See how much interest a loan will cost you, or how much an investment will earn, year by year
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years
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years
Recurring contributions
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Estimates only — not financial advice.
Add one-time or recurring extra payments toward principal to see how much cumulative interest they save.
Compare cumulative interest across up to 3 rates/terms for the same loan amount.
Year
Principal Paid
Interest
Balance
Try an example:
Total cumulative interest
$0.00
Loan amount$0.00
Total contributions$0.00
Total interest$0.00
Interest saved (extra payments)$0.00
Calculation History
No calculations saved yet. Click "Calculate" to save your first one.
For guidance only — actual loan and investment terms vary. Currency selection changes the displayed symbol/format only; no live exchange-rate conversion is performed.
Interest accumulates very differently depending on which side of it you’re on, a loan racks up cumulative interest against you, while an investment compounds it in your favor. This cumulative interest calculator handles both directions in one tool: see exactly how much interest a loan will cost you over its full term, or how much an investment with regular contributions will actually earn, year by year.
How to use this cumulative interest calculator
Choose Loan Interest or Investment Growth mode, and select your Currency.
Loan Interest: in Details, enter the Loan amount, Annual interest rate, Loan term, and Payment frequency (Monthly, Bi-Weekly, or Annually).
Investment Growth: in Details, enter the Initial deposit, Annual interest rate, Duration, and Compounding frequency, plus optional Recurring contributions (amount, frequency, and whether they’re added at the Start or End of each period).
In Loan mode, switch to Extra Payments to add one-time or recurring extra payments toward principal and see how much cumulative interest they save.
Switch to Compare Rates to test up to 3 rates/terms against the same amount side by side.
Switch to Yearly Table for a full year-by-year breakdown of principal paid (or deposited), interest, and balance.
Click Calculate to see your results, or try a built-in example.
What this cumulative interest calculator does
Rather than showing only a single total interest figure, this calculator tracks interest accumulation year by year, in whichever direction applies to your situation. In Loan Interest mode, it shows how much interest you’ll pay in total, and how extra payments change that figure. In Investment Growth mode, it shows how much interest (growth) accumulates on your initial deposit plus any recurring contributions, compounded at your chosen frequency. Both modes share the same underlying yearly-breakdown structure, just applied in opposite directions.
Loan Interest: how cumulative interest builds against you
Periodic Payment = P × r × (1+r)n ÷ [(1+r)n − 1]
where P = loan amount, r = periodic interest rate, n = total number of payments
Cumulative Interest = Total of all payments made − Original loan amount
In the early years of a loan, most of each payment goes toward interest rather than principal, which is why cumulative interest accrues quickly at first and slows down as the balance shrinks. The Yearly Table makes this visible directly, showing how the split between principal paid and interest shifts year over year across the loan’s full term.
Extra payments: cutting cumulative interest directly
Adding extra payments toward principal reduces the balance that future interest is calculated against, which compounds into meaningfully lower cumulative interest over the life of the loan, sometimes substantially, depending on how early and how much extra is applied. This calculator’s Extra Payments tab lets you add one-time lump sums or recurring extra amounts and see the resulting cumulative interest savings directly, rather than needing to estimate the effect yourself.
Investment Growth: how cumulative interest builds in your favor
Future Value = P(1 + r/n)nt + recurring contributions compounded at each period
where P = initial deposit, r = annual rate, n = compounding periods per year, t = years
Cumulative Interest (Growth) = Final Value − Initial Deposit − Total Contributions
Investment mode adds a layer loan mode doesn’t need: recurring contributions, with their own frequency and timing. This reflects how real investing usually works, an initial deposit plus regular ongoing contributions, rather than a single lump sum left untouched.
Contribution timing: start of period versus end of period
Whether a recurring contribution is added at the Start or End of each compounding period changes how much time that specific contribution has to earn interest before the period’s compounding is applied. A contribution added at the start of a period earns interest for that full period; one added at the end doesn’t, since compounding for that period has already occurred by the time it lands. Over many periods, this timing difference compounds into a real, calculable difference in final value, which is why this calculator asks you to specify it explicitly rather than assuming one or the other.
Comparing rates or terms side by side
The Compare Rates tab lets you test up to 3 different rate/term combinations against the same loan amount (or investment setup), showing cumulative interest for each side by side. This is useful for seeing directly how much a rate difference, or a shorter versus longer term, actually changes your total interest cost or total investment growth, rather than running each scenario separately and comparing the numbers by hand afterward.
Reading the Yearly Table
Whichever mode you’re in, the Yearly Table breaks the full term down year by year, showing Principal Paid (or contributed), Interest, and Balance for each year. This makes the compounding effect, whether working against you on a loan or for you on an investment, visible as a trajectory rather than a single end-of-term number, which is often more useful for understanding when interest accumulation actually accelerates or slows.
A note on accuracy
These are estimates only, not financial advice. Actual loan terms, fees, and investment returns vary by lender, account type, and market conditions, none of which this calculator can predict. Use it to compare scenarios and understand the mechanics of interest accumulation, but confirm exact figures with your lender or financial institution before making a financial decision.
Frequently asked questions
Why does cumulative interest on a loan build up faster in the early years?
Early loan payments are calculated against a larger remaining balance, so a bigger share of each payment goes toward interest rather than principal. As the balance shrinks over time, more of each payment shifts toward principal, which is why the Yearly Table shows interest accumulation slowing down in later years even though the payment amount itself stays the same.
Does contribution timing (start vs. end of period) really make a meaningful difference?
Yes, over many compounding periods. A contribution added at the start of a period earns interest for that entire period, while one added at the end doesn’t, since that period’s compounding has already been applied. This difference compounds across the full investment duration into a real, calculable gap in final value.
How do extra payments reduce cumulative interest on a loan?
Extra payments reduce the principal balance immediately, which means future interest, always calculated against the current balance, is calculated against a smaller number from that point forward. This compounding reduction is why even modest extra payments, especially made earlier in a loan, can meaningfully cut total cumulative interest.
Can I use this calculator for both a mortgage and a savings account?
Yes, that’s exactly the two modes it supports. Loan Interest mode handles any installment loan (mortgage, auto, personal), while Investment Growth mode handles a deposit account or investment with optional recurring contributions, whichever direction matches your actual situation.
