The formula
Each share = amount × share ÷ sum of shares
Where it comes from
The ownership share is the percentage each flat represents of the whole building, set in the deed of division according to floor area and other factors. Shared costs are split in proportion to that share unless the bylaws say otherwise, so a large top-floor flat pays more than a small ground-floor one for the same lift.
How to work it out by hand
- Find each flat's ownership share in the deeds
- Add them all up
- Divide each one by that total
- Multiply by the amount of the cost
What is worth knowing
If you enter the shares for the whole building they should add up to a hundred, and you get a warning here when they do not. But the split still works with only a few flats, because it is proportional: what you cannot do is treat a partial result as anyone's real bill. And there are two common exceptions: bylaws can exempt ground-floor flats from lift costs, and certain works are split by a meeting resolution rather than by share.