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Compound interest calculator
See how an initial deposit and monthly contributions grow over time, or work out how much to save to reach a goal. Any compounding frequency, yearly raises and inflation. Free, no sign-up, and nothing you type leaves your browser.
- Monthly contributions
- Compounding frequency
- Savings goal
- Today’s money
In today’s money · 3% inflation a year
From year 10, the interest you earn each year is more than you put in.
At 7.00% a year, money left alone doubles about every 10.2 years.
Growth over 20 years
- Contributed
- Interest
- Balance
- 5% to 9%
- Goal
Point at the chart to see any year. The dashed line marks year 10, when the interest earned beats what you put in that year.
Year by year
| Year | Contributions | Interest | Total contributed | Total interest | Balance |
|---|---|---|---|---|---|
| 1 | $6,000.00 | $890.15 | $16,000.00 | $890.15 | $16,890.15 |
| 2 | $6,000.00 | $1,372.46 | $22,000.00 | $2,262.61 | $24,262.61 |
| 3 | $6,000.00 | $1,888.53 | $28,000.00 | $4,151.14 | $32,151.14 |
| 4 | $6,000.00 | $2,440.73 | $34,000.00 | $6,591.87 | $40,591.87 |
| 5 | $6,000.00 | $3,031.58 | $40,000.00 | $9,623.45 | $49,623.45 |
| 6 | $6,000.00 | $3,663.79 | $46,000.00 | $13,287.24 | $59,287.24 |
| 7 | $6,000.00 | $4,340.26 | $52,000.00 | $17,627.49 | $69,627.49 |
| 8 | $6,000.00 | $5,064.07 | $58,000.00 | $22,691.56 | $80,691.56 |
| 9 | $6,000.00 | $5,838.56 | $64,000.00 | $28,530.12 | $92,530.12 |
| 10 | $6,000.00 | $6,667.26 | $70,000.00 | $35,197.38 | $105,197.38 |
| 11 | $6,000.00 | $7,553.97 | $76,000.00 | $42,751.34 | $118,751.34 |
| 12 | $6,000.00 | $8,502.74 | $82,000.00 | $51,254.09 | $133,254.09 |
| 13 | $6,000.00 | $9,517.93 | $88,000.00 | $60,772.02 | $148,772.02 |
| 14 | $6,000.00 | $10,604.19 | $94,000.00 | $71,376.21 | $165,376.21 |
| 15 | $6,000.00 | $11,766.48 | $100,000.00 | $83,142.70 | $183,142.70 |
| 16 | $6,000.00 | $13,010.14 | $106,000.00 | $96,152.83 | $202,152.83 |
| 17 | $6,000.00 | $14,340.85 | $112,000.00 | $110,493.68 | $222,493.68 |
| 18 | $6,000.00 | $15,764.71 | $118,000.00 | $126,258.39 | $244,258.39 |
| 19 | $6,000.00 | $17,288.24 | $124,000.00 | $143,546.62 | $267,546.62 |
| 20 | $6,000.00 | $18,918.41 | $130,000.00 | $162,465.03 | $292,465.03 |
| Year | Contributions | Interest | Total contributed | Total interest | Balance | At 5% | At 9% |
|---|
How it works
How the calculator works
The calculator follows your money month by month, the way an account or a fund statement would. Every month it repeats the same steps, so the final balance always equals what you put in plus the interest it earned.
- Interest is added every month at the monthly rate that matches your annual rate and compounding choice. At 7% compounded annually that is 0.5654% a month.
- Your contribution goes in at the end of the month by default. Choose the start of the month if you invest on payday, and it earns one extra month of interest.
- Every 12 months your contribution grows by the yearly increase you set, if any.
- In today’s money each amount is divided by the inflation built up until the month it happens, so you see what it would buy today.
Compounding
Annual rate and compounding frequency
The same “12% a year” can mean different things. Savings accounts usually quote an effective rate that already includes compounding: the APY in the United States and the AER in the United Kingdom. Loans and some products quote a nominal rate plus how often it compounds, such as “12% compounded monthly”. The more often interest compounds, the more a nominal rate is really worth.
| Compounding | Periods a year | Monthly rate | Effective annual rate |
|---|---|---|---|
| Annually | 1 | 0.9489% | 12.00% |
| Semiannually | 2 | 0.9759% | 12.36% |
| Quarterly | 4 | 0.9902% | 12.55% |
| Monthly | 12 | 1.0000% | 12.68% |
| Daily | 365 | 1.0049% | 12.75% |
Which one should I pick?
