Simple Interest Calculator

Put $10,000 into an account paying 5% simple interest for 3 years and you will earn exactly $1,500 in interest - $500 every year - leaving a final balance of $11,500. Because simple interest is charged only on the original principal, the amount never changes from one year to the next. That predictability is exactly what makes a simple interest calculator useful: you can see the full cost or return of a loan or deposit at a glance.

The simple interest formula

Simple interest uses one short equation:

where P is the principal (your starting amount), r is the annual interest rate written as a decimal (5% → 0.05), and t is the time in years. I is the interest earned or owed and A is the final amount. In our example: 10,000 × 0.05 × 3 = 1,500, so A = 10,000 + 1,500 = 11,500.

Simple interest vs compound interest

The key difference is what the interest is calculated on. Simple interest always uses the original principal, so you earn a flat amount each period. Compound interest adds each period's interest back to the balance, so future interest is earned on a larger and larger amount. Over short periods the two are close, but the gap widens over time. On $10,000 at 5% for 3 years, simple interest earns $1,500 while annual compounding earns about $1,576 - and over 30 years compounding pulls far ahead. To see that curve for your own numbers, run the same figures through the Compound Interest Calculator and compare the two totals side by side.

Where simple interest is used

Simple interest shows up more often than people expect. Many auto loans, some personal and student loans, short-term promissory notes, and certain bonds and Treasury bills use simple interest on the outstanding principal. By contrast, most savings accounts, credit cards and mortgages compound. Always check whether a rate is simple or compound before comparing two offers.

Working with partial years and rates

• Convert the rate to a decimal: divide the percentage by 100, so 7.25% becomes 0.0725.
• Partial years: 6 months is 0.5 years, 18 months is 1.5 years - enter the fraction directly.
• Days: divide days by 365 (or 360 if your lender uses a 360-day year) to get the time in years.
• Solve backwards: if you know any three of P, r, t and I, you can rearrange the formula to find the fourth - or let the Interest Rate Calculator solve for the rate directly.

How to use this calculator

Getting an answer takes about thirty seconds. You only need three numbers, and the tool handles the arithmetic and the year-by-year breakdown for you.

1. Enter the principal. This is the starting amount - the money you deposit, lend, or borrow. In our running example it is $10,000.
2. Enter the annual interest rate. Type it as a percentage (for example, 5). The calculator converts it to the decimal form (0.05) for you before applying the formula.
3. Enter the time in years. Use whole numbers for full years, or a decimal for partial periods - 0.5 for six months, 1.25 for fifteen months.
4. Read the results. You will see the total interest, the final amount, the interest earned per year, and a table showing the balance at the end of each year.
5. Compare with compounding. The side-by-side comparison shows how much annual compounding would add, so you can judge whether the "simple" assumption fits your situation.

A second worked example

Suppose you sign a $5,000 personal note for a friend at 4% simple interest to be repaid in 18 months. First convert the time: 18 months is 1.5 years. Then apply the formula: 5,000 × 0.04 × 1.5 = $300 in interest, for a final repayment of $5,300. Notice that the annual interest is 5,000 × 0.04 = $200, and half of that ($100) is earned in the final six months - simple interest accrues at a steady, straight-line pace, which makes a half-year easy to prorate.

Now compare a short-term saving scenario. Park $2,000 in a 6-month promissory note at 3% simple interest: that is 2,000 × 0.03 × 0.5 = $30. The same rate over a full year would have paid $60. Because the relationship between time and interest is perfectly linear, doubling the time exactly doubles the interest - something that is not true once compounding enters the picture.

Key terms explained

• Principal (P): the original sum of money that interest is calculated on. With simple interest, the principal never changes - interest is not added back to it.
• Interest rate (r): the annual percentage charged or paid, expressed as a decimal in the formula. A "nominal" rate is the stated rate before any compounding is considered.
• Term or time (t): how long the money is borrowed or invested, measured in years to match the annual rate.
• Interest (I): the dollar amount earned or owed - the output of P × r × t.
• Final amount (A): principal plus interest, the total you end up with or have to repay.
• Day-count convention: the rule a contract uses to turn days into a fraction of a year, most often 365 days (actual) or 360 days (banker's year).

