Compound interest calculator

Enter a one-off starting amount and, if you like, a regular monthly contribution, and this tool shows what they grow to once interest starts earning interest of its own. You get the final balance split into the money you put in and the interest on top, a year-by-year table and a simple chart. Every figure is yours to change, and the maths runs entirely in your browser — nothing is saved or sent anywhere.

Your result

–Final balance
–Total contributions
–Total interest

Year by year
Year	Contributions	Interest	Balance at year end

This calculator needs JavaScript because the calculation runs entirely in your browser – so your inputs never leave your device. Two fully worked examples are shown further down this page.

What your result means

The final balance is what you would have at the end: your starting amount, plus every contribution you made, plus all the interest credited along the way. Total contributions is the part that came from you — the starting amount and all the monthly payments — with no interest included.

Total interest is the gap between the two: final balance minus what you paid in. That gap is compounding at work, because it includes interest that has itself earned interest. The longer the term and the higher the rate, the larger that share becomes next to what you contributed.

The year-by-year table under the result shows how the balance, contributions and interest build up one year at a time.

How this calculator works

Compounding means interest is added to your balance and then earns interest itself. From that point on you are earning interest on interest, which is what makes a balance curve upwards over time rather than rise in a straight line.

With monthly crediting, the annual rate is divided by twelve to give a nominal monthly rate. Each month the tool adds your contribution either at the start of the month, so it earns that month's interest, or at the end, after interest has been worked out, depending on the timing you choose. Interest is then credited on the running balance.

With annual crediting, the balance at the start of each year earns the full annual rate once. Contributions paid in during the year earn simple, pro-rata interest for the months that remain — a convention common to traditional savings accounts. Across twelve equal monthly payments that comes to 6.5 months of interest per year if you pay at the start of the month, or 5.5 months if you pay at the end. From the following year those contributions are part of the fully compounded balance.

Internally the calculation is kept unrounded and only rounded to the nearest penny for display, so a hand calculation using rounded intermediate figures can differ by a few pence. The rate you enter is a nominal annual rate for the chosen frequency, not a standardised AER, and it is an assumption you set rather than a live market rate.

Formula and variables

Balance(n) = P · (1 + i)^n — lump sum, annual creditingBalance(n) = P · (1 + i/12)^(12·n) — lump sum, monthly creditingContributions(n) = C · (q^m − 1) / (q − 1) with q = 1 + i/12, m = 12·n — monthly contribution at month end

P: Starting amount (principal)
Balance(n): Final balance after n years
i: Annual interest rate as a decimal (4% is 0.04)
n: Duration in years
C: Monthly contribution
q: Monthly factor 1 + i/12
m: Number of months (12 × n)

Worked examples

Example 1: a lump sum left to grow

Example inputs
Starting amount	€5,000.00
Monthly contribution	€0.00
Interest rate (example assumption)	6% p.a.
Duration	15 years
Interest crediting	Annually

Example results
Final balance	€11,982.79
Total contributions	€5,000.00
Total interest	€6,982.79

With annual crediting the €5,000.00 earns 6% a year and nothing is added along the way: €5,000.00 × 1.06^15 ≈ €11,982.79. So the full €5,000.00 is your own money, while €6,982.79 is interest — more than the starting amount itself over 15 years. The 6% rate is an example assumption, not a current market rate.

Example 2: a starting amount plus monthly saving

Example inputs
Starting amount	€1,000.00
Monthly contribution (end of month)	€200.00
Interest rate (example assumption)	5% p.a.
Duration	20 years
Interest crediting	Monthly

Example results
Final balance	€84,919.37
Total contributions	€49,000.00
Total interest	€35,919.37

You begin with €1,000.00 and pay in €200.00 at the end of each month for 20 years, so your own money adds up to €49,000.00 — that is €1,000.00 plus 240 × €200.00. With monthly compounding the balance reaches €84,919.37, which means €35,919.37 of it — more than 40% of the total — is interest earned along the way. The 5% rate is an example assumption, not a current market rate.

Assumptions

The interest rate stays constant for the whole term (your chosen scenario — real rates move).
The monthly contribution stays the same and is paid on time.
Interest is credited exactly at the chosen frequency (annual or monthly) and compounds from then on.
Tax, fees and inflation are not included.
Figures are calculated without rounding and shown rounded to the nearest penny.

Limitations of this calculator

A model calculation, not a forecast: the return you actually get depends on the real product.
The rate is a nominal annual rate for the chosen frequency, not a standardised AER, and no specific account or its terms are modelled.
Inflation reduces what the final balance can buy — this tool deliberately shows nominal figures only.
Tax on interest or investment income is not applied (version 1 calculates without tax).

Common misconceptions

Treating the rate as guaranteed: The rate is your assumption, not a current or guaranteed offer. Real rates vary and can change over the term.
Confusing compound with simple interest: Simple interest is paid only on what you put in. Here, interest already credited keeps earning too, so the interest share grows faster over time.
Reading the balance as spending power: The final balance is a nominal figure with no inflation. If prices rise, it buys less than the same amount would today.
Assuming tax is included: No tax is deducted. Interest can be taxable depending on where you live and the account, which lowers the net result.
Expecting monthly and annual crediting to match: For the same annual rate, monthly crediting ends a little higher because interest starts compounding sooner.

Frequently asked questions

What interest rate should I enter?

Whatever fits your situation — the rate from a specific account, or a cautious estimate of your own. The pre-filled figure is an example, not a live market rate, and the tool neither knows current offers nor recommends any product.

Why does monthly crediting give more than annual?

With monthly crediting, interest is added twelve times a year and starts earning interest straight away. For the same nominal annual rate that ends slightly higher than crediting once a year. Which one applies depends on the account's terms.

Start or end of the month — which should I pick?

Choose start of the month if you usually pay in near the beginning, so that contribution earns interest during the month; choose end of the month if you pay later. The difference is small but grows with the term and the rate.

Does it account for tax?

No. This version calculates without tax. Interest and investment income can be taxable depending on where you live and the account, so it is worth checking your own position separately.

What about inflation?

Not included — the figures are nominal. If prices rise, the final balance buys less than the same amount would today. A separate inflation calculator is planned for this site.

Are my inputs saved?

No. Everything is worked out in your browser. Your inputs are not stored and are not sent to any server.

Sources and further reading

Official and independent sources on this topic. The links open each website in a new tab; no content is loaded from them into this page.

What Is Compound Interest?U.S. Securities and Exchange Commission (Investor.gov)Plain-language explanation of interest earning interest, from the US securities regulator's investor-education service.
Future Value of AnnuitiesMathematics LibreTexts (Business Mathematics, J. Olivier)Reference for the future value of regular contributions, including start-of-period versus end-of-period timing.

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Last reviewed: 26/07/2026 · All calculations run in your browser – inputs are not stored. ·How we check our calculators