> ## Documentation Index
> Fetch the complete documentation index at: https://docs.truemath.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Unit numbers

> Numeric values that carry a unit. TrueMath converts compatible units automatically and rejects incompatible ones.

A **unit number** is a numeric value with a unit attached — `30 yr`, `5.2 m`, `375000 USD`, `60 mph`. The unit is part of the value, which lets TrueMath convert between compatible units and catch dimensional mistakes.

## Writing unit numbers

Write the value followed by a unit suffix. The space is optional — `5m` and `5 m` are identical:

```
5 m
3.5 kg
400000 USD
45 deg
12 m/s
```

Both simple and compound suffixes work. The suffix must be a recognized unit; see [Units](/math/units) for the full catalog. Writing a unit *inside an activity equation* has one extra wrinkle — a bare suffix can collide with a variable name — covered in [Writing equations](/authoring/writing-equations).

## Behavior

* **Automatic conversion.** Compatible units convert when combined: `1m + 10cm` yields a length, with no manual conversion.
* **Dimensional safety.** Incompatible combinations are rejected: `5m + 3kg` is an error. See [Combining types](/authoring/writing-equations#combining-types).
* **Derived dimensions.** Multiplying and dividing units produces compound units — a distance over a time is a velocity; a cost over an area is a cost-per-area.

## Precision

Like all values, unit numbers are carried at full precision through every step; rounding happens only at display. A value's unit and decimal display are governed by the [variable's](/concepts/variables) format. See [Units and
precision](/concepts/units-and-precision).