Simple Interest Calculator: Find Interest, Total, Rate Or Time

Simple interest is the easiest kind of interest to calculate, and it shows up in car loans, short-term personal loans, some bonds and many school math problems. This guide covers the formula, how to handle months and days, how to solve for any missing value, and how simple interest compares with compound interest. Every number was checked in Python.

Disclaimer: This guide explains how interest math works. It is not financial advice. Loan and account terms vary, so check your agreement or ask a qualified professional before making financial decisions.

What Simple Interest Is

Simple interest is interest calculated only on the original amount of money, called the principal. It does not earn interest on past interest. So the interest amount is the same every year.

If you put $10,000 into an account that pays 5 percent simple interest, you earn $500 every year. After 5 years, you have earned $2,500, and the balance is $12,500. The growth is a straight line.

Compound interest is different. It adds each period's interest to the balance, so the next period's interest is calculated on a bigger number. Over long periods, that difference becomes large, as the comparison section below shows.

The Simple Interest Formula

The simple interest formula is I = P × r × t. To find the total amount at the end, add the interest back to the principal: A = P + I, or A = P(1 + rt).

Two conversions trip people up. First, change the percent to a decimal by dividing by 100: 5 percent becomes 0.05, and 7.25 percent becomes 0.0725. Second, the time must be in years when the rate is per year. The next section shows how to convert months and days. If you need help with the percent step, the percentage calculator does it instantly.

How To Use The Simple Interest Calculator

The simple interest calculator above handles the most common questions in three tabs.

1. Interest and Total: enter the principal, rate and time to see the interest and the final total.
2. Solve for Any Value: find the rate, time or principal when you know the other values.
3. Simple vs Compound: see how much more compound interest would earn over the same period.

Pick the time unit before you type. If you choose months or days, the calculator converts to years for you.

How To Calculate Simple Interest Step By Step

The steps are the same for every problem: convert the rate to a decimal, convert the time to years, then multiply.

Example 1: $1,000 at 5 percent for 3 years

1. Rate: 5% = 0.05.
2. Time: already 3 years.
3. Interest: $1,000 × 0.05 × 3 = $150.
4. Total: $1,000 + $150 = $1,150.

Example 2: $25,000 at 7 percent for 4 years

1. Rate: 7% = 0.07.
2. Interest: $25,000 × 0.07 × 4 = $7,000.
3. Total: $25,000 + $7,000 = $32,000.

Quick yearly answers. For one year, the interest is just principal × rate. That is how you answer questions like "What is 4 percent interest on $10,000?" The answer is $10,000 × 0.04 = $400 per year. For 6 percent on a $30,000 loan, it is $1,800 per year.

Simple Interest For Months And Days

When the time is not in whole years, convert it before you multiply. Divide months by 12, and divide days by the number of days your lender uses for a year.

Months

For $5,000 at 6 percent for 9 months, the time is 9 ÷ 12 = 0.75 years. The interest is $5,000 × 0.06 × 0.75 = $225.

You can also find the monthly interest first. At 6 percent per year, the monthly rate is 6 ÷ 12 = 0.5 percent. One month of interest on $5,000 is $25, and 9 months is $225.

Days

For $2,000 at 8 percent for 90 days, the answer depends on how many days the lender counts in a year.

Some loan agreements, especially business loans, use a 360-day year, which slightly increases the interest. Your loan documents should say which method applies. The calculator's day-count toggle lets you check both.

Solving For Rate, Time Or Principal

The same formula can be rearranged to find any missing value. Divide the interest by the other two values.

These come up in real questions. "What rate do I need to earn $1,200 in 3 years on $8,000?" gives r = 1,200 ÷ (8,000 × 3) = 0.05, or 5 percent. "How long until $1,000 at 5 percent earns $150?" gives 3 years.

To find how long it takes to double your money with simple interest, set the interest equal to the principal. Then t = 1 ÷ r. At 5 percent, that is 1 ÷ 0.05 = 20 years.

Finding The Principal From The Total

Sometimes you know the final amount and need the starting amount. Rearrange A = P(1 + rt) to get P = A ÷ (1 + rt).

You want $12,000 in 4 years from an account paying 5 percent simple interest. How much should you deposit now?

1. Find 1 + rt: 1 + 0.05 × 4 = 1.20.
2. Divide: $12,000 ÷ 1.20 = $10,000.

Check: $10,000 × 0.05 × 4 = $2,000 in interest, and $10,000 + $2,000 = $12,000. This "present value" idea is the basis for comparing money today with money later.

How Simple Interest Grows Year By Year

Simple interest adds the same dollar amount every year, so the balance grows in a straight line.

For $10,000 at 5 percent:

That steady pattern is what makes simple interest easy to plan around. You always know exactly what one year will cost or earn.

Simple Interest Vs Compound Interest

Simple interest is calculated on the principal only. Compound interest is calculated on the principal plus any interest already added. In the first year they match, but compound interest pulls ahead over time.

For $10,000 at 5 percent, with yearly compounding:

The rule is easy to remember:

• As a saver, compound interest is better, because your money grows faster.
• As a borrower, simple interest is better, because you never pay interest on interest.

For compound growth, money doubles in about 72 ÷ rate years, a shortcut called the rule of 72. At 5 percent, that is about 14.4 years, compared with 20 years for simple interest. The scientific calculator handles the powers in the compound formula, A = P(1 + r)ᵗ.

Where Simple Interest Is Used

Simple interest is common in short-term and installment borrowing. It is less common for savings accounts, which usually compound.

When you see "simple interest" on a loan, it usually means the interest is figured on what you still owe. As you pay down the balance, each month's interest gets smaller. Credit cards are the main exception most people meet: their interest usually compounds, so carrying a balance costs more than the simple formula would suggest.

How Simple Interest Works On A Car Loan

On a simple interest car loan, each month's interest is based on the balance you still owe. Your payment stays the same, but more of it goes to principal over time as the balance shrinks.

Take a $20,000 loan at 6 percent for 60 months:

1. The monthly rate is 0.06 ÷ 12 = 0.005.
2. The first month's interest is $20,000 × 0.005 = $100.
3. The fixed monthly payment for this loan works out to about $386.66.
4. In month 1, $100 goes to interest and $286.66 goes to principal.
5. Over the full loan, the total interest is about $3,199.36.

The Consumer Financial Protection Bureau explains that with simple interest, extra payments lower your principal and the interest you owe later. With precomputed interest, the total interest is set up front, so paying early may not save as much.

Add-On Interest: A Costly Look-Alike

Some loans use "add-on" interest, which applies the simple interest formula to the full original amount for the whole term, even as you pay it down. On the same $20,000 at 6 percent for 5 years, add-on interest is $20,000 × 0.06 × 5 = $6,000. That is nearly twice the $3,199.36 a true simple interest loan would charge.

The difference comes from what the rate is applied to. Simple interest on a declining balance charges less each month. Add-on interest charges on money you have already paid back. Always check which method a loan uses before you sign.

Exact Interest Vs Ordinary Interest

Business math books give the two day-count methods their own names. Exact interest uses a 365-day year. Ordinary interest, sometimes called the banker's rule, uses a 360-day year. The 360-day method was popular before calculators, because 360 divides evenly by 12 months of 30 days.

On $10,000 at 6 percent for 106 days:

The 360-day method always gives a little more interest, which favors the lender. On a large commercial loan, that gap can add up, so it is worth checking which method a contract uses.

Counting The Days Between Two Dates

For date-based problems, count the days from the start date to the end date. Most lenders count the end date but not the start date, so the loan runs for the number of nights in between.

From March 1 to June 15 is 106 days: 30 days left in March after the 1st, plus 30 in April, 31 in May and 15 in June. The calculator's "by date" option does this count for you, including leap years.

In Excel or Google Sheets, =DAYS(end, start) returns the number of days, and =YEARFRAC(start, end) returns the fraction of a year. You can then write the full formula as =PrYEARFRAC(start, end). YEARFRAC uses a 30/360 basis by default, so add a basis argument, such as 3 for actual/365, if your loan uses exact days.

Simple Interest Word Problems

Most word problems are the same formula in different clothes. Pick out P, r and t, then decide what the question is asking for.

Borrowing from a relative. You borrow $3,000 from a family member at 4 percent simple interest and repay it in 18 months. The time is 18 ÷ 12 = 1.5 years. The interest is $3,000 × 0.04 × 1.5 = $180, so you repay $3,180.

Saving for a goal. You put $5,000 in a 2-year certificate that pays 4.5 percent simple interest, paid at the end. You earn $5,000 × 0.045 × 2 = $450.

Comparing two loan offers. For $8,000 over 3 years, Offer A charges 6.5 percent simple interest with no fee. Offer B charges 6 percent plus a $150 fee. Offer A costs $8,000 × 0.065 × 3 = $1,560. Offer B costs $8,000 × 0.06 × 3 + $150 = $1,590. Offer A is $30 cheaper, even though its rate is higher. This is the same reason APR exists: it rolls fees into one comparable rate.

How Extra Payments Cut Simple Interest

Because simple interest is charged on what you still owe, every extra dollar of principal you pay stops costing interest right away. The effect adds up faster than most people expect.

On the same $20,000 car loan at 6 percent for 60 months, adding $100 to each $386.66 payment changes the result:

The extra $100 a month pays the loan off 13 months early and saves about $755 in interest. With add-on or precomputed interest, the same extra payments might save much less, which is why the loan type matters.

Simple Interest Vs Simple Discount

A simple discount loan takes the interest out up front, so you receive less than the face amount but repay the full face amount. U.S. Treasury bills work in a similar way: you pay less than face value and get the full amount back at maturity.

Compare a $1,000 loan at 5 percent for 1 year:

The dollar interest is the same, but with the discount loan you only get $950 to use. Paying $50 for $950 is a true rate of 50 ÷ 950 ≈ 5.26 percent. When a quoted rate is a "discount rate," the real cost is a little higher than it looks.

Questions To Ask Before Taking A Simple Interest Loan

A few questions reveal how much a loan will really cost. The answers should be in the loan agreement or the lender's disclosure.

• Is interest charged on the declining balance, or added on to the original amount? Declining-balance simple interest costs far less.
• Is interest calculated daily or monthly, and on a 360-day or 365-day year? Daily accrual rewards paying early in the month.
• Is there a prepayment penalty? Without one, extra payments save interest.
• What is the APR, not just the interest rate? The APR includes required fees, which makes offers easier to compare.
• How are payments applied? Most simple interest loans apply each payment to interest owed first, then to principal.

Monthly And Daily Interest Rates

To find a monthly or daily simple interest rate, divide the annual rate by 12 or by the number of days in the lender's year.

This helps you estimate the cost of paying late or early. On a $15