Sinking funds: how to budget for irregular expenses without a shock

I read a lot of ledgers, and the same story appears in most of them around the end of the year. The monthly budget balances from January to October. Then December arrives with gifts, travel, a heating bill half again the size of October's, and an insurance renewal in the post, and the whole thing goes onto a card. The budget was not wrong. It just never had a line for the expenses that only happen once or twice a year.

A sinking fund is that line. This post explains what one is, how it differs from an emergency fund, and — because "how much should I put in a sinking fund each month" is the question people actually search — works the amounts out month by month on a real list of bills, including the awkward first year when you start late.

There is a reason to do it this week rather than in November. On 26 August, Ofgem confirmed that the energy price cap rises 4% from 1 October, taking a typical UK dual-fuel bill from £1,663 to £1,723 a year, just as the heating months begin. In the US, the Federal Reserve raised rates by a quarter point on 16 September, which makes putting December on a card slightly dearer than it was. And Christmas is thirteen weeks away. Every one of those is a known cost with a known date, which is exactly what a sinking fund is for.

What a sinking fund is, and what it is not#

A sinking fund is money you set aside every month, in advance, for a specific expense you know is coming: the car insurance due in March, the holiday in December, the tyres that will need replacing next summer. When the bill arrives, the money is already there, and the month it lands in looks like any other month.

It is not an emergency fund. An emergency fund covers the things you cannot see coming — a lost job, a broken boiler, a client who does not pay — and it is sized from your essential spending, as the emergency fund post works through. A sinking fund covers the things you can see coming. If you use the emergency fund for Christmas, you have not had an emergency; you have had a planning gap, and you have made the real emergency more expensive when it comes.

The test is simple. If the same "surprise" has happened before and will happen again, it is a bill with a long gap between payments, and it belongs in the budget as one.

Step 1: find the irregular expenses in your ledger#

Do not do this from memory. Memory keeps the big ones and drops the rest, and the rest is where the shock lives. Open the last twelve months of entries and look for anything that appears once, twice or four times a year: insurance premiums, vehicle tax, servicing and tyres, annual software and membership renewals, school costs, dental and vet visits, gifts, travel, the winter jump in heating bills. If you keep a Business book too, read it as well — it usually hides an accountant's bill and a tax payment.

For each item write down what it costs (last year's amount, rounded up a little), when it is due, and how sure you are. A renewal date is a fact; a car service is an estimate. That is fine. The fund does not need to be exact, it needs to exist.

While you are in the renewals, cancel what you no longer use. A March 2026 survey of 1,272 US adults found 59.9% were paying for at least one subscription they did not use, at an average of $26.79 a month. An annual renewal you have forgotten is a sinking fund you are filling for nothing.

Step 2: divide by the paydays left, not by twelve#

Every guide says "divide the annual cost by twelve". That is the right answer in the second year. In the first year it is wrong for anything due soon, because you do not have twelve months — you have the paydays between today and the bill.

The honest formula is: amount ÷ the number of paydays before the due date. Then, once a bill has been paid, its fund restarts at one twelfth and stays there.

Here is that formula applied to one person's list, starting with the first payday on 1 October 2026:

The whole list comes to $3,352 a year, which is $279 a month if it were spread evenly. But in October, November and December she has to set aside $738 a month, because the holiday and the heating top-up are close and their funds are empty. Then the monthly total falls as each bill is paid and its fund restarts at one twelfth: $513 in January, $371 in February and March, $306 in April and May, and $279 from June onward — where it stays, for good, as long as the list does not change.

Check the first cycle adds up: three months at $738, one at $513, two at $371, two at $306 and $279 in June is $3,352 in contributions against $3,352 in bills. Nothing borrowed, nothing left over, and no month next year that costs more than $279 in irregular bills.

Notice how the heating row works. The extra $90 a month is not due on one date; it arrives across four winter bills. Saving $90 from October means the first winter bill in December is met from two months of contributions, and each later one from the month before it. A seasonal cost is just a sinking fund that pays out in instalments.

When the catch-up number is too big#

$738 a month for three months is a lot on a $3,400 take-home, and that is the point at which most people abandon the whole idea. Do not. The sinking fund has already done its most valuable job: it has told you in September, with thirteen weeks to act, that December costs $900 plus $90 of heating. That is a very different situation from finding out on 20 December.

Three ways to bring the number down, in order:

1. Fund by due date. Put whatever you have towards the nearest bills first — holiday, heating, then the January renewals. Insurance in March can start in December and still be fully funded at $285 a month (four paydays). The car service can wait until spring.
2. Change the bill, not the plan. The holiday line is the only one you set yourself. $900 was last year's figure; $600 funded at $200 a month is still a Christmas, and it is a Christmas that does not carry interest into February.
3. Use the money that stops. If the 50/30/20 split is your frame, the first-year catch-up comes out of the 30% wants bucket, temporarily. Once June arrives and the total settles at $279, the wants bucket gets its money back.

What you should not do is skip the fund because the first year is lumpy. That is what starting late looks like, and it only happens once.

Where the money sits, and how to keep it honest#

One savings account is enough. You do not need five accounts for five funds; you need five named goals in your ledger, each with a target, a due date and a running balance, and one account to hold the money. The account must be separate from the card you spend from — sinking fund money next to grocery money becomes grocery money — and it must be instantly accessible, because these bills have dates. The rate it earns barely matters; $3,352 held for an average of six months earns tens of dollars, not hundreds. Sharing the account with your emergency fund is fine, as long as the ledger keeps the goals apart so you always know which dollars are for March and which are for a bad day.

Two rules keep it honest. First, the transfer goes out on payday, automatically — a fund topped up "when there is something left" is empty in December. Second, when a bill is paid from a fund, record the payment against that goal in the same entry, so the balance drops and the fund restarts. A fund whose balance never moves has quietly become general savings, and general savings are the thing people spend.

If your income is irregular, size the paydays from the floor income you budget from, not the good months. The zero-based budget for variable income shows how to set that floor; the sinking funds simply sit inside it as one more fixed line.

Reviewing the list#

Bills change. Insurance renews at a different price, the heating estimate turns out high or low, a new annual subscription appears. When a bill is paid, compare what you had saved to what it cost and adjust next year's twelfth. Once a year, run the whole exercise again from the ledger; it takes twenty minutes when the goals already exist.

What to ask an advisor about it#

Inside Azno the list, the arithmetic and the goals are three questions rather than a spreadsheet:

• "Which expenses in the last twelve months appeared only once or twice, and how much did each cost?" — a read of your entries grouped by payee and category, with the totals shown, so the list comes from the ledger rather than from memory.
• "Set up sinking-fund goals for these five bills with their due dates, and tell me what I need to set aside each month from 1 October." — this comes back as a proposal card per goal, with the amount, the target and the per-payday figure worked out; nothing is written until you approve it.
• "Which of these will not be fully funded by its due date at $450 a month, and in what order should I fund them?" — the same due-date ordering as above, done on your numbers.

If you have not set your books up yet, creating an account and entering last year's irregular bills, even roughly, is enough to answer all three before the first winter bill arrives.