Simple & Compound Interest Calculator (Free)

Use this free simple and compound interest calculator to find the interest you earn and the final balance. Enter the principal, annual rate, and time, choose simple or compound interest (and the compounding frequency), and see the result with a growth chart.

How to use it

1. Enter the principal, the annual interest rate (%), and the time in years.
2. Choose simple or compound interest (and, for compound, how often it compounds).
3. Read the interest earned and the final amount, and see the balance grow.

The formulas

Simple: I = P·r·t. Compound: A = P(1 + r/n)^(n·t), where n is the number of times it compounds per year.

Frequently asked questions

What is the formula for compound interest?

The compound interest formula is A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate as a decimal, n is the number of times interest compounds per year, and t is the time in years. The interest earned is A − P. For $500 at 4% compounded annually for 2 years, A = 500 × 1.04² = $540.80, so the interest is $40.80.

Is compound interest calculated monthly or yearly?

It depends on the account or loan. Interest can compound yearly, quarterly, monthly or daily, and the terms will say which. Many savings accounts compound daily or monthly, while textbook problems often use annual compounding. More frequent compounding gives a slightly higher balance: $1,000 at 6% for one year grows to $1,060.00 compounded yearly but about $1,061.68 compounded monthly.

How do you calculate quarterly compound interest?

Use n = 4 in the formula, so divide the annual rate by 4 and multiply the number of years by 4. For $2,000 at 8% compounded quarterly for 1 year, the rate per quarter is 2%, so A = 2,000 × 1.02⁴ ≈ $2,164.86. That is $164.86 in interest, a little more than the $160 you would earn with simple interest.

How do you calculate compound interest without a calculator?

Work one period at a time: find the interest on the current balance, add it, and repeat. For $200 at 10% compounded annually for 2 years, year one adds $20 to make $220, and year two adds 10% of $220, which is $22, for a final balance of $242. Friendly rates like 5% or 10% keep the multiplication simple when you work by hand.

How do you calculate compound interest in Excel?

Type the formula with your numbers, such as =1000*(1+0.05/12)^(12*3) for $1,000 at 5% compounded monthly for 3 years, which gives about $1,161.47. You can also use the built-in FV function: =FV(0.05/12, 36, 0, -1000) returns the same amount. Running the same numbers through this calculator is a quick way to check that your spreadsheet formula is right.

Is compound interest a calculator or non-calculator topic?

Compound interest can appear on both calculator and non-calculator math papers, including GCSE exams. On a non-calculator paper the numbers are chosen to be easy, such as 10% for 2 years, so you can work year by year. On a calculator paper you are more likely to use the multiplier method, for example 1.03⁵ for 3% growth over 5 years.

Read the full lesson: learn the method step by step.

How to use the Simple & Compound Interest Calculator for homework

The Simple & Compound Interest Calculator is most useful when you treat it as a learning check, not just a shortcut to the final answer. Start by copying the original problem carefully, including signs, exponents, decimal points, fractions, parentheses, and units. Then enter the values in the same order the problem gives them. A small typing change can completely change the result, especially in algebra, statistics, geometry, and probability problems.

Before you press the button to calculate, make a quick estimate or prediction. The estimate does not need to be exact. Its job is to help you notice impossible answers. If a distance becomes negative, a probability is bigger than 1, an angle looks too large, or a decimal point seems misplaced, go back and check the input before trusting the final result.

Before you enter the problem

• Rewrite the problem in a clean line so every value is easy to see.
• Use parentheses around grouped expressions, especially in fractions and exponents.
• Keep units with the numbers while you work, even if the calculator only asks for the numbers.
• Check whether the problem wants an exact value, a decimal approximation, or a rounded answer.
• Look for restrictions such as positive values only, a chosen interval, or a required domain.

How to read the result

After the calculator gives a result, read more than the final line. If steps, tables, graphs, or intermediate values are shown, use them to understand how the answer was built. That is especially important when you are studying for a quiz or test, because teachers often give more credit for a correct process than for an unsupported number.

Try to identify the main idea behind the result. For example, ask yourself which formula was used, which operation changed the expression, which value controlled the graph, or which assumption made the answer possible. When you can explain that idea in your own words, the tool has helped you learn the skill instead of only checking one problem.

Common mistakes to avoid

Turn the answer into practice

One good way to study is to solve the problem by hand first, then use the Simple & Compound Interest Calculator to check your work. If your answer is wrong, do not erase everything immediately. Find the first line where your work stops matching the calculator’s logic. That line is usually where the real misunderstanding happened.

1. Work the problem on paper and circle your final answer.
2. Use the calculator to check the result.
3. If the answers match, write one sentence explaining the method.
4. If they do not match, compare each step until you find the first difference.
5. Redo a similar problem without the tool to make sure the correction sticks.

When to use a calculator and when to work by hand

Use the Simple & Compound Interest Calculator when you want to check a long calculation, explore a pattern, test a graph, or confirm a result after practicing. Work by hand when the assignment asks for steps, when you are learning a new method, or when the test will not allow a digital tool. The strongest students use both: hand work to build understanding and calculators to check accuracy.

If you are preparing for a timed test, practice some problems without the tool and some with it. That balance helps you build speed while still understanding the math. Over time, the goal is to need the calculator less often for routine steps and use it more strategically for checking, exploring, and confirming your reasoning.

Quick accuracy checklist

Before you leave the page, use the Simple & Compound Interest Calculator result to answer three quick questions: What was the input? What rule, formula, or method connects the input to the result? What would change if one number in the problem changed? These questions turn a single calculator answer into a reusable math skill.

For homework, write the original problem, the calculator’s result, and one short explanation of the method in your notebook. For test review, cover the result and try to reproduce the setup on your own. If you can set up a similar problem without looking back at the calculator, you are much closer to owning the skill.