A sinking fund is money you set aside gradually for an expense you already know is coming. Not an emergency — the opposite. Insurance renewals, car servicing, Christmas, the annual software licence, the dentist, replacing a laptop that is clearly on its last year. Each one is predictable in size and roughly predictable in date, and each one is capable of ruining a month if you meet it with a single payment.
The term comes from company accounting, where a sinking fund sets aside cash to repay a debt or replace an asset. The household version is the same idea with smaller numbers: spread a lumpy cost across the months before it instead of the month it lands.
Sinking fund vs. emergency fund
They are often confused, and mixing them is why emergency funds get drained by things that were never emergencies.
- An emergency fund covers what you did not see coming: a job loss, a medical bill, a boiler that dies in January.
- A sinking fund covers what you did see coming: the car service that happens every year at roughly the same time.
- Emergency funds should be boring and untouched; sinking funds are meant to be spent, on schedule.
- If you find yourself raiding the emergency fund twice a year for the same bill, you needed a sinking fund.
How to work out the monthly amount
The arithmetic is deliberately simple: take the expected cost, divide by the number of months until it is due, and set that aside each month.
Monthly contribution = expected cost ÷ months remaining until the bill lands.
A 1,200 insurance premium due in ten months needs 120 a month. A 600 car service due in six needs 100. The honest version of this exercise is uncomfortable the first time, because adding up every sinking fund you actually need often reveals several hundred a month that your budget never accounted for. That number was always there — it was just hidden in the months it ambushed you.
Which costs deserve one
- Insurance paid annually or twice a year, where paying monthly costs more.
- Vehicle costs: servicing, tyres, road tax, the test.
- Property: maintenance, service charges, the boiler fund if you own.
- Health: dentist, optician, anything your cover does not include.
- Gifts and holidays — the predictable ones, which is most of them.
- Replacement cycles: phone, laptop, washing machine. Divide the price by its realistic lifespan in months.
Where to keep the money
In an account separate from your everyday spending, but not locked away — you need it on the day the bill arrives. One savings account per fund is cleanest if your bank allows sub-accounts or pots; otherwise a single account with a spreadsheet that tracks what each portion is for works perfectly well. What matters is that the money does not look available when you check your current account balance.
Why this flattens your budget
Plot your running balance across a year with annual bills included and you get a sawtooth: a steady climb, then a cliff in March, then another in August. The monthly maths is healthy, but the timing is not, and timing is what causes the overdraft. Sinking funds turn each cliff into a gentle, constant slope you have already paid for.
This is also the cleanest way to find out whether your budget genuinely works. Many budgets that fail are not overspent — they are under-provisioned for the lumpy stuff, and the owner only discovers it eleven months a year too late.
Project your own sawtooth with the cumulative budget calculator: add your annual bills as one-off entries on the months they actually land, and watch what they do to the running balance before they do it for real.
Try Cumulative Budget →Common questions
How many sinking funds should I have?
As many as you have predictable lumpy costs, but start with the three that have hurt most in the last two years. A dozen funds tracked badly are worse than three tracked properly.
What if the bill arrives before the fund is full?
Pay what you can from the fund and the remainder from the budget, then restart the fund the next month with the correct monthly figure. The first year of any sinking fund is always partial — that is normal, and it still absorbs most of the shock.
Should the money earn interest?
If it can do so without a notice period, yes, but do not optimise this. On a few hundred held for a few months the interest is small, while missing the bill because the money was locked up is expensive.