- Annually when the rate is an APY, an AER or the expected yearly return of a fund. This is the default.
- Monthly when the product says “compounded monthly”, common for loans and some savings accounts.
- Daily, quarterly or semiannually when the account or loan terms say so.
Not sure? Use Annually. It never overstates the result.
Formula
The compound interest formula
Without contributions, compound interest is the familiar A = P(1 + r)n. With a monthly contribution you add the future value of that series of payments. First convert the annual rate into a monthly one:
r = (1 + i ÷ m)m/12 − 1
Then, with contributions at the end of each month:
FV = P × (1 + r)n + PMT × [(1 + r)n − 1] ÷ r
With contributions at the start of each month, multiply the second part by (1 + r). At a 0% rate the formula simplifies to FV = P + PMT × n.
- P
- Initial deposit
- PMT
- Monthly contribution
- i
- Annual rate, as a decimal (7% = 0.07)
- m
- Times interest compounds a year (1, 2, 4, 12 or 365)
- r
- Monthly rate
- n
- Number of months
Four things that make compounding work for you
Every figure below comes from this calculator at 7% a year, compounded annually, with contributions at the end of each month.
Start early
$300 a month for 40 years grows to about $741,000. Over 30 years it reaches about $351,000. Start ten years later with double the amount, $600 a month, and you still end with less: about $702,000.
Try it in the calculatorRaise your contribution
$500 a month for 30 years ends at about $585,000. Raise it by 3% each year and you end at about $802,000, having put in $285,000 instead of $180,000.
Try it in the calculatorWatch the fees
$10,000 plus $500 a month for 30 years: about $661,000 at 7%, but about $545,000 at 6%. A 1% yearly fee costs roughly $116,000 here.
Try it in the calculatorThink in today’s money
With 3% inflation a year, $1,000,000 thirty years from now buys about what $412,000 buys today. Switch on today’s money to plan with real purchasing power.
Try it in the calculator
The questions people ask most
What is compound interest?
Interest calculated on your deposit plus the interest it has already earned. With simple interest, $1,000 at 7% earns $70 every year, so after 30 years you have $3,100. With compound interest, the second year earns $74.90 because it is calculated on $1,070, and after 30 years you have $7,612.26.
Which compounding frequency should I choose?
Use the one in your account or product terms. If the rate is an APY, an AER or an expected yearly return, choose Annually, because that rate already includes compounding. If the terms say “compounded monthly” or “compounded daily”, choose that option.
What is the difference between APR and APY (or AER)?
APR is a nominal yearly rate that does not include compounding. APY, used in the United States, and AER, used in the United Kingdom, show what you earn in a year once compounding is included. A 12% APR compounded monthly is a 12.68% APY.
What is the rule of 72?
A quick way to estimate how long money takes to double: divide 72 by the yearly rate. At 8% that gives 9 years, which matches the exact answer. At 7% it gives 10.3 years against an exact 10.2. The further the rate is from 8%, the less precise the rule becomes.
How does the savings goal tab work?
Enter the amount you want to reach and choose what to work out: the monthly amount needed in the years you set, or how long it takes with the monthly amount you can save. With a yearly increase, the monthly amount shown is what you save in the first year. Switch on “My goal is in today’s money” to raise the goal by inflation, so it buys then what it buys now.
What does the rate range show?
The same calculation at your rate minus and plus the points you choose, for example 5%, 7% and 9%. It shows how sensitive the result is to the rate. It is not a probability: each line assumes that rate every year.
Should contributions go at the start or the end of the month?
Choose the start if you invest as soon as you are paid. Each contribution then earns one more month of interest. The end of the month is the more cautious choice and the one most calculators use.
Why show results in today’s money?
Because prices rise. A balance 30 years from now looks large, but it buys less than the same amount today. Today’s money divides every amount by the inflation built up until that date, so you can compare the result with what things cost now.
Does the calculator include taxes and fees?
No. Tax rules depend on your country and the type of account. To allow for fees, lower the rate by the yearly fee. For a rough view of taxes on growth, lower the rate by the share that tax would take.
What rate should I use?
For a savings account or a fixed deposit, use the rate it pays. For investments, returns are not guaranteed, so try a cautious rate and a more hopeful one and compare the two results.
Is my data stored or sent anywhere?
No. All the calculations run in your browser. Nothing is sent to a server and nothing is saved. Copy link puts your numbers in the web address, so only the people you share it with can see them.