The Sinking Fund Method: Budgeting for Irregular Costs Like MOTs and Car Tax

Irregular costs - an MOT, car tax renewal, a boiler service, back-to-school spending - are individually predictable but collectively easy to be caught out by, precisely because they don’t arrive monthly like a mortgage or a phone bill. A sinking fund solves this by turning irregular costs into a smooth monthly saving habit.

What a sinking fund actually is

A sinking fund is money set aside in advance, in regular small instalments, specifically earmarked for a known future cost - rather than trying to find a lump sum when the bill actually arrives. It’s a well-established concept in business and public finance, applied here to ordinary household budgeting.

Common irregular costs well suited to this method

• Car costs: MOT (currently up to £54.85 for a standard car, though many garages charge less), annual car tax (varying by vehicle emissions and age), servicing, and an eventual replacement or repair fund.
• Home maintenance: boiler servicing, appliance repairs or replacement, and general upkeep.
• Seasonal costs: Christmas (see our dedicated article), back-to-school spending (see our dedicated article), and summer holidays (see our dedicated article) - all genuinely predictable in timing even if the exact amount varies year to year.
• Annual subscriptions or memberships billed yearly rather than monthly, where paying annually is often cheaper but creates a larger single outflow.
• Birthdays and gifts across the year, which collectively add up to a meaningful sum even though no single gift feels large in isolation.

How to calculate the monthly amount

For each cost, estimate the annual total (using last year’s actual cost as a starting point where available) and divide by 12 to get a monthly saving target - this works just as well layered on top of a baseline monthly figure if your income itself is variable or freelance, rather than assuming a steady salary to divide against. For costs that occur less than annually (like a boiler replacement, which might be needed once every 10–15 years), estimate the total cost and divide by the expected number of years, then by 12, to get a genuinely smoothed monthly figure.

Where to actually hold sinking fund money

Since sinking fund money needs to be accessible when the specific cost falls due - not locked away or exposed to investment risk - an easy-access savings account is the standard choice (see our dedicated Saving series article comparing Cash ISAs and easy-access accounts). Some people use a single account with mental or app-based sub-categories for each sinking fund purpose; others prefer genuinely separate accounts or ‘pots’ for each one.

Why this differs from a general emergency fund

A sinking fund is for costs you know are coming, even if you don’t know the exact date or amount - an emergency fund is for costs you don’t expect at all (see our dedicated article on sizing an emergency fund). Keeping the two conceptually and, ideally, physically separate avoids a known, planned expense like an MOT quietly eating into the buffer meant for genuine emergencies.

A worked example

Say your combined estimate for MOT, car tax, and servicing is £600 a year. Saving £50 a month specifically for this means the bill, whenever it lands, is already covered - rather than being an unplanned £600 hit to that month’s budget that competes with everything else due at the same time.

The bottom line

Most ‘unexpected’ costs that derail a monthly budget aren’t actually unexpected at all - they’re predictable costs that simply don’t arrive monthly. A sinking fund for each recurring irregular cost turns these from periodic shocks into a smooth, unremarkable part of your regular budget.