Compound Interest Calculator
Compound interest grows your money on both your deposits and the interest they have already earned. Enter a starting amount, a regular contribution, a rate and a time horizon to see your future value, an interactive growth chart, the split between what you deposited and what compounding added — and the result in inflation-adjusted today’s dollars.
$300,851 future value
In today’s dollars: $166,574 after 3% inflation — what your balance can actually buy.
Total deposited: $130,000 (start $10,000 + $120,000 added) · Interest earned: $170,851
Interest is 131% of what you put in — that gap is compounding at work.
Yearly breakdown
| Year | Deposited | Interest | Balance |
|---|---|---|---|
| 1 | $16,000 | $919 | $16,919 |
| 2 | $22,000 | $2,339 | $24,339 |
| 3 | $28,000 | $4,294 | $32,294 |
| 4 | $34,000 | $6,825 | $40,825 |
| 5 | $40,000 | $9,973 | $49,973 |
| 6 | $46,000 | $13,782 | $59,782 |
| 7 | $52,000 | $18,299 | $70,299 |
| 8 | $58,000 | $23,578 | $81,578 |
| 9 | $64,000 | $29,671 | $93,671 |
| 10 | $70,000 | $36,639 | $106,639 |
| 11 | $76,000 | $44,544 | $120,544 |
| 12 | $82,000 | $53,455 | $135,455 |
| 13 | $88,000 | $63,443 | $151,443 |
| 14 | $94,000 | $74,587 | $168,587 |
| 15 | $100,000 | $86,971 | $186,971 |
| 16 | $106,000 | $100,683 | $206,683 |
| 17 | $112,000 | $115,820 | $227,820 |
| 18 | $118,000 | $132,486 | $250,486 |
| 19 | $124,000 | $150,790 | $274,790 |
| 20 | $130,000 | $170,851 | $300,851 |
How compound interest works
Compound interest is interest earned on both your original money and the interest it has already earned. The future value of a lump sum is A = P · (1 + r/n)n·t, where P is the principal, r the annual rate, n the number of times interest compounds per year and t the number of years. The interest earned is simply A − P.
Example: $10,000 at 7% compounded monthly for 20 years grows to about $40,000 — four times your money, with roughly $30,000 of that being pure interest. The longer the horizon, the more the curve bends upward, which is why starting early matters more than starting big.
Compounding frequency: daily vs monthly vs annual
The more often interest compounds, the slightly higher your return, because interest starts earning interest sooner. At 7%, daily compounding edges out annual compounding by a fraction of a percent a year — small per year, but it adds up over decades. Use the Compounding selector above to see the exact difference for your numbers; the headline updates instantly.
Adding regular contributions
Most wealth is built by steady deposits, not a single lump sum. When you add a monthly or annual contribution, each deposit starts its own compounding clock. The calculator uses the future-value-of-a-series formula and lets you set whether deposits land at the start or end of each period, plus an optional annual step-up so your contributions grow with your income.
Putting in $500 a month at 7% for 20 years turns $120,000 of deposits into roughly $260,000 — about $140,000 of it interest you never deposited. The stacked chart above shows exactly how much of your balance is money you put in versus money the market added.
Real returns: adjust for inflation
A dollar in 20 years buys less than a dollar today, so a big nominal balance can be misleading. Calcuris shows your inflation-adjusted (real) future value — what your money will actually buy in today's dollars — something the most-visited compound-interest calculators leave out. Set your expected inflation rate and compare the nominal headline with the real figure beneath it; the gap is the part of your growth that just keeps pace with prices.
The Rule of 72: how long to double your money
For a quick estimate, divide 72 by your annual return to get the years it takes to double. At 7%, that's about 72 ÷ 7 ≈ 10.3 years to double; at 9%, roughly 8 years. It's an approximation of the compound-growth formula, handy for back-of-the-envelope planning before you run the exact numbers here.
Simple vs compound interest
Simple interest is paid only on your original principal, so $10,000 at 7% simple earns a flat $700 every year — $14,000 of interest over 20 years. Compound interest pays on the growing balance, earning roughly $30,000 over the same period on the same deposit. The difference is compounding, and it widens every year.
APR vs APY (effective annual rate)
APR is the stated annual rate before compounding; APY (or effective annual rate) folds the compounding in, so it's the number that reflects what you actually earn. A 7% APR compounded monthly is about a 7.23% APY. When you compare savings accounts or investments, compare APYs — and enter the stated APR plus the compounding frequency above to see your effective return.
Frequently asked questions
How do you calculate compound interest?
Use A = P · (1 + r/n)^(n·t): principal times one plus the periodic rate, raised to the number of compounding periods. For deposits added over time, add the future value of that series. Enter your figures above and the calculator does both, with a year-by-year breakdown.
What is the formula for compound interest?
For a lump sum, A = P(1 + r/n)^(nt), where P is the principal, r the annual rate, n the compounds per year and t the years; interest earned = A − P. With regular deposits you add PMT · [((1 + r/n)^(nt) − 1) / (r/n)], adjusted for whether deposits fall at the start or end of each period.
How much is $1,000 worth in 20 years with compound interest?
At a 7% return compounded monthly, $1,000 grows to about $4,000 in 20 years with no further deposits — roughly quadrupling. At 5% it's about $2,700, and at 10% about $7,300. Change the rate above to see your figure.
What will $10,000 be worth in 20 years?
About $40,000 at 7% compounded monthly with no extra deposits, of which roughly $30,000 is interest. Add a monthly contribution and it grows far more — $10,000 plus $500/month at 7% reaches roughly $300,000 in 20 years.
How do I calculate compound interest monthly?
Set the compounding frequency to monthly: n = 12, so the periodic rate is r/12 and the exponent is 12 × years. The calculator above defaults to monthly compounding; switch to daily, quarterly or annual to compare.
Is compound interest calculated daily or monthly?
It depends on the account. Many savings accounts compound daily, most loans and investments monthly, and bonds often semi-annually. More frequent compounding earns slightly more — at 7%, daily beats annual by a fraction of a percent a year. Pick the frequency that matches your account above.
How much will I have if I invest $500 a month?
Investing $500 a month at a 7% return compounds to roughly $260,000 over 20 years and about $570,000 over 30 years — from $120,000 and $180,000 of actual deposits respectively. The rest is compound growth. Enter $500 as a monthly contribution above to see your timeline.
What is the difference between simple and compound interest?
Simple interest is paid only on the original principal; compound interest is paid on the principal plus all previously earned interest. Over 20 years at 7%, $10,000 earns about $14,000 simple but roughly $30,000 compound — the difference is the interest your interest earns.
How long does it take to double your money?
Use the Rule of 72: divide 72 by your annual return. At 7% that's about 10.3 years to double; at 9%, about 8 years; at 6%, about 12 years. It's a close approximation of the exact compound-growth formula used by this calculator.
What is APY, the effective annual rate?
APY (annual percentage yield) is the real annual return once compounding is included, unlike APR which is the stated rate before compounding. A 7% APR compounded monthly is about a 7.23% APY. Compare accounts by APY, not APR.
Researched & verified by the Calcuris Data & Research Team. How we build and check our tools →