See how much you build up by saving regularly — enter starting amount, monthly deposit, rate and years to see your final balance and interest earned.
Compare interest rates on savings accounts.
Estimates only. Assumes constant rate and monthly compounding. Not advice or an offer.
Enter a starting amount, a monthly deposit, the annual interest rate and the number of years. The calculator applies monthly compounding and shows the final balance, how much you contributed and how much you earned in interest.
The key insight is the gap between "total contributed" and "final balance" — that gap is pure interest. At a 5% rate over 30 years, the interest earned can easily exceed the amount you deposited. Starting earlier — even with smaller amounts — has a bigger impact than depositing more later.
Interest earned not only on your contributions but also on previously earned interest. The longer you save, the more powerful this effect becomes.
FV = P × (1+r)ⁿ + PMT × ((1+r)ⁿ − 1) / r, where r = monthly rate and n = number of months. Monthly compounding is assumed.
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Use the AER offered by your savings account. UK easy-access savings accounts currently offer around 4–5% AER.
Compound interest means you earn interest not only on the money you deposited, but also on the interest already added — growth on growth. Its power is exponential, which human intuition consistently underestimates: the curve looks flat for years and then climbs steeply. The calculator above compounds monthly and supports a starting lump sum plus regular monthly deposits, which is how most people actually save.
The core formula for a lump sum is FV = P × (1 + r/12)12t, where P is the starting amount, r the annual rate and t the number of years. Regular deposits each compound from the moment they are paid in, so earlier deposits do disproportionate work — the first year’s contributions may end up worth several times the last year’s.
Save £200 a month at 5% for 10 years: you deposit £24,000, but the pot grows to about £31,056 — £7,056 earned by the money itself. Stretch the horizon and the effect dominates: £1,000 up front plus £100 a month at 7% for 20 years means £25,000 of deposits growing to about £56,131. More than half the final pot is growth, not saving.
Divide 72 by the annual return to estimate how many years money takes to double. At 6%, roughly every 12 years; at 8%, every 9. It also works in reverse for inflation: at 3% inflation, cash halves its purchasing power in about 24 years — which is why long-term savings need to earn more than inflation, not just more than zero.
Less than people think at savings-account rates: 5% compounded monthly is equivalent to about 5.12% annually. Frequency matters more at high rates — which is precisely why credit-card debt (compounding daily or monthly at 20%+) grows so viciously.
For cash savings, use the rate your account actually pays. For long-horizon investments, common planning assumptions are 5–7% nominal for diversified equity funds — but they are assumptions, not promises, and real (after-inflation) returns are what buy things.
Both, depending which side you are on. It is the engine of pensions and the reason unpaid credit-card balances spiral. The same formula in this calculator explains both — try entering a card’s APR to see what the balance does in five years.