Compound interest calculator with regular deposits
Project savings and investment growth with regular deposits and any compounding frequency, and see exactly how much of the balance is interest.
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- Nothing you enter is sent
- Free, no account
You deposit
$160,000.00
Interest earned
$302,290.03
Interest share of final balance
65.4%
Effective annual rate
7.229%
nominal 7%
- Starting balance2.2%$10,000.00
- Deposits32.4%$150,000.00
- Compound interest65.4%$302,290.03
What the final balance is made of.
- Starting balance$10,000
- Deposits$150,000
- Interest$302,290
Formula and working
Growth of the starting balance
FV₀ = P × (1 + r/m)^(m·t) FV₀ = 10,000 × (1 + 7% ÷ 12)^(12 × 25) = $57,254.18
Growth of the deposits
FVₚ = PMT × [((1 + i)^n − 1) / i] i = rate per deposit period = 0.00583333 n = 12 × 25 = 300 deposits FVₚ = 500 × [((1 + 0.00583333)^300 − 1) ÷ 0.00583333] = $405,035.85
Projected balance
FV = FV₀ + FVₚ FV = $10,000.00 start + $150,000.00 deposits + $302,290.03 interest = $462,290.03
Effective annual rate
EAR = (1 + r/m)^m − 1 EAR = (1 + 7% ÷ 12)^12 − 1 = 7.229%
When deposits and compounding happen at different frequencies, the deposit-period rate is derived from the nominal rate as (1 + r/m)^(m/c) − 1, which reproduces the exact compounded growth between deposits.
| Year | Start balance | Deposits | Interest | End balance |
|---|---|---|---|---|
| 1 | $10,000.00 | $6,000.00 | $919.19 | $16,919.19 |
| 2 | $16,919.19 | $6,000.00 | $1,419.38 | $24,338.58 |
| 3 | $24,338.58 | $6,000.00 | $1,955.73 | $32,294.31 |
| 4 | $32,294.31 | $6,000.00 | $2,530.85 | $40,825.16 |
| 5 | $40,825.16 | $6,000.00 | $3,147.55 | $49,972.70 |
| 6 | $49,972.70 | $6,000.00 | $3,808.82 | $59,781.53 |
| 7 | $59,781.53 | $6,000.00 | $4,517.90 | $70,299.43 |
| 8 | $70,299.43 | $6,000.00 | $5,278.24 | $81,577.68 |
| 9 | $81,577.68 | $6,000.00 | $6,093.55 | $93,671.22 |
| 10 | $93,671.22 | $6,000.00 | $6,967.79 | $106,639.02 |
| 11 | $106,639.02 | $6,000.00 | $7,905.24 | $120,544.25 |
| 12 | $120,544.25 | $6,000.00 | $8,910.45 | $135,454.70 |
| 13 | $135,454.70 | $6,000.00 | $9,988.32 | $151,443.02 |
| 14 | $151,443.02 | $6,000.00 | $11,144.12 | $168,587.14 |
| 15 | $168,587.14 | $6,000.00 | $12,383.47 | $186,970.62 |
Showing 1–15 of 25 rows
Assumptions
- A constant annual rate for the whole period. Real investment returns vary year to year, so treat this as an average-case projection.
- Deposits are the same size every period and never missed.
- No tax on interest, no account fees and no inflation adjustment — use the inflation calculator to see the result in today’s money.
- Interest is credited at the end of each compounding period and immediately starts earning interest itself.
How to use the compound interest calculator
- Enter your starting balance. Set it to zero if you are starting from nothing and only making regular deposits.
- Enter the regular deposit and how often you make it. The annual equivalent is shown underneath so you can sanity-check it.
- Set the annual interest rate and the compounding frequency. The effective annual rate appears next to the frequency, which is the number to compare between accounts.
- Choose the number of years, and switch deposits to the start of each period if that matches your arrangement.
- Read the projection, then use the stacked chart to see the moment interest starts contributing more than your own deposits.
