Sinking funds: the end of 'surprise' expenses

It's a regular Tuesday, you open the mail, and there it is: car registration, $240, due in three weeks. You knew it existed. But "knowing it exists" and "having $240 set aside for it" are two very different things, and the gap between them is where most budgets quietly fall apart.

They're not surprises. They're irregular

Here's the reframe that changes everything: car registration is not a surprise. Neither are the holidays, your renter's insurance renewal, back-to-school season, or the vet visit your dog will almost certainly need this year. These expenses don't show up monthly, so monthly budgets ignore them. Then they arrive, the budget breaks, and it feels like a personal failure.

It isn't. Nobody naturally budgets every month for something that happens once a year. That's not a character flaw, it's just a blind spot built into how most people think about money: in months. The expenses think in years. A sinking fund is the translator between the two.

The fix is one division problem

A sinking fund is a small pot of money you build up on purpose, a little every month, for a specific future expense. The math is genuinely this simple:

Monthly amount = total cost ÷ number of months until it's due

Divide, set that amount aside each month, and when the bill arrives, the money is already sitting there waiting for it. The expense didn't get cheaper. It got boring. Boring is the goal.

Three worked examples

Car registration: $240, due every March

$240 ÷ 12 months = $20 a month. That's it. Twenty dollars a month, every month, and March goes from a budget-wrecker to a non-event. If you're starting in June with nine months to go, it's $240 ÷ 9 ≈ $27 a month for the first year, then $20 a month forever after.

Holidays: $900, every December

Gifts, travel, the work party outfit, the extra groceries. Add up what last December actually cost you. Say it's $900. Starting in January, that's $900 ÷ 12 = $75 a month. Starting in June, you've got seven months including December, so $900 ÷ 7 ≈ $130 a month. Steeper, but still a number you can plan around, instead of a four-figure January credit card statement you can't.

Vet visits: about $400 a year, on no schedule at all

This one's sneaky because the timing is random even though the average isn't. Last year it was a $150 checkup and a $250 ear infection. So budget the average: $400 ÷ 12 ≈ $34 a month. The ear infection will still happen on a random Thursday. The difference is that future-you opens the vet bill, shrugs, and pays it from the fund.

Notice the pattern: none of these monthly amounts hurt. $20, $75, $34. What hurts is meeting the full bill cold.

Where to put the money

The money needs to live somewhere your everyday spending can't accidentally eat it. Three good options, in order of simplicity:

1. A separate savings account. Many banks let you open extra savings accounts for free. Name it after the expense ("Registration", "December") so the balance has a job.
2. One savings account with a list. Keep a simple note of what's inside: "$140 registration, $375 holidays, $102 vet." The account is one pot; the list keeps the pots honest.
3. A protected floor in the account you already use. If moving money between accounts is the step you'll skip, mark a portion of your existing balance as off-limits instead. OneTruth Money does this with Keep in Account, a floor you set so your spendable number never includes the money you've earmarked, and a Goal gives the fund a finish line and a date if you want one.

Whichever home you choose, the test is the same: when you check what you can spend, the sinking fund money shouldn't be in that number.

Your December self will thank your June self

Sinking funds are really a gift exchange between two versions of you. Your June self gives up $130 a month, which stings a little. Your December self receives a fully funded holiday season, zero new debt, and a January with no financial hangover. Every sinking fund works this way: a small, manageable kindness now in exchange for a calm, unbothered later.

This is also why sinking funds are so good for couples and families. When the irregular expenses are planned and visible, nobody gets ambushed, and nobody has to be the one who says "we can't afford it" in the moment. The plan already answered the question months ago. Nobody's the bad guy; the math did the talking back in June.

Here's a quick-start checklist you can copy:

1. Scroll back through 12 months of statements and list every expense that isn't monthly.
2. Next to each one, write the total cost and the month it's due.
3. Divide each cost by the number of months until it lands.
4. Pick one fund to start. Choose the soonest deadline or the one that scares you most.
5. Move this month's share somewhere your daily spending can't reach it, and set a repeating reminder (or an automatic transfer) for the rest.

You don't need all of them on day one. One fund, started today, beats a perfect system started someday.

Try this today

Pick the very next non-monthly expense headed your way, whether that's registration, the holidays, or the dog's annual checkup. Divide its cost by the number of months between now and the due date. Then open a savings bucket, name it, and move this month's share before the day ends. That's the whole assignment: one expense, one division problem, one transfer, and you've officially ended your first "surprise."

OneTruth Money content is education, not financial advice. Your situation is yours — when in doubt, talk to a fiduciary advisor.