The question.
If two people share a home and a partner stays over many nights each month, what would be a reasonable contribution toward the variable costs of the household?
There is no universal legal or social rule that answers that question. The calculator therefore does something narrower: it creates a baseline based on how many person-days are spent in the household during the month.
The method.
The calculation compares the guest's overnight presence with the total resident person-days for the month. The resulting ratio is applied only to the costs the user chooses to include. Electricity, water, gas or shared household supplies can be included; housing cost is optional.
Because the user controls the included costs, the result is an estimate for discussion rather than a statement about what someone legally or morally owes.
Why it is useful.
The point is not to turn a relationship into an accounting exercise. It is to replace a vague argument with a visible set of assumptions that both people can change. If the assumptions do not feel fair, they can be adjusted rather than hidden.