For the popular single based 28mm figures, the size of the base determines how packed together the troops will be. Smaller bases allow more troops to fit into a small area. This determines the unit's formation density (explained later).

Close order foot	20mm base
Loose order foot	25mm base
Open order foot	40mm base *
Close order mounted	20mm base **
Loose order mounted	25mm base
Open order mounted	40mm base

* two per base, placed one behind the other to make two ranks - or alternatively, mounted on 20mm bases in a checkerboard pattern
** an unusual troop type, most cavalry is based in loose formation - and since these bases don't exist, use 40mm (2 per base)