calculatorpro

Compound Interest Calculator

What your savings grow to, year by year — including anything you add along the way. It shows the effective rate as well as the headline one, because compounding quietly makes them different.

  • Free
  • No account needed
  • Nothing leaves your browser
£
%
£
Refine
Money goes in at the

Paying in at the start earns one extra period of interest each time. Over a long term the difference is real.

Loaded with an example. Type over any field.

Final balance

£75,218.25

£5,000.00 grows to £75,218.25 over 20 years, paying in £150.00 monthly

Paid in
£41,000.00
Interest earned
£34,218.25
Effective yearly rate
5.116%
Interest as a share
45.5%

The working

  1. Rate per period: 5% ÷ 12 = 0.4167%
  2. Periods: 12 × 20 years = 240
  3. Paid in: £5,000.00 + (£150.00 × 240) = £41,000.00
  4. Interest earned: £75,218.25 − £41,000.00 = £34,218.25
  5. Compounding monthly makes 5% worth 5.116% a year

How it grows

  • Paid in£41,000
  • Interest earned£34,218

The green band is interest. It starts as a sliver and widens as interest itself starts earning interest — which is the whole point, and why the line bends rather than running straight.

Year-by-year table
Balance, contributions and interest for each year
YearOpeningPaid inInterestClosing
1£5,000.00£1,800.00£297.64£7,097.64
2£7,097.64£1,800.00£404.96£9,302.59
3£9,302.59£1,800.00£517.77£11,620.36
4£11,620.36£1,800.00£636.35£14,056.71
5£14,056.71£1,800.00£761.00£16,617.71
6£16,617.71£1,800.00£892.02£19,309.73
7£19,309.73£1,800.00£1,029.75£22,139.48
8£22,139.48£1,800.00£1,174.53£25,114.00
9£25,114.00£1,800.00£1,326.71£28,240.71
10£28,240.71£1,800.00£1,486.68£31,527.39
11£31,527.39£1,800.00£1,654.83£34,982.22
12£34,982.22£1,800.00£1,831.59£38,613.80
13£38,613.80£1,800.00£2,017.38£42,431.19
14£42,431.19£1,800.00£2,212.69£46,443.88
15£46,443.88£1,800.00£2,417.99£50,661.86
16£50,661.86£1,800.00£2,633.79£55,095.65
17£55,095.65£1,800.00£2,860.63£59,756.27
18£59,756.27£1,800.00£3,099.07£64,655.35
19£64,655.35£1,800.00£3,349.72£69,805.06
20£69,805.06£1,800.00£3,613.19£75,218.25

Check this before you rely on it. A free everyday calculator, provided as a guide — not tax, accounting, financial or legal advice. The working is shown above so you can verify it against your own figures. See our terms of use.

How it works

STEP 1

Enter what you are starting with

A starting amount, the interest rate, and how many years. Add a regular deposit if you pay in monthly — most people do, and it changes the answer far more than the rate does.

STEP 2

Choose how often it compounds

Yearly, quarterly, monthly or daily. More often is better for a saver, and the calculator shows what the headline rate is actually worth once compounding is counted.

STEP 3

Read the split

The chart separates what you paid in from what the interest earned. Watching the green band widen is the clearest picture of what compounding does.

What this doesn't cover

Knowing where a calculator stops is what makes the rest of it trustworthy.

  • TaxInterest is shown gross. Depending on your allowances and the account, some may be taxable — an ISA would not be. This is arithmetic, not tax advice.
  • InflationFigures are in today’s pounds with no adjustment. A balance in 30 years will not buy what the same number buys today.
  • Rate changesThe rate is assumed fixed for the whole term. Real savings rates move, so treat a long projection as a shape rather than a promise.
  • Fees and withdrawalsNo platform fees, no charges and no money taken out. Any of those reduce the result, and fees compound against you exactly as interest compounds for you.

Common questions

What is compound interest?

Interest paid on your interest as well as on your original money. In year one you earn interest on the balance you put in; in year two you earn it on that balance plus the first year’s interest, and so on. That is why the line bends upward rather than running straight.

What is the compound interest formula?

A = P(1 + r/n)^(nt) — where P is the starting amount, r the annual rate as a decimal, n how many times a year it compounds, and t the years. This calculator works period by period instead, which gives the same answer and also produces the year-by-year table.

Why is 5% compounded monthly worth more than 5%?

Because each month’s interest starts earning interest itself. 5% compounded monthly returns 5.116% over a year. That figure — the effective annual rate — is the one to compare between accounts, and it is shown alongside the result.

Does it matter whether I pay in at the start or end of the month?

Yes, more than people expect. Money paid in at the start of each period earns one extra period of interest every time, and over decades that compounds into a real difference. The Refine section lets you switch between the two.

Is the interest shown before or after tax?

Before. Whether you actually pay tax depends on the account and your allowances — interest inside an ISA would not be taxed. The figure here is gross, so treat it as the ceiling.

Why does my bank’s figure differ slightly?

Usually rounding. Banks often round to the penny at each step; this calculator keeps full precision throughout and rounds only when displaying, which avoids drift building up over a long term. Day-count conventions can also differ slightly on daily compounding.

Related calculators

Percentage Calculator

Percentage of a number, increase, decrease and difference.

VAT Calculator

Add or remove VAT at 20%, 5% or a rate you set.