What a sinking fund actually is
A sinking fund is a dedicated pool of savings you build up gradually toward a specific, known future expense — car repairs, annual insurance premiums, holiday gifts, a wedding, a vacation. Unlike a general emergency fund, which covers unpredictable events, a sinking fund is for expenses you can see coming: you know roughly when they'll happen and roughly how much they'll cost. The entire point is to avoid being surprised by a bill you actually should have expected.
The math behind "how much per month"
At its simplest, without any interest, the required contribution is just division: target amount minus what you've already saved, divided by the number of periods until your deadline. Need $1,200 for a repair fund in 12 months with nothing saved yet? That's $100/month, no more complicated than that.
Where interest changes the number
If your sinking fund sits in a savings account earning even a modest interest rate, the required contribution drops slightly, because the growing balance does part of the work for you. The formula that accounts for this is the same one used to solve for a loan payment, just pointed the opposite direction — solving for the periodic deposit that grows an account to a target future value:
PMT = (target − saved × (1+r)^n) × r / ((1+r)^n − 1)
where r is the interest rate per period and n is the number of periods remaining. At a 6% annual rate paid monthly, that same $1,200 goal over 12 months needs about $97.28/month instead of $100 — a small but real difference that compounds further the longer your timeline and the higher your rate.
Why existing savings matter more than you'd think
If you already have some money set aside toward a goal, that head start also grows over time, further reducing what you need to contribute going forward. $600 already saved toward a $1,200 target with no interest cuts your monthly requirement in half, to $50/month. Add interest, and the existing balance grows on its own between now and your deadline, trimming the requirement even further.
Why tracking multiple funds matters
Most people have more than one thing they're saving toward at once — car maintenance, holiday spending, an annual subscription renewal, a friend's wedding gift. Lumping all of these into one undifferentiated savings account makes it easy to lose track of which goal is actually funded and which is falling behind. Separating them, even just conceptually with individual targets, dates, and progress tracking, keeps each goal honest on its own timeline.
Sinking funds versus your emergency fund
It's worth being explicit about the difference between a sinking fund and an emergency fund, because mixing the two undermines both. An emergency fund exists for the genuinely unpredictable — a job loss, an unplanned medical bill, a sudden car breakdown — and should stay liquid and untouched by any planned spending. A sinking fund is the opposite: it's for expenses you already know are coming, just spread out so no single month has to absorb the full cost. If you dip into your emergency fund for a holiday gift budget you should have been sinking-funding all year, you've quietly turned a predictable expense into an emergency, which defeats the purpose of having both.
Automate it once the number is set
Once you know the exact periodic contribution a goal requires, the most reliable way to actually hit it is to automate a transfer for that amount right after payday, into an account earmarked (even just mentally, or via a named sub-account) for that specific fund. Manually deciding each month whether to move money is where sinking funds most often quietly fail — the math only works if the contribution actually happens on schedule.
Bottom line
A sinking fund turns a vague future expense into a concrete number you can automate today. The math is straightforward once it's laid out, but doing it by hand for several goals at once — each with a different date, different starting balance, and possibly different interest assumption — gets tedious fast. Running each goal through the same formula keeps every fund honest and on track.
Frequently Asked Questions
It's the periodic payment that grows both your existing savings and your regular contributions to exactly your target by your deadline. With no interest, a $1,200 target over 12 months needs exactly $100/month; with 6%/year interest, the same goal needs about $97.28/month since the growing balance does part of the work.
Yes — the Sinking Fund Savings Calculator supports unlimited concurrent funds, each with its own name, target amount, target date, current saved amount, contribution frequency, and progress bar.
Yes — enter an optional annual interest rate and the required contribution is recalculated to reflect the growth your balance will earn between now and your target date, which slightly lowers what you need to set aside each period.
Yes — the Sinking Fund Savings Calculator computes the exact monthly or weekly amount needed for any target and date, and lets you track multiple goals with progress bars at once. It's a one-time $4.99 purchase — no subscription, no account required.
Yes — every fund you add is stored only in your own browser's local storage. Nothing is uploaded to any server, and your funds will still be there the next time you open the tool on the same device and browser.