Compound Interest Calculator

Future value$40,000.00 contributed over 120 months; estimated interest $15,290.66 at 5% annual rate with 12 compounding period(s) per year.

Compound growth proof · Starting balance plus end-of-month deposits over 120 months.

$55,290.66Future value · 10 years$40,000.00Contributed · $250.00/mo$15,290.66Interest earned · 12x compounding5%Rate basis · Nominal planning rate

Starting balancecurrencyAnnual interest rate%Time horizonyears

Live math canvas

Your numbers, formula and explanation together

Compound Interest Calculator: $55,290.66 future value. $40,000.00 contributed over 120 months; estimated interest $15,290.66 at 5% annual rate with 12 compounding period(s) per year.

Formula applied

The exact method behind this answer

CalculationTime keeps the method visible so the number can be checked instead of blindly trusted.

Periodic rate = annual rate ÷ compounds per year. Compound periods = years × compounds per year. Future value of principal = P × (1 + r)^n. Contributions are accumulated monthly by applying interest between end-of-month deposits.

Apply the formulaPeriodic rate = annual rate ÷ compounds per year. Compound periods = years × compounds per year. Future value of principal = P × (1 + r)^n. Contributions are accumulated monthly by applying interest between end-of-month deposits.$55,290.66 future value$40,000.00 contributed over 120 months; estimated interest $15,290.66 at 5% annual rate with 12 compounding period(s) per year.

Your live breakdown

Current inputs in the calculation

These values come from the controls above and update when the calculator changes.

Starting balance: 10,000 currency
Annual interest rate: 5 %
Time horizon: 10 years
Monthly contribution: 250 currency/month
Compounding frequency: 12 times/year

Resulting answer

$55,290.66 future value

$40,000.00 contributed over 120 months; estimated interest $15,290.66 at 5% annual rate with 12 compounding period(s) per year.

Answer: $55,290.66 future value
Live support: $40,000.00 contributed over 120 months; estimated interest $15,290.66 at 5% annual rate with 12 compounding period(s) per year.

Assumptions used

What this answer assumes

Nominal projection only. Test lower-rate and no-contribution scenarios before relying on a target balance.

The entered annual rate is a nominal planning rate, not a guaranteed return.
Regular contributions are treated as end-of-month deposits.
Compounding frequency controls the starting balance growth and the monthly simulation uses the equivalent monthly rate from the entered annual rate.
Taxes, fees, inflation, account minimums, early-withdrawal penalties and changing market returns are not included.

Master’s Tip

How to use the result well

Master’s Tip: separate the return you can control from the return you are assuming. Contribution size, fees and time horizon are usually more controllable than the future interest rate, so test at least one lower-rate scenario before planning around the headline result.

Printable record

What belongs in the saved calculation

Save the inputs, result, formula, assumptions, page URL and date together so the calculation can be reviewed later.

Starting balance: 10,000 currency
Annual interest rate: 5 %
Time horizon: 10 years
Monthly contribution: 250 currency/month

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Formula

Periodic rate = annual rate ÷ compounds per year. Compound periods = years × compounds per year. Future value of principal = P × (1 + r)^n. Contributions are accumulated monthly by applying interest between end-of-month deposits.

Worked example

Start with 10,000, add 250 at the end of each month, use 5% annual interest and compound monthly for 10 years. The calculator applies the monthly rate for 120 months and adds 250 after each month, giving about 55,290.66 total.

Professional note

Regional and unit assumptions

Standard or basis: transparent compound-interest arithmetic using a nominal annual rate and user-entered compounding frequency. This is not financial advice, a bank quote, tax guidance or an investment-performance guarantee.

Assumptions and limitations

The entered annual rate is a nominal planning rate, not a guaranteed return.
Regular contributions are treated as end-of-month deposits.
Compounding frequency controls the starting balance growth and the monthly simulation uses the equivalent monthly rate from the entered annual rate.
Taxes, fees, inflation, account minimums, early-withdrawal penalties and changing market returns are not included.

Methodology & Accuracy

How this calculator is checked

CalculationTime pages are built around visible arithmetic: the formula, assumptions, worked example and practical limitations are shown so the result can be checked rather than simply trusted.

Formula used

Standard or basis

Where a calculator follows a named legal, trade or industry standard, that standard is cited visibly. Otherwise the page uses transparent general arithmetic and states its limits.

Master's Tip

Questions

How do you calculate compound interest?

For a lump sum, multiply the principal by (1 + periodic rate) raised to the number of periods. Regular contributions are added over time and then earn interest for the remaining periods.

What does compounding frequency mean?

Compounding frequency is how often interest is added to the balance. Monthly compounding uses 12 periods per year, quarterly uses 4, and annual uses 1.

Are monthly contributions added before or after interest?

This calculator treats monthly contributions as end-of-month deposits, so each deposit starts earning interest after it is added.

Does this include tax or inflation?

No. The result is a nominal arithmetic projection before tax, fees, inflation or changing rates.

Can the actual result be different?

Yes. Real savings and investments can have variable rates, fees, tax rules, missed contributions and market losses. Use the result as a planning estimate only.

Calculation note

Compound interest is the arithmetic of interest earning interest. It appears in savings accounts, loans, bonds, investments and long-term planning, but the same formula can produce very different real outcomes once fees, tax, inflation and variable returns are included.

Compounding turns time into a multiplier

Simple interest adds interest only on the original principal. Compound interest adds interest to the growing balance, so later periods can earn interest on earlier interest. That is why time horizon is a central input rather than a small detail.

Contribution timing changes the result

A monthly saving plan is not the same as a single lump-sum deposit. This page treats contributions as end-of-month deposits so the timing is visible and repeatable rather than hidden inside a vague growth estimate.

Nominal growth is not purchasing power

A future balance can look large while taxes, fees and inflation reduce what it can buy. The calculator deliberately shows nominal compound arithmetic first, then warns users not to treat the projection as a guaranteed financial outcome.

Sources and further readingInvestor.gov: Compound Interest Calculator and Explanation Consumer Financial Protection Bureau: Interest and fees