# 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
• 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 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
• diffusion constants
• have units of area / time, i.e. length * length / time