Compound interest calculator

How to use the calculator

1. Choose a currency. It changes the symbol only; the calculator never converts between currencies.
2. Under Money in, enter the Starting amount and what you’ll add, then choose How often you add, from every week to every year. Pick “End of each period” or “Start” for when each payment goes in, and use Increase what you add each year if you’ll raise it with pay rises.
3. Under Growth, enter the Interest or return per year, how often it’s Compounded and the number of Years.
4. Leave Show the result in today’s money switched on to see the balance after Inflation per year, which starts at 2.5%.

The defaults (US$1,000 to start, US$100 a month, 6% a year compounded monthly, 20 years) are an illustration, not a forecast. The Copy link to these numbers button gives you a link with your own inputs in it.

How the numbers are worked out

Each period, the balance earns that period’s interest and then the payment goes in. The rate for one payment period, i, comes from the yearly rate (r), the number of times a year interest compounds (n) and the number of payments a year (p). With a starting amount P and N payments of C, the balance after the last payment is P × (1 + i)^N + C × ((1 + i)^N − 1) ÷ i. With the defaults, r is 6%, n and p are both 12, and N is 240 (20 years of monthly payments):

You’d pay in $25,000 (the $1,000 plus 240 payments of $100), so $24,514 of the balance is interest. The effective yearly rate, (1.005)¹² − 1, is 6.17%; it’s the figure the results list as APY/AER.

Paying at the start of each period multiplies the payments line by (1 + i), because each payment earns one more period of interest. When payments rise each year, the calculator works through the periods one at a time instead of using the formula.

How compounding works

Compounding means earning interest on interest. On its own, the US$1,000 starting amount grows to about $3,310 in 20 years at 6% compounded monthly. Simple interest, paid on the original $1,000 only, would add $60 a year, or $1,200 over 20 years, for $2,200 in all.

The effect starts slowly. In year 1 the defaults earn about $95 of interest against $1,200 of payments; from year 12 the interest each year is more than the payments, and in year 20 it’s about $2,840. Run the calculator for 21 years and the total interest ($27,602) passes everything you’ve paid in ($26,200). A late start gives up those final, biggest years, which is what the cost of waiting to invest calculator measures.

Two percentage points either way move the 20-year result a long way. On the same $25,000 paid in, 4% a year gives about $38,900 and 8% gives about $63,800.

When you pay in counts too, though by less. Paying at the start of each period gives every payment one extra period of interest, which is worth $231 on the defaults ($49,745 instead of $49,514). Raising the payment by 3% a year does more: the balance reaches about $61,900, with about $33,200 paid in. Payments every week or every two weeks work the same way, so you can match the calculator to your payday and enter the amount per payment rather than a monthly total.

APR, APY and AER, and how often interest compounds

US savings accounts quote an interest rate, which is the yearly rate before compounding, and an annual percentage yield (APY), which counts compounding over a year. Both are defined in Regulation DD, which also lets a bank call the interest rate an annual percentage rate. UK accounts show a gross rate and an AER (annual equivalent rate), which works like the APY. To compare two accounts, compare their APYs or AERs.

The formula is (1 + r ÷ n)ⁿ − 1. The same 6% gives these results at each setting of Compounded, with the other defaults unchanged:

Daily compounding beats yearly by about $1,050 over 20 years, while moving from 6% to 7% adds about $6,600 (the balance becomes $56,131). If you only know an account’s APY or AER, enter it as the rate and set Compounded to Yearly. That matches the gross rate at the account’s real compounding: £5,000 plus £200 a month at 4% AER, compounded yearly, gives £19,319 after five years, and the matching gross rate of about 3.93% compounded monthly lands within a pound of it.

Inflation and today’s money

A balance 20 years away will buy less than the same number buys today. The calculator divides by (1 + inflation)^years, so at 2.5% a year the default $49,514 is worth about $30,200 in today’s money. That 2.5% sits a little above the 2% target the government sets for the Bank of England and the 2% the Federal Reserve aims for over the longer run.

Recent inflation has been higher than both targets. UK consumer prices rose 3.1% in the 12 months to August 2026, and US prices rose 3.7% in the 12 months to September 2026. At 3.7% a year for 20 years, the default balance would be worth about $23,900 in today’s money. Savings that earn less than inflation lose buying power: an account paying 0.37% while prices rise 3.7% buys about 3.2% less each year.

Savings rates and investment returns

A savings rate is whatever your bank sets, and it can change at any time. On 21 September 2026 the FDIC’s national averages were 0.37% for US savings accounts and 1.73% for 12-month CDs, while the Federal Reserve’s target range was 3.75% to 4.00% after its rise on 16 September. In the UK, Bank Rate has been 3.75% since 18 December 2025, and the next decision is due on 5 November 2026. Your own account’s rate is the one to enter, and the savings goal calculator works backwards from a target to the monthly amount.

Investments don’t pay a rate. Share prices and dividends move from year to year and some years are negative, while the calculator assumes the same return every year, so a 6% input shows what 6% in every year would produce. Nobody knows what returns will be over the next 20 years, us included. Fees come out of the return too; the investment fee calculator shows what 0.45% or 2.1% a year takes from a pot.

Scroll is education, not advice. Our lessons and calculators explain how money works in general. They don’t know your circumstances, so they can’t tell you what to do with yours. For that, speak to a qualified, regulated adviser in your country.

The Rule of 72

Divide 72 by the yearly rate and you get roughly the number of years money takes to double. At 6%, that’s 72 ÷ 6 = 12 years. The exact figure is ln 2 ÷ ln(1 + APY): 11.9 years at 6% compounded yearly, and 11.6 years at 6% compounded monthly, which is what the results show next to the rule. Between 3% and 12% the rule stays within about half a year of the exact answer (24 years against 23.4 at 3%, and 6 against 6.1 at 12%).

Examples in dollars, pounds and rupees

These people are made up; the numbers come from the calculator.

Maya, in the US, starts with US$1,000 and adds $25 every week ($1,300 a year) at 6% compounded monthly. After 20 years she has about $53,500, of which $27,000 is her own money.

Tom, in the UK, has £5,000 in a cash ISA paying 4% AER and adds £200 a month. Entered as 4% compounded yearly, that’s £19,319 after five years: £17,000 paid in and £2,319 of interest, or about £17,100 in today’s money at 2.5% inflation.

Ananya, in India, starts with ₹50,000 and adds ₹5,000 a month for 15 years, assuming 8% a year with Compounded set to Yearly. She ends with ₹18,46,640 (about ₹18.5 lakh), of which ₹9,50,000 is her own money. The Reserve Bank of India’s inflation target is 4%, and at that rate her balance is worth about ₹10,25,000 in today’s money. The 8% is an assumption that no investment promises.