What Is a Sinking Fund? Definition and Example | Cash Book
Quick answer
A sinking fund is money set aside in regular amounts for a specific, expected cost that falls due later: annual insurance, car servicing, a holiday, a new laptop, December gifts. Divide the cost by the number of months until it is due, save that much each month, and the bill is fully funded when it lands. A $1,200 insurance premium due in 12 months is $100 a month. An emergency fund covers what you cannot predict. A sinking fund covers what you can, and it is spent on purpose, on a date you already knew.
The month was going fine. Then the garage calls about the brakes, or the insurer's annual premium lands on the fourteenth, and the month is not going fine any more. Nothing about either was a surprise. The car has had brakes the entire time you have owned it and the insurance renews on the same date every year, and yet both arrive as shocks, because an ordinary month has no room in it for $600. A sinking fund is the small, boring fix: money put aside a little at a time for a cost you already know is coming, so the bill meets money that is waiting for it.
Divide the bill by the months
Take the cost. Take the number of months until it is due. Divide. Then move that amount into savings each month and treat it as already spent, pay the bill from the fund when it arrives, and start the next cycle the same day.
Car insurance is $1,200, due in 12 months: $100 a month. The car service and tires are estimated at $600 in 8 months: $75 a month. A summer holiday will cost around $1,500 and is 10 months away: $150 a month. Holiday gifts, $400 in 4 months: $100 a month. Total, $425 a month into sinking funds, which reads like a great deal of money until you notice it is the same $3,700 you were going to spend anyway, arriving evenly instead of in four shocks.
Paying each of those out of the month it lands in is precisely what breaks budgets in March, July and December, and it is not a rare problem: the Federal Reserve's 2025 household survey found that a major vehicle repair or replacement was the most common unexpected expense, reported by 30 percent of adults, followed by home and appliance repairs at 22 percent. Some of those are genuine emergencies. Plenty are predictable costs that nobody funded.
And when the car service comes out of emergency savings, the emergency fund shrinks for something that was never an emergency, which leaves you rebuilding the one pot that is supposed to stay full, over months you had not planned to spend rebuilding anything.
The contribution is not an expense
Two logging moments, and one of them is routinely got wrong. The monthly contribution is not spending. It moves money from one place you own to another, and your net worth does not shift an inch. The bill, when it is finally paid, is the expense.
So in Cash Book the contribution is a transfer between accounts, checking to savings, which leaves the net-worth number alone and keeps the month's spending total honest. When the insurance is paid from savings, log it once, from the savings account, with an Insurance tag, and the month then shows a $1,200 expense that was funded twelve months in advance.
Tags and budgets do the rest. One tag per fund, or a single Sinking funds category with tags underneath it. A recurring per-tag budget of $100 on the Insurance tag turns the plan into a progress bar on the Budgets screen, filling toward the annual figure, and the savings goal calculator does the arithmetic for any target and date. Because the amounts are fixed and monthly, the subscriptions and recurring bills guide applies to contributions too: same rhythm, same tag, same yearly review.
Last year's price
Premiums rise. Service costs rise. A fund built on what you paid last year comes up short at exactly the wrong moment, and adding 5 to 10 percent to the estimate covers most of that gap; if it still falls short, the difference comes out of the current month and the monthly amount goes up for the next cycle.
Timing is the other one. A $1,200 bill three months away is $400 a month, which no ordinary budget absorbs quietly, so the fund wants to start on the day the last bill was paid rather than in the month you remember the next one is coming.
Then there is the balance itself, sitting there looking available. It is not. A $900 savings balance made of four assigned funds is $0 of spending money in a convincing disguise. And logging the contributions as expenses and then the bill as well counts the same $1,200 twice, which makes a well-run year read like a disaster on the Insights screen.
A sinking fund is the planned cousin of the emergency fund, and the thing that makes fixed annual expenses behave like monthly ones, with the rest of the terms in the glossary.
Nothing about the March bill was ever a surprise.
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Frequently asked questions
What is the difference between a sinking fund and an emergency fund?
An emergency fund covers costs you cannot predict: a job loss, a medical bill, a broken boiler. A sinking fund covers costs you can predict but do not pay monthly: insurance, servicing, holidays, gifts. Both are savings, but a sinking fund is spent on schedule, on purpose, while an emergency fund is meant to sit untouched.
How many sinking funds should I have?
As many as you have irregular costs worth planning for, which for most households is between 3 and 6. Common ones are car costs, annual insurance, holidays, gifts, home repairs and technology replacement. Too many becomes a chore; a single fund with a note of what it covers works fine for smaller items.
Do sinking funds need separate bank accounts?
No. Many people keep one savings account and track the split with a note, a spreadsheet or a per-purpose tag. What matters is that the money is counted as assigned, so a $600 savings balance is understood as $400 for insurance and $200 for gifts, not as $600 available. Separate accounts help if the balance tempts you.
What happens if the cost is more than the fund?
Pay the difference from the current month's spending or from the emergency fund, then raise the monthly amount so the next cycle covers it. Insurance premiums and service costs tend to rise, so a fund built on last year's price is usually a little short. Adding 5 to 10 percent to the estimate avoids that.
Should I log a sinking fund contribution as an expense?
As a transfer, not an expense. Moving $100 from checking to savings does not reduce what you own, so logging it as spending makes the month look worse than it is. When the insurance bill is finally paid from the fund, that is the expense, and it is logged once, from the savings account, with the right tag.
What this is based on
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