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.
Money
Compound Interest Calculator
Project compound growth from a starting balance, regular contributions, interest rate, compounding frequency and time horizon.
Calculator
Working calculator
Live result55290.66 future value10000.00 start + 30000.00 contributions + 15290.66 estimated interest
Formula used
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.
This is the method behind the answer, so the result can be checked rather than simply trusted.What-if check
Rate and contribution sensitivity
Same starting balance and time horizon, with the annual rate and monthly contribution varied around the current inputs. This keeps the target balance from depending on one optimistic assumption.
| Annual rate | Future value | Change |
|---|---|---|
| 3.00% | 48,428.89 | -6,861.77 |
| 5.00% | 55,290.66 | Current rate |
| 7.00% | 63,367.82 | +8,077.15 |
| Monthly contribution | Future value | Change |
|---|---|---|
| 0.00 | 16,470.09 | -38,820.57 |
| 250.00 | 55,290.66 | Current contribution |
| 350.00 | 70,818.89 | +15,528.23 |
Visual proof
Balance split
The bar separates money paid in from estimated compound interest, so the projection is not mistaken for interest alone.
Printable calculation report
Result: 55290.66 future value. Assumption: The entered annual rate is a nominal planning rate, not a guaranteed return.
- Starting balance
- 10000
- Annual interest rate
- 5
- Time horizon
- 10
- Monthly contribution
- 250
- Compounding frequency
- 12
- Page/date context
- 2026-05-16 UTC page version
- Page URL
- https://calculationtime.com/calculators/compound-interest-calculator
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
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.
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
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.
Standard or basis
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.
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
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.
Related calculators
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.