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Compound Interest Calculator (Savings Growth)

See how savings grow with compound interest and regular deposits, or work out how long it takes to reach a savings goal.

$
$

How much you add, and how often.

% a year

A placeholder, not a quote. Use the rate from your account.

Work out
years
Balance after 10 years$31,806.61
Total deposited$25,000.00
Interest earned$6,806.61

$1,000.00 at 4.5% a year is 0.375% a month (rate ÷ 12), over 120 months. Deposits of $200.00 a month are added at the end of each month. Balance = start × (1 + r)^n + deposit × ((1 + r)^n − 1) ÷ r = $31,806.61. Deposited $25,000.00, so $6,806.61 is interest.

Interest is added each month at the yearly rate divided by 12. Tax on interest, bank fees and changes in the rate are not included. Check the rate and terms with your bank.

Fig. 1 What the savings grow to.

Every formula is tested against worked examples. How we check

YearsBalanceInterest earned
5 years$14,680.91$1,680.91
10 years$31,806.61$6,806.61
20 years$80,080.34$31,080.34
30 years$155,724.93$82,724.93

Fig. 2 The same deposits and rate over different times.

Guide

How compound interest works

Compound interest means the interest you have already earned starts earning interest too. This calculator adds interest every month at the yearly rate divided by 12, and adds your regular deposit at the end of each month. The balance after n months is start × (1 + r)n + deposit × ((1 + r)n − 1) ÷ r, where r is the monthly rate.

For example, $1,000.00 plus $200.00 a month at 4.5% for 10 years grows to $31,806.61. You deposit $25,000.00, so $6,806.61 is interest. The rate and amounts are placeholders, not a quote.

Time does most of the work

Doubling the time on the example, from 10 to 20 years, takes the balance from $31,806.61 to $80,080.34, and the interest from $6,806.61 to $31,080.34. The table under the result shows 5, 10, 20 and 30 years for your own figures.

Rate and regular deposits

A rate one point higher (5.5% instead of 4.5%) lifts the example to $33,632.59. Adding $50.00 more a month instead lifts it to $39,366.51. With no deposits, the starting $1,000.00 alone would grow to $1,566.99. Switch the deposit between weekly, monthly and yearly to match how you save.

Working out how long to reach a goal

Choose Time to reach a goal to find how long it takes. On the example, reaching $10,000.00 takes 3 years 5 months. If you add nothing and the rate is zero it can never be reached, and the calculator says so.

What is not included

Tax on interest, account fees, changes in the rate and inflation are not included. Many accounts pay a rate that changes, so treat the result as an estimate. Banks often quote the rate that includes bonus conditions, so check what you must do to earn it.

Common questions

How often is the interest added?

This calculator adds interest every month at the yearly rate divided by 12. Your account may add it daily, monthly or yearly, so the real figure can differ a little.

Is the deposit added at the start or end of the month?

At the end. A deposit at the start of each month earns one month more interest, so it would give a slightly higher balance.

Does this include tax on the interest?

No. Interest is usually taxable in some way, and the rules differ by country and by your circumstances, so check with your tax authority.

What if the rate changes?

Run it again with the new rate, your current balance and the years left. It assumes one rate for the whole time.

What next?