Weekly, fortnightly and monthly are different rhythms — not rough equivalents.

The same recurring cost can look very different

A bill can be expressed in any planning view, but the conversion has to preserve the same annual total. That is the simplest way to avoid drift when different parts of a budget use different frequencies.

Why “four weeks in a month” causes trouble

Four weeks is 28 days. Across a year, twelve four-week blocks account for only 48 weeks. A real year has 52 weeks, so recurring-cost conversions based on a four-week month do not preserve the annual total.

Normalise for the view, not the source

If groceries are $300 every fortnight, keep them recorded as $300 fortnightly. If a subscription is $30 monthly, keep it monthly. A normalised budget can then show both in a weekly, fortnightly or monthly planning view without altering what you actually entered.