See how much sooner a car, student or personal loan is paid off – and how much interest you save – with an extra monthly payment or a one-off lump sum.
By TanLabs · Formulas cross-checked against the free spreadsheet · Updated 2026-10-31 · Runs in your browser – nothing you type is sent anywhere
With loan balance $20,000, APR 7.5%, months left 48, extra per month $100, one-off lump sum now $0:
Paid off in 39 months – 9 months early
Interest saved: $634 · Regular payment $484/mo · Interest with extra $2,578
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Regular payment M = B·r ÷ (1 − (1 + r)^−N), r = APR ÷ 12. Each month: interest = balance × r; balance = balance + interest − (M + extra). A lump sum comes off the balance first.
$20,000 at 7.5% APR with 48 months left (regular payment about $484):
| Extra payment | Paid off in | Total interest | Interest saved |
|---|---|---|---|
| None | 48 months | $3,212 | $0 |
| $50/mo | 43 months | $2,859 | $353 |
| $100/mo | 39 months | $2,578 | $634 |
| $200/mo | 33 months | $2,157 | $1,055 |
| $500/mo | 22 months | $1,457 | $1,755 |
| None + $2,000 lump sum | 43 months | $2,553 | $659 |
$100 a month extra saves about $630 and ends the loan 9 months early. A $2,000 lump sum paid today saves even more ($659) and needs no monthly discipline.
A dollar paid earlier saves more interest, so a lump sum today beats the same money spread over the year. Monthly extras are easier to keep up.
Usually, but ask. Some lenders apply extra to the next payment instead. Mark the payment "principal only" if you can.
Some loans have a prepayment penalty. Check your loan agreement first; the CFPB explains what to look for.
Usually not. On most installment loans the payment stays the same and the loan ends sooner.
Compare the APR with what your money earns safely. Paying off a 7.5% loan is a guaranteed 7.5% return.
The maths is the same, but mortgage payments include escrow. Enter only principal and interest.
Sources: CFPB – what is a prepayment penalty?