes the intent unambiguous even without mixed-number shorthand.
Convert the result, not the job
Calculate using the measurements in front of you. Then choose a compatible output unit. A length can become feet, inches, meters, or millimeters. An area needs an area unit; a volume needs a volume unit.
A displayed value may be rounded to fit the selected precision. Converting a quantity should use its stored value, not a value copied from a rounded label.
When a result will become a cut, purchase, or layout mark, choose the rounding direction deliberately. Rounding up a count of bags is a different operation from rounding a length to the nearest sixteenth of an inch.
Imperial and US customary details
The international foot is exactly 0.3048 meters. A cubic yard is 27 cubic feet. These relationships let a volume calculation move between dimensional units without changing its physical meaning. See the NIST guide to SI units for unit definitions and conversion guidance.
Gallon-based quantities need a named convention. A US liquid gallon and an imperial gallon are different units. Read the unit label and the helper’s assumptions; never treat a bare word gallon as a universal volume.
Temperature needs special care
An absolute temperature and a temperature difference are different concepts. Celsius and Fahrenheit absolute scales have an offset. A temperature difference has a scale factor but no offset.
The domain permits conversion between absolute Celsius, Fahrenheit, and Kelvin temperatures, and rejects arithmetic on absolute temperatures. Temperature-difference units (dC, dF, and dK) can participate in arithmetic. Do not multiply an absolute Fahrenheit reading as though it were an ordinary length or count.
Errors are useful information
- Incompatible units: check whether an input is a length, area, or volume.
- Incomplete expression: finish the operation or close the parentheses.
- Division by zero: inspect the denominator and any missing field that became zero.
- Invalid measurement: enter a finite number and respect the helper’s allowed range.
Correct the input and calculate again. An error should not leave an old result looking like the answer to a new, invalid expression.