# Introduction to SciMath Units¶

A large number of units used in science and engineering are available for use, and scimath.units makes working with and converting among them easy.

## Getting Started¶

We can import unit objects from the scimath.units submodules. (See list of available units for a listing by submodule.) There are submodules for many categories of physical quantities.

```
>>> from scimath.units.length import foot, inch, meter
>>> 3 * foot
0.9144000000000001*m
>>> foot / inch
12.0
>>> foot / meter
0.3048
```

Unit objects display natively in SI fundamental units and can be used to generate unit conversion factors or to see how units relate to one another.

```
>>> from scimath.units.pressure import pascal
>>> pascal * meter ** 2
1.0*m*kg*s**-2
```

**Caution**: If you are using this tool to produce conversion factors, remember
that the factor is the *inverse* of what it looks like. Above, we divided one
foot by one meter to get the ratio 0.3048. That is, `foot / meter`

yields the
number of meters per foot. Be careful to think through the logic clearly when
developing conversion factors.

You can define your own arbitrary units and use them for calculating conversion factors:

```
>>> from scimath.units.length import inch
>>> from scimath.units.force import lbf
>>> from scimath.units.pressure import torr
>>> my_psi = 2 * lbf / inch ** 2
>>> my_psi / torr
103.44718363855331
```

Internally, they are stored as a *derivation* of fundamental physical
quantities expressed in the SI system

```
>>> my_psi
13789.509579019157*m**-1*kg*s**-2
```

Conversions can be made between units with the same derivation using
`convert()`

:

```
>>> from scimath.units.api import convert
>>> from scimath.units.force import lbf, newton
>>> convert(1, lbf, newton)
4.44822
```

However, adding incompatible units raises an `IncompatibleUnits`

exception:

```
>>> from scimath.units.electromagnetism import volt
>>> from scimath.units.mass import kilogram
>>> 1 * volt + 2 * kilogram
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "scimath/units/unit.py", line 62, in __add__
raise IncompatibleUnits("add", self, other)
scimath.units.unit.IncompatibleUnits: Cannot add quantities with units of 'm**2*kg*s**-3*A**-1' and 'kg'
```