Compound Interest Calculator with Monthly Contributions

Project how savings and investments grow with compound interest, regular contributions, step-ups, inflation and tax. Solve for the deposit needed to reach a goal and compare return scenarios.

Built and maintained by Shehzad Munir ·

Estimates only, not financial, tax or investment advice. Results depend on the figures you enter and on rules that change; check with a qualified professional before making decisions.

Compound Interest Calculator
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The compound interest formula

See what your money grows to with compound interest. Add a starting balance, regular weekly, monthly, quarterly or yearly deposits and any compounding frequency from daily to continuous. Adjust for inflation and tax on gains, raise deposits every year, work out how much to save to hit a goal by a date, and compare savings, bond and stock-market returns side by side.

For a single deposit, A = P × (1 + r/n)^(n×t), where P is the principal, r the annual rate, n how many times a year interest compounds and t the number of years. Regular deposits add the future value of an annuity: PMT × ((1 + i)^N − 1) ÷ i, multiplied by (1 + i) if you deposit at the start of each period. This calculator simulates month by month, so it also handles step-ups, yearly tax and mixed schedules exactly.

Features

  • Daily to continuous compounding, effective annual yield
  • Weekly, monthly, quarterly or yearly contributions at start or end of period
  • Yearly contribution step-up, tax on gains and inflation adjustment
  • Savings goal solver: deposit needed and time to target
  • Compare up to 5 return scenarios on one chart
  • Stacked growth chart, year-by-year table, CSV and PDF export
  • Rule of 72 doubling time and 4% withdrawal income

How to use Compound Interest Calculator

  1. 1

    Enter your starting amount

    Type the initial deposit and pick your currency.

  2. 2

    Set the rate, time and compounding

    Enter the expected annual return, the number of years and how often interest compounds (daily, monthly, yearly or continuously).

  3. 3

    Add regular contributions

    Choose how much you add and how often, whether deposits land at the start or end of each period, and optionally increase them every year.

  4. 4

    Adjust for inflation and tax

    Switch on inflation to see the result in today's money, and set a tax rate on gains if your account is taxable.

  5. 5

    Plan a goal or compare scenarios

    Use Savings goal to find the deposit needed for a target, or Compare rates to see different returns side by side. Export the yearly table as CSV or PDF.

Common use cases

  • A saver checks what $300 a month becomes over 20 years in an index fund.
  • A parent works out the monthly deposit needed for a college fund by age 18.
  • Someone compares a high-yield savings account with a bond fund and stocks.
  • An investor sees the real value of a retirement pot after inflation.
  • A young worker sees the cost of starting to invest five years later.
  • A planner tests how raising contributions with each pay rise changes the result.

Detailed guide

How compounding frequency changes the result

The more often interest compounds, the higher the effective annual yield (EAY or APY). At 5%, annual compounding yields exactly 5%, monthly yields 5.12%, daily 5.13% and continuous 5.127%. The difference is small at low rates but grows with time and higher returns, which is why savings accounts quote APY.

CompoundingEffective yield at 5%Effective yield at 10%
Annually5.00%10.00%
Quarterly5.09%10.38%
Monthly5.12%10.47%
Daily5.13%10.52%
Continuously5.13%10.52%

The Rule of 72

Divide 72 by the annual return to estimate how many years money takes to double: about 10 years at 7.2%, 7.2 years at 10% and 18 years at 4%. The calculator shows both the rule-of-72 estimate and the exact doubling time for your rate and compounding.

Why starting early matters

Compounding rewards time more than amount. Someone who invests $300 a month from 25 to 65 at 7% ends with roughly $790,000, while starting at 35 gives about $366,000, less than half, despite only ten fewer years of deposits. The "cost of waiting" insight shows this for your own numbers.

Inflation and real returns

A 7% return with 3% inflation is about a 4% real return. Turn on the inflation adjustment to see your future balance in today's money, which is the number that matters for planning retirement income. The 4% withdrawal figure gives a rough monthly income your final balance could support.

See how a loan's interest compounds against youFree up money to invest by clearing debt firstFind subscriptions to cancel and invest instead

Frequently asked questions

How do I calculate compound interest?

Use A = P(1 + r/n)^(nt) for a lump sum, where P is the principal, r the annual rate, n compounding periods per year and t the years. $10,000 at 7% compounded monthly for 20 years grows to about $40,387. Add contributions in the calculator to include regular deposits.

How much will $500 a month be worth in 20 years?

At a 7% annual return compounded monthly, $500 a month for 20 years grows to about $260,000, of which $120,000 is your deposits and around $140,000 is interest. At 10% it is roughly $380,000.

What is the difference between simple and compound interest?

Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus the interest already earned, so growth accelerates over time. Over long periods compound interest produces far larger balances.

Does compounding frequency make a big difference?

It makes a small difference. At 5%, monthly compounding gives an effective 5.12% a year versus 5.00% with annual compounding. The calculator shows the effective annual yield for whatever frequency you choose.

How much do I need to save each month to reach my goal?

Open the Savings goal tab, enter the target, years, starting amount and expected return, and the tool solves for the exact monthly (or weekly, quarterly, yearly) deposit needed, plus a table for other time frames.

What is the Rule of 72?

It estimates doubling time: 72 ÷ annual return = years to double. At 8%, money doubles in about 9 years. The calculator also shows the exact doubling time for your rate.

Should I adjust for inflation?

Yes for long-term planning. Inflation reduces what your future balance can buy. Turn on the inflation option to see results in today's money, which gives a realistic picture of retirement or education savings.

What return should I assume for stocks, bonds and savings?

Historically, broad stock indexes have returned around 7–10% a year before inflation, bonds 3–5% and high-yield savings 3–5% depending on rates. Returns are not guaranteed, so use the Compare rates tab to test a cautious and an optimistic case.

Do deposits at the start of the month earn more?

Yes, slightly. Money deposited at the start of each period earns one extra period of interest, which is called an annuity due. Switch the timing option to see the difference.

Is the calculator free and private?

Yes. It runs entirely in your browser with no signup. Inputs are saved locally on your device, and you can share a link that reopens the same calculation.

Does it include monthly contributions?

Yes. Enter a monthly deposit and it's added every month before the next round of interest, so you see the combined effect of saving and compounding.

Sources

The figures and rules on this page are based on these primary sources:

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