# Units & formulas

> Give number fields units, show values in imperial and metric, and compute fields with formulas that check units before they save.

Source: https://www.speclatch.com/guide/units-formulas
Plan: Every plan.
Last verified: 2026-11-11

## What to know first

- **Fields that know their unit.** A **Number** field can carry a unit, such as psi, GPM, in or °F. Spec sheets print both systems.
- **Formulas that check units.** A **Calculated** field computes from other fields. The editor refuses math that mixes kinds, like a pressure plus a length.
- **Your own functions.** Name a formula once on **Functions**, then call it from any field.
- **Reference tables.** Values from a chart or a standard, such as a rating by temperature, read with `lookup()`.
- **Curves.** Data series hold X/Y points for each record, such as a performance curve.

- [Give fields units and write your first formula](https://www.speclatch.com/guide/units-formulas/write-a-formula): Give a number field a unit, add a Calculated field, write a formula that checks units, and see the result in both systems.
- [Define a reusable function](https://www.speclatch.com/guide/units-formulas/define-a-function): Write a calculation once, give it a name, and use it in any calculated field. When the math changes, you fix it in one place.
- [Performance curves](https://www.speclatch.com/guide/units-formulas/performance-curves): Define a data series for a table, type each record's curve points, and let calculated lines draw themselves from the record's values.
- [How formulas work](https://www.speclatch.com/guide/units-formulas/formula-language): Field keys and operators, unit checks, temperatures, tests and choices, linked records, reference tables, your own functions and curves, each with an example.
- [Built-in functions, with examples](https://www.speclatch.com/guide/units-formulas/functions): Every function a formula can call without defining it, from if() and round() to num(), qty() and lookup(), with worked examples in real units.

## Where you see it

Admin and Engineering members set units and formulas on **Fields**, under **Data model**. **Functions** and **Reference tables** sit beside it, readable by everyone. Curves live in each record's **Curves** section. In the grid, a calculated value carries a small ƒ; hover it to see the formula.

![The Functions page with a function selected: its formula, a live calculator with one box per input, and the fields that use it.](https://www.speclatch.com/guide/units-formulas/overview-1.png)

*A function's page shows its formula, a calculator to try it, and where it is used.*

## An example

Take a 0 to 300 psi pressure gauge, accurate to 0.5 % of span.

- Inputs: **Range, low end** = 0 psi, **Range, high end** = 300 psi, **Accuracy (% of span)** = 0.5.
- **Span** is `range_max - range_min`, which gives 300 psi (20.7 bar).
- **Accuracy (±)** is `span * accuracy_pct / 100`, which gives 1.5 psi (0.103 bar).

The editor refuses `range_max + 5`, because 5 has no unit. Write `range_max + qty(5, "psi")` instead.

## FAQ

### Which units can a field use?

The unit picker groups units by kind: length, mass, pressure, flow, power, temperature and more. Pick the unit you enter values in. SpecLatch pairs it with a unit in the other system, such as psi with bar or GPM with m³/h. **Change**, under the unit, picks other display units.

### Why won't my formula save?

The red message under the editor names the problem. Usually the two sides of a `+` or `-` are different kinds of value, or a plain number needs a unit through `qty()`. Some results have a unit SpecLatch can't store, such as flow times head. Read those inputs as plain numbers with `num()`, do the math, then give the result its unit with `qty()`.

### When do calculated values update?

On their own, and nobody types into them. A calculated field recomputes when one of its inputs changes or a function it calls is edited. It also recomputes when a linked record it reads changes, and after an import.

### Can a formula give a word instead of a number?

Yes. An `if()` that returns labels in quotes gives a choice, for example `if(ratio < 0.4, "Pass", ratio < 0.8, "Check", "Fail")`. The first true test wins. The result filters and prints like a single choice field.

### Who can change units and formulas?

Admin members, and Engineering members with Confidential visibility, edit fields, functions and reference tables. Everyone can open **Functions** and **Reference tables** to read how a value is computed. Which values each person sees still follows the field tiers.