Super Boring Tools

Search tools

Find a tool by name or what it does.

Compound Interest Calculator

Project how a balance grows with compound interest and monthly contributions, year by year.

Investment

Calculated in your browser as you type. Nothing is uploaded.

Compounding frequency

Monthly contributions are grouped to the chosen frequency and applied at the end of each period.

Result
31,998.32
Final balance after 10 years
22,000.00
Total contributed
9,998.32
Total interest earned

Assumes a constant annual rate and steady contributions. Real returns vary.

Year by year
YearContributed to dateInterest to dateBalance
111,200.00539.5011,739.50
212,400.001,168.0113,568.01
313,600.001,890.0615,490.06
414,800.002,710.4417,510.44
516,000.003,634.2019,634.20
617,200.004,666.6021,866.60
718,400.005,813.2324,213.23
819,600.007,079.9126,679.91
920,800.008,472.7929,272.79
1022,000.009,998.3231,998.32

How to calculate compound interest

  1. Enter the starting numbers

    Type a starting amount, an annual interest rate, and how many years to grow, up to 60.

  2. Add a monthly contribution

    Optionally set a monthly contribution; leave it at zero to grow the starting amount alone.

  3. Pick a compounding frequency

    Choose monthly, quarterly, or yearly compounding. The result recalculates the moment you switch.

  4. Read the result

    The final balance, total contributed, and total interest appear instantly, with a year-by-year table below.

Why use this tool

Monthly contributions modeled

Add a regular monthly deposit and it compounds alongside the starting amount, applied at the end of each compounding period.

Three compounding frequencies

Switch between monthly, quarterly, and yearly compounding and see how often interest is added change the outcome.

Year-by-year growth table

Every year shows the amount contributed to date, the interest earned to date, and the running balance, so you can watch the curve steepen.

Interest vs contributed split

The final balance is split into what you put in and what compounding added, so the effect of interest is never hidden inside one number.

No signup, no server

The math runs locally as you type; there is nothing to create an account for and nothing uploaded.

About this tool

Compound interest is interest earned on interest. Each period, the balance earns a return, and that return joins the balance, so the next period earns on a slightly larger amount. Over a few years the effect is modest; over decades it dominates. This calculator applies the standard compound growth formula to a starting amount and, if you set one, a monthly contribution, then reports the final balance, the total you put in, and the interest earned on top, with a year-by-year table showing how the split shifts over time.

Starting early beats contributing more later. At 7% compounded monthly, 200 a month for 30 years grows to about 244,000, and only 72,000 of that is deposits. Wait ten years and contribute 300 a month for 20 years instead: the same 72,000 goes in, but the balance reaches only about 156,000. Identical money, very different outcomes, purely because the earlier deposits spent longer compounding. The years input goes up to 60 for exactly this kind of comparison.

The frequency toggle changes less than most people expect. At 5%, monthly compounding gives an effective annual return of about 5.12% against 5% for yearly compounding; the gap is real but rarely decisive. The calculator assumes one constant rate and unbroken contributions applied at the end of each period, so treat the output as a projection for comparing scenarios, not a forecast of any investment. Everything runs in your browser as you type, and nothing is uploaded. For debt moving in the other direction, the loan calculator works through repayments, and the percentage calculator covers one-off percentage changes.

Frequently asked questions

What formula does the compound interest calculator use?
The starting amount grows by the standard compound formula: the balance times (1 + rate per period) raised to the number of periods. Contributions are grown with the matching future value of a series of regular deposits at the same per-period rate. At a 0% rate the result is simply everything you put in.
When are contributions applied?
At the end of each compounding period. With quarterly compounding, three monthly contributions are treated as one deposit made at the end of the quarter. Contributing at the start of each period would give a slightly higher result; the difference is small at typical rates.
How much does the compounding frequency matter?
Less than the rate or the time horizon. At 5% annual interest, monthly compounding yields an effective 5.12% per year against 5% for yearly compounding. Switch the pills and watch the final balance; the gap is real but rarely decisive.
Will my investment actually grow like this?
The calculator assumes one constant annual rate and unbroken contributions for the whole horizon. Real returns vary year to year and can be negative, so use the output to compare scenarios, not as a forecast of any specific investment.
Which currency does it use?
None. The calculator works in plain numbers, so the result is in whatever currency you typed. Formatting adds thousands separators and two decimals but no symbol.
Is anything sent to a server?
No. The math runs in your browser as you type. Nothing is uploaded, stored, or logged.

Related tools