How do I calculate the number of units I can fit in to a commercial conversion?

Take the gross floor area, for example, 100sqm. Take 20% of this off and assume it will be used for communal areas such as stairs and hallways. This gives us 80sqm. Then, see how many units of your chosen type (over the minimum space standards) can be added into this space such as 1 bed flats, 2 bed flats and more.