How to use the sinking fund calculator
- List each thing you are saving for, such as car repairs, gifts or an annual insurance bill, with the amount needed and the months until it is due.
- Enter any money you have already saved for that fund. Leave it at 0 if you are starting fresh.
- Add an interest rate if the money sits in an account that pays interest, or leave it at 0.
- Read the monthly deposit for each fund and the one total to move into savings each month.
Formula
Without interest: deposit = (amount needed - already saved) / months
With interest: deposit = (amount needed - saved x (1 + i)^n) x i / ((1 + i)^n - 1)
i = yearly rate / 12, n = months, deposits at the end of each month
A fund that is already full after growth needs $0. Each deposit is rounded to the nearest cent.
Three planned expenses
Car repairs: $1,200 needed in 12 months with $200 saved, so ($1,200 - $200) / 12 = $83.33. Holiday gifts: $900 in 10 months with nothing saved, so $90.00. Annual insurance: $1,800 in 8 months with $300 saved, so $187.50.
Together that is $360.83 a month. With a 6% yearly rate the car fund needs $80.07 a month and the gift fund $87.99, because interest does part of the work.
Why sinking funds help
Big bills feel like emergencies when they arrive all at once, even though you knew they were coming. A sinking fund spreads a known cost over the months before it is due, so each payment is small and the bill is already covered when it lands. It is different from an emergency fund, which is for surprises.
Many people keep each fund as a separate savings bucket or sub-account and move the total on payday. This tool gives you the number for each bucket and the combined transfer. If you want one big goal with a deadline, the savings goal calculator is the closer fit, and the emergency fund calculator covers the cushion for surprises.
Choosing amounts and dates
Base the amount on last year's actual bill or a quote, and round up. For irregular costs such as repairs, use a yearly average of what you have spent and treat it as a standing monthly set-aside. Use the due date of the bill, not the date you hope to pay it.
If a fund is short because you started late, the deposit rises. That is the tool showing you the real cost of waiting. You can also lower the target or move the date and watch the deposit change.
Assumptions and limits
- Deposits are level and made at the end of each month.
- Interest, when entered, compounds monthly at the yearly rate / 12. The rate is your number, not a promise from any bank.
- Taxes on interest and changes in the rate are not modeled.
- Because deposits are rounded to the cent, a fund can end a few cents above or below its goal.
Frequently asked questions
What is a sinking fund?
It is money set aside in small regular amounts for a known future expense, such as a repair, a holiday or a yearly bill. The goal is to have the full amount ready by the due date without a shock to your monthly budget.
How is a sinking fund different from an emergency fund?
A sinking fund is for costs you know are coming and can date. An emergency fund is a general cushion for surprises such as job loss or an unplanned repair that you cannot schedule.
Should I include interest?
Only if the money sits in an account that pays it. For short periods and small balances the effect is small, so entering 0 gives a safe, slightly higher deposit.
What if my bill is due in less than a month?
The tool needs at least 1 month. For a bill due sooner, the deposit is simply the amount still needed, so you can enter 1 month.