# Units¶

## Fundamental Units¶

To describe a spatial model we need to define the following fundamental units:

`amount`

(e.g. Mole)`length`

(e.g. metre)`time`

(e.g. second)

Volume is not a fundamental unit. However, for user convenience we treat volume as a fundamental unit

`volume`

(e.g. litre)

This allows the use of units such as cm for length and mMol/mL for concentration, instead of the equivalent but less common mMol/cm^3.

## Derived Units¶

All quantities in the model have units that can be written as some combination of these fundamental units:

- species concentrations
- have units of
`amount`

/`volume`

- have units of

- reactions
*inside*a compartment - describe the rate of change of the concentration of species
- have units of concentration / time, i.e.
`amount`

/`volume`

/`time`

- reactions
- reactions
*between*two compartments - describe that rate at which a unit amount of species crosses a unit area of the
*membrane* - where the membrane is the area where the two compartments touch each other
- have units of amount / membrane-area / time, i.e.
`amount`

/`length`

/`length`

/`time`

- describe that rate at which a unit amount of species crosses a unit area of the

- reactions
- diffusion constants
- have units of area / time, i.e.
`length`

*`length`

/`time`

- have units of area / time, i.e.