How the calculation works
Solve for the deposit needed to reach a future fund
A sinking fund builds toward a known future amount through regular deposits and compound interest. Instead of asking how much a fixed payment will grow to, the sinking-fund problem works backward from the required future target and solves for the deposit.
The calculator separately grows any starting balance and then determines the regular deposit needed to fill the remaining gap.
This is the end-of-period deposit formula. For beginning-of-period deposits, divide the required payment by (1 + r).
Sinking fund versus loan amortization
A sinking fund accumulates assets while loan amortization reduces a debt. The mathematics is closely related: both use periodic rates, level payments, and time-value-of-money factors, but the cash flows move in opposite directions.
Beginning versus end-of-period deposits
A deposit made at the beginning of each period earns interest for one additional period compared with an end-of-period deposit. Therefore, the required beginning-of-period deposit is slightly lower when the interest rate is positive.
Using a starting balance
If money is already set aside, the calculator compounds that balance through the full term before solving for the additional deposits required. If the starting balance alone is projected to exceed the target, the required periodic deposit becomes zero.
Need a complete loan schedule?
For detailed loan work, Amortization Pro creates full payment-by-payment schedules, supports extra principal and changing rates, saves loan files, exports CSV data, and prints reports. The Windows software is a $25 one-time purchase.