Sinking Fund Calculator
Calculate monthly contributions needed to build a sinking fund for a future expense.
What this calculator does
A sinking fund calculator answers one planning question: to have a certain amount by a certain date, how much must I put aside each month? Enter the target, an expected annual interest rate, and the number of years you have.
It returns the required monthly contribution, the target balance, and how much of that balance comes from interest rather than from you. Over short horizons the interest share is small — the purpose of a sinking fund is not growth but turning one large bill into a series of manageable ones.
When to use it
Use it for any cost you can already see. The annual car insurance renewal, a boiler with a few years left, replacing a car, a holiday, a wedding, or the excess on a home insurance claim. All are predictable enough in timing and size to plan against.
It is equally useful for the negative answer. If the monthly figure it produces is plainly unaffordable, better to know that now than in two years — early enough to lengthen the timeline, reduce the target, or accept that the purchase is not going to happen.
Understanding the inputs
Target amount should be the full cost including VAT and delivery, both of which are routinely left out of mental estimates. For a car, include the first year's insurance and road tax. For building work, add a contingency of at least ten percent because quotes rarely survive contact with the property.
Annual return should reflect where the money will genuinely sit — currently 4 to 5 percent for a competitive easy access account, near nothing in a current account. Do not assume investment returns for dated money. Years to save takes decimals, so eighteen months should be entered as 1.5.
How is this calculated?
Monthly Payment = FV × r / ((1+r)^n − 1) where r = monthly rate, n = months.
A worked example
Say you need £6,000 in two years for a new boiler and fitting, held in an easy access account paying 4.5 percent. The monthly rate is 0.375 percent across 24 contributions.
The required monthly payment is £239.39. Over two years you contribute £5,745 yourself and interest provides the remaining £255. Had the money sat in a current account paying nothing, you would need £250 a month instead — the interest is worth about £11 a month, useful but clearly not the reason to do this.
Limitations and assumptions
The calculation assumes a fixed interest rate and contributions made reliably at the end of each month. Easy access rates move with the base rate, so a headline rate today may not last two years. It also assumes your target is accurate, and targets for building work are commonly understated.
It does not model inflation on the cost itself, which matters over longer horizons — a kitchen quoted at £12,000 today may cost £15,000 in five years. Nor does it account for tax on interest above your personal savings allowance. For anything beyond five years, revisit the target annually.
Common Questions
- What is a sinking fund?
- Money set aside gradually for a known, dated expense — a boiler replacement, a car, a wedding, the annual car insurance premium. The defining feature is that the cost is expected. A sinking fund is what stops a predictable bill from being met with a credit card in the week it arrives.
- How does it differ from an emergency fund?
- An emergency fund covers the genuinely unforeseen — redundancy, an urgent repair — and otherwise stays untouched. A sinking fund covers the foreseeable and is meant to be spent on schedule. Blurring the two means raiding your safety net for a bill you always knew was coming.
- How many sinking funds should I run?
- One for each predictable irregular cost, though they can share a single account. Common ones are car replacement and MOT, home maintenance at roughly one percent of the property value a year, annual insurance premiums, Christmas, and holidays. A simple spreadsheet tracking each pot within one balance is enough.
- Where should the money be held?
- For horizons under two years, an easy access savings account or a Cash ISA — protected by the FSCS up to £85,000 per banking licence. For three to five years, fixed-rate bonds or NS&I products can add yield. Money with a fixed date should not be in the stock market.
- Should I use a Cash ISA?
- It depends on whether you would otherwise pay tax on the interest. The personal savings allowance shelters £1,000 of interest for basic rate taxpayers, £500 for higher rate, and nothing for additional rate. Below those thresholds a taxable account paying a better headline rate often wins outright.
- How much difference does the interest rate make?
- Modest over short horizons. Saving £6,000 across two years, a 5 percent account earns around £283 of interest against roughly £57 in a 1 percent one. Worth capturing, but the monthly contribution does the heavy lifting. Over ten years the rate matters far more and is worth actively shopping for.
- What if the monthly figure is unaffordable?
- There are exactly three levers: lengthen the timeline, cut the target, or increase income. Lengthening is usually simplest — stretching a £4,500 goal from 12 months to 20 months reduces the monthly amount by around 40 percent. Pulling a lever deliberately is better than quietly missing the target.
- Should I save or clear debt first?
- Compare the rates honestly. Credit card debt at 24 percent costs far more than savings at 5 percent earn, so clearing it comes first. The exception is a small starting buffer, because without one the next unexpected bill goes straight back on the card and nothing has changed.
- Should contributions be automated?
- Yes. A standing order timed for the day after payday removes the monthly decision, and the decision is where most savings plans quietly collapse. Many banks now let you create named savings pots or spaces, which makes it easier to keep several sinking funds distinct without opening multiple accounts.
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