The formula, and why frequency matters
A lump sum grows by FV = P × (1 + r/m)^(m·t), where r is the nominal
annual rate, m is the number of compounding periods per year and t
is the number of years. Regular deposits add a second term, the future value of an annuity:
FV = PMT × [((1 + i)^n − 1) / i].
Compounding more often raises the effective rate, because interest starts earning interest sooner. The gain is real but modest, and it shrinks as frequency rises — the jump from annual to monthly is far larger than the jump from monthly to daily.
| Compounding | Periods / year | Effective rate on 7% nominal | 10,000 after 20 years |
|---|---|---|---|
| Annually | 1 | 7.0000% | 38,697 |
| Quarterly | 4 | 7.1859% | 40,064 |
| Monthly | 12 | 7.2290% | 40,387 |
| Weekly | 52 | 7.2458% | 40,514 |
| Daily | 365 | 7.2501% | 40,547 |
What a lump sum becomes
10,000 left alone, with no further deposits:
| Annual return | 5 years | 10 years | 20 years | 30 years | 40 years |
|---|---|---|---|---|---|
| 3% | 11,593 | 13,439 | 18,061 | 24,273 | 32,620 |
| 5% | 12,763 | 16,289 | 26,533 | 43,219 | 70,400 |
| 7% | 14,026 | 19,672 | 38,697 | 76,123 | 149,745 |
| 10% | 16,105 | 25,937 | 67,275 | 174,494 | 452,593 |
Compounded annually, no deposits, before tax and inflation.
What 500 a month becomes
Deposits only, no starting balance, compounded monthly:
| Annual return | 10 years | 20 years | 30 years | 40 years |
|---|---|---|---|---|
| 4% | 73,625 | 183,387 | 347,025 | 590,981 |
| 6% | 81,940 | 231,020 | 502,258 | 995,745 |
| 8% | 91,473 | 294,510 | 745,180 | 1,745,504 |
At 8% over 40 years you deposit 240,000 and finish with roughly 1.75 million. Over 10 years you deposit 60,000 and finish with 91,473. Same deposit, same rate — the difference is entirely time.
How long does money take to double?
The rule of 72 says: divide 72 by the annual percentage rate. It is a good mental shortcut in the 6–10% range and drifts at the extremes.
| Rate | Rule of 72 | Exact years |
|---|---|---|
| 2% | 36.0 | 35.00 |
| 4% | 18.0 | 17.67 |
| 6% | 12.0 | 11.90 |
| 8% | 9.0 | 9.01 |
| 10% | 7.2 | 7.27 |
| 12% | 6.0 | 6.12 |
Read the projection honestly
A constant rate is a modelling convenience, not a forecast. A savings account rate changes whenever the central bank moves; an investment return arrives as a jumble of good and bad years that happens to average out. Two adjustments make the number more useful: subtract your platform and fund fees from the expected return, and run the final figure through an inflation calculator so you can see it in today's money. A 40-year projection at 7% nominal is roughly a 4.4% projection in real terms if inflation runs at 2.5%.
Questions
What does compounding frequency actually change?
It sets how often earned interest is added to the balance and starts earning interest itself. At 7% nominal, annual compounding gives 7.00% effective, monthly gives 7.23% and daily gives 7.25%. The tool shows the effective annual rate next to your choice.
What if my deposits and my compounding happen at different frequencies?
That is handled exactly. The tool converts the nominal rate into the equivalent growth over one deposit period using (1 + r/m)^(m/c) − 1, so monthly deposits into a quarterly-compounding account come out right.
Should I choose deposits at the start or the end of the period?
End of period (an ordinary annuity) is the safer default and matches most standing orders. Start of period earns one extra period of interest on every deposit, which is worth a percent or so over a long run.
Does this account for inflation and tax?
No — the projection is in nominal money before tax. Run the final figure through the inflation calculator to see it in today’s money, and subtract your fund fees from the expected return to approximate charges.