A sinking fund is a division: what the thing will cost, divided by the months until you need it. Every guide on this subject tells you that, and it is right. Almost none of them tell you the next number — what all of your funds cost per month once you add the divisions together. On the budget below, £1,280 in and £1,315 of plan, four completely ordinary sinking funds come to £160 a month, and £90 of it does not exist.
The division everyone teaches
Take the target, divide by the months until the date, put that in every month. The worked examples are everywhere and they are all correct: £1,000 for Christmas over ten months is £100 a month; £720 of car insurance renewing in eight months is £90 a month; £1,800 for a holiday next summer is £150 a month.
You may already be doing one without calling it that. Here is a real row from the household this article uses:
| Category | Budgeted | Available |
|---|---|---|
| Car Insurance | £35.00 | £35.00 |
Thirty-five pounds, every month, in a household that has never used the phrase “sinking fund”. The insurance premium is £420 a year and there are twelve months to find it in. £420 ÷ 12 = £35. The division was already done; it just did not have a name.
What makes it a sinking fund rather than a bill is that the money stays. The row does not empty at the end of the month — it carries, and by the twelfth month it is holding the whole premium, ready for the renewal that is going to arrive whether you planned for it or not.
The number nobody prints
So do it properly. Ask this household what else it would name, and you get three more, each with a real date:
| Fund | Target | Months to the date | Monthly |
|---|---|---|---|
| Car insurance | £420 | 12 | £35 |
| Christmas | £240 | 4 (it is September) | £60 |
| MOT | £180 | 6 (due in March) | £30 |
| Birthday | £35 | 1 (next month) | £35 |
| £160 a month |
Nothing in that table is unreasonable. It is not twelve funds, it has no holiday in it, no “self-care”, no new car. It is a car, a Christmas, an MOT and one birthday — and it costs a hundred and sixty pounds every month.
That is the number. It is the number that decides everything else, and it is the one that does not appear in the guides, because the guides work one fund at a time and stop.
Two of them already exist
Look again at the four. The car insurance is already in the budget at £35. So is the birthday, at £35. Between them that is £70 of sinking funds this household has been paying for years without ever using the word.
Which leaves £90 a month — Christmas and the MOT — that this budget has never found.
Where the £90 has to come from
Here is the same budget at the top of the month, before a single new fund exists:
| Take-home, monthly | £1,280.00 |
| What the month costs | £1,315.00 |
| To Be Budgeted | −£35.00 |
The header is already negative, and it is negative by exactly the birthday: £35 that is planned and not paid for. Every envelope is funded at its plan; the plan costs more than the money that arrived. There is no surplus for £90 to come out of. There is no surplus for £9 to come out of.
This is the point at which most people conclude they are bad at this. They are not. A budget cannot hold a number it does not have, and neither can a list of sinking funds. The total is not a failure — it is information, and it is the first honest number in the whole exercise.
The division has exactly two dials
Go back to it: target ÷ months = monthly. There are only two things you can change.
Everyone reaches for the months first, because more time means smaller payments. But look at what the months actually are in that table. Christmas is in December whatever anybody decides. The MOT is due the month the last one ran out. The birthday does not move.
For almost every fund on a normal list, the months are the one thing you do not own.
Which leaves the target — and the target is the only number in a sinking fund you actually control.
So the target comes down
Christmas is not £240 because £240 is what Christmas costs. It is £240 because somebody sat down and typed £240. Cut it in half and the same four months ask for £30 instead of £60.
That is not a smaller Christmas because you failed. It is the size of Christmas decided in September, when you can still do something about it, instead of on a credit card in December, when you cannot. The MOT gets the same treatment: £180 was an estimate, and if the real bill is £120 then the monthly is £20, not £30.
Keep cutting targets until the total fits what is genuinely spare. That total, not the list, is your sinking fund plan.
The rule, in three lines
- Write down every fund you want, what it will cost, and the date you need it.
- Divide each target by the months until that date, then add the monthlies up before you start a single one.
- Compare that total with what is actually spare after your bills and your spending. It will not fit. Cut targets — not dates — until it does, and add the next fund only when a target finishes.
Your dates will not be these dates, so do the arithmetic on your own calendar. And the method is arithmetic, so it works on paper, in a spreadsheet, or in an app; nothing here depends on a product.
Two limits worth saying plainly
If the total still does not fit after you have cut every target, you do not have a sinking fund problem, and no amount of dividing will fix it. That is a different conversation about income and outgoings.
If you are behind on rent or council tax, do not start a Christmas fund. Ring StepChange or Citizens Advice. Both are free and independent, and being behind on those two is not a budgeting problem.
This is education about a budgeting method, not financial advice.
Doing it in Zeroed
I built Zeroed for budgets like this one. Every category’s balance carries over month to month, which is the whole sinking-fund mechanism: money you put in a row stays in that row until you spend it, so the car insurance envelope really is holding £420 by the twelfth month. Categories can also carry a target with a date, and the app works out what that target needs each month.
One payment, no subscription, works offline, and your data never leaves your device. But the arithmetic above is the point, and it works wherever you do it. The savings goal calculator does one fund’s division on your own target and date, free, in the browser.
What to do tonight
Write your funds down with a cost and a date beside each one. Divide. Add the answers up. Then look at the total before you name a single fund — because that total, not the list, is what you are actually deciding.
Frequently asked
What is a sinking fund, in plain terms? Money put aside monthly for a cost with a known date: the target divided by the months until you need it. Car insurance at £420 a year is £35 a month, and the row keeps its balance until the renewal instead of emptying at the end of the month.
What sinking funds should I have in the UK? Only the ones with a real date and a real cost: car insurance, the MOT, Christmas, a birthday, the car tax. Add the monthlies up before you start a single one. On the budget above four ordinary funds came to £160 a month, and the list only fits once the targets are cut to what is genuinely spare.
Where should I keep sinking fund money? Anywhere you can name the row and see its balance: a notebook column, a spreadsheet, a savings pot or a category in an envelope app. The account matters far less than the division, and this article does not recommend accounts.
Is a sinking fund the same as an emergency fund? No. A sinking fund has a date and a target; an emergency fund covers the costs you cannot date. Keeping the MOT inside the emergency fund is how that pot gets emptied in March, exactly as planned.