Money
Interest Calculator – Simple & Compound Interest
Last updated: May 31, 2026
Written by Blake Boege
An interest calculator is a financial calculation tool used to determine the amount of interest earned on savings or paid on a loan over time. It supports both simple interest, which is calculated strictly on the initial principal (I = P * r * t), and compound interest, where interest earned is added back to the principal, earning additional interest in subsequent periods. The calculator handles compounding frequencies ranging from daily to annual, as well as continuous compounding and optional recurring deposits, helping users model investment growth and loan repayment strategies.
Calculate simple and compound interest using our free, easy-to-use interest calculator. Toggle between compound growth and simple interest math.
Quick Answer
Calculate interest on savings, loans, or investments. Toggle between simple interest or compounding growth with customizable compounding frequencies.
Select Interest Type
Choose between simple interest or compounding growth
The amount you start with. · e.g. 10,000
Nominal annual rate. · e.g. 7
e.g. 20
Compounding frequency
Added at the end of each month. · e.g. 100
Future balance after 20 years
$92,480.05
Contributed $34,000.00 · interest earned $58,480.05
Growth summary (first three + final year)
- Year 1$11,962.16
- Year 2$14,066.16
- Year 3$16,322.27
- Year 20$92,480.05
Estimate only. Real returns, taxes, fees, and contribution timing can change the final number.
Examples
Simple Interest: $10,000 at 5% for 3 years
= $1,500 interest ($11,500 total)
Compound Interest: $10,000 at 7% for 20 years ($100/mo contribution)
≈ $85,620 balance ($51,620 interest earned)
How it works
How it works
Use this interest calculator to determine your returns or loan costs based on a specific interest rate. The tool supports two modes for calculating simple interest and compound growth:
- Compound Interest Mode: Calculates the growth of principal over time with customizable compounding frequencies and optional monthly contributions, compounding interest onto previous interest.
- Simple Interest Mode: Computes interest using the formula I = P × r × t, where interest is calculated only on the original principal.
Worked Examples
1. Compound Interest Example:
If you invest $10,000 at a 7% annual interest rate, compounded monthly, for 20 years, contributing an additional $100 per month:
- Total principal contributed: $10,000 (starting) + $24,000 (monthly contributions) = $34,000
- Future Balance: $85,619.64
- Total Interest Earned: $51,619.64
2. Simple Interest Example:
If you borrow or invest $10,000 at a 5% simple annual interest rate for 3 years:
- Interest (I) = Principal ($10,000) × Rate (0.05) × Time (3 years)
- Interest Earned/Owed: $1,500
- Total Value: $10,000 + $1,500 = $11,500
Specialized Interest Tools
For deep-dives into a specific method of calculating interest, try our dedicated tools:
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Frequently asked questions
Simple interest is calculated solely on the principal (the initial amount of money). Compound interest is calculated on the principal plus any interest that has accumulated in previous periods, meaning you earn interest on your interest.
Compound interest works by adding earned interest back into the principal balance at regular intervals (such as monthly or annually). In the next period, the interest rate is applied to this new, larger balance, leading to exponential growth over time.
A higher compounding frequency is better for savers because interest is added to the balance more often, allowing compound interest to build faster. For example, daily compounding yields a slightly higher return than monthly or annual compounding at the same nominal interest rate.
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