Interest Calculator

Calculate simple interest, compound interest, and continuous compounding in one smart tool. You can solve for final amount, interest earned, principal, rate, time, or even the recurring contribution needed to hit a target. Includes step-by-step formulas, a growth chart, a timeline table, and comparison views.

Background

Interest describes how money grows over time when a rate is applied to a starting balance. With simple interest, growth is linear because interest is calculated only on the original principal. With compound interest, interest can earn interest, so growth becomes faster over time. This calculator helps students and everyday users see both the math and the meaning behind the result.

Enter values

Calculator mode

Interest model

Compound Most common savings and investment growth model Simple Interest applies only to the original principal Continuous Uses the continuous compounding model Compare modes See simple, compound, and continuous side by side

Solve for

Compound interest is the best default because it covers the most common real-world savings and growth scenarios.

Core values

Currency

Principal

Target / final amount

Annual rate (%)

Time

Time unit

Compounding frequency

Recurring contribution

Contribution frequency

Recurring contributions are supported for compound and continuous growth when solving for final amount, interest earned, or required recurring contribution.

Display & analysis options

Show growth chart

Show schedule table

Show step-by-step explanation

Show comparison panel

Schedule interval

Display rounding

Quick picks

Presets fill the form and calculate immediately.

Result

No results yet. Enter values and click Calculate.

How to use this calculator

Choose an interest model: simple, compound, continuous, or compare.
Choose what you want to solve for: amount, interest, principal, rate, time, or recurring contribution.
Enter the known values such as principal, rate, time, and optional recurring contributions.
Pick a compounding frequency for compound interest.
Click Calculate to see the answer, chart, comparison view, and timeline table.

How this calculator works

Simple interest grows linearly because interest is applied only to the original principal.
Compound interest grows faster because each compounding period can earn interest on past interest.
Continuous compounding models the limiting case where compounding happens continuously.
For recurring deposits, the calculator estimates growth using the effective rate per contribution period.
The comparison view lets you quickly see how the same principal, rate, and time behave under different interest models.

Formula & Equations Used

Simple interest: I = Prt and A = P(1 + rt)

Compound interest: A = P(1 + r/n)^(nt)

Continuous compounding: A = Pe^(rt)

Future value of recurring contributions: FV = PMT × [((1 + i)^m − 1) / i]

Effective annual yield: APY = (1 + r/n)^n − 1 for discrete compounding, and APY = e^r − 1 for continuous compounding.

Example Problems

Example 1 — Compound interest

Invest \$5,000 at 5% annual interest, compounded monthly, for 10 years.

Use A = P(1 + r/n)^(nt).
Substitute: A = 5000(1 + 0.05/12)^(12×10).
Evaluate the growth factor.
The final amount is approximately \$8,235.05.

Example 2 — Simple interest

Find the amount after 4 years if \$2,000 earns 6% simple interest.

Use A = P(1 + rt).
Substitute: A = 2000(1 + 0.06×4).
Compute: A = 2000(1.24) = 2480.

Example 3 — Reach a target with recurring contributions

Start with \$1,000 and grow to \$10,000 in 5 years at 6% compounded monthly. Solve for the recurring monthly contribution.

First grow the starting principal to its future value.
Then use the annuity growth formula to solve for PMT.
The calculator returns the monthly deposit needed to hit the goal.

Simple vs Compound Interest

With simple interest, growth is steady because interest is always based on the original principal. With compound interest, each new interest amount can also earn interest, so the balance grows faster over time. That difference becomes much more obvious over longer time periods or with more frequent compounding.

Compound Frequency Comparison

For the same annual rate, daily compounding usually produces slightly more growth than monthly compounding, which usually produces slightly more than annual compounding. The difference is often small over short periods, but it can add up over time.