Who this calculator is for

A simple interest calculator is useful any time interest is charged on a fixed principal rather than a growing balance. It fits borrowers checking the true cost of an auto loan or a short personal loan, private lenders drafting a promissory note for family or a small business, students learning the I = P × r × t formula for a finance or math class, and savers evaluating a fixed-coupon instrument such as a Treasury bill or a simple-interest certificate. If your account compounds - as most savings accounts and credit cards do - reach for the compound interest tool instead, since simple interest will understate the real growth or cost. Comparing a fixed-rate certificate of deposit? The CD Calculator applies the bank's APY and compounding for you.

What changes the interest you earn or owe

Only three inputs move the result, and each one moves it in a straight line. Raising any single input while holding the others fixed changes the interest proportionally:

• Principal: doubling the starting amount doubles the interest. A larger loan or deposit scales the dollar interest one-for-one.
• Rate: a higher annual rate raises interest proportionally. Going from 4% to 6% adds 50% more interest over the same period.
• Time: a longer term adds interest at a constant pace - each extra year on a $10,000 deposit at 5% adds exactly $500, no more and no less.
• Day-count convention (for day-based loans): a 360-day year nudges day-count interest slightly higher than a 365-day year because each day counts for a bigger slice of the year.

What does not change a simple-interest result is the passage of earlier periods. Unlike compound interest, money you have already earned is never re-invested, so year five earns no more than year one.

Tips to get the most from the result

• Confirm the interest method first. Ask the lender or bank in writing whether the rate is simple or compound - it changes the math, not just the wording.
• Shop the rate. Because interest scales directly with the rate, even a half-point lower rate is real money on a multi-year balance.
• Mind the day count. On short notes, check whether the contract uses a 360- or 365-day year before you agree to the figure.
• Account for taxes. Interest income is generally taxable, so subtract your marginal tax rate to estimate what you actually keep.
• Compare like with like. Convert every offer to the same basis - simple-to-simple, or APY-to-APY - before deciding which is cheaper or pays more.

Limitations and assumptions

This tool models the cleanest possible case: one fixed principal, one fixed annual rate, and straight-line interest with no compounding. It does not account for recurring deposits or withdrawals, interest that compounds within the term, taxes on interest income, origination fees or points, late-payment penalties, variable rates, or inflation eroding the real value of your money. It also assumes interest accrues continuously and evenly across the term rather than being credited at specific dates. For loans with a repayment schedule, the outstanding balance falls over time, so real interest paid is usually lower than a flat P × r × t on the full original principal - treat the result as a clear baseline, then layer your own terms on top.

How it compares to related calculators

This page answers one focused question: "how much interest does a fixed principal earn or cost at a flat annual rate?" The moment your situation adds compounding, regular contributions, or a repayment schedule, a sister tool will give you a more accurate number. Here is how to pick the right one:

• If your money earns interest on interest, use the Compound Interest Calculator - it bends the curve upward as earlier interest starts compounding.
• For a general "interest earned or owed over any period" calculation that can switch between methods, the broader Interest Calculator is the catch-all.
• To project a savings balance with regular monthly deposits, use the Savings Calculator or set a target with the Savings Goal Calculator.
• For a loan with monthly payments rather than a single lump sum, the Loan Calculator amortizes the balance and shows total interest paid.
• When you know the interest but need the rate, the Interest Rate Calculator solves for r directly.
• To compare a quoted simple rate against a yield that already includes compounding, convert with the APY Calculator or check a loan's true cost with the APR Calculator.

Choosing the matching tool matters because the same nominal rate produces different totals depending on the method. Simple interest is the clean baseline; the calculators above layer compounding, payments, and fees on top of it.

Sources

• Consumer Financial Protection Bureau - What is a simple-interest loan?
• Consumer Financial Protection Bureau - Interest rate and APR basics
• IRS - Topic No. 403, Interest Received (taxable interest income)
• U.S. Department of the Treasury (TreasuryDirect) - Treasury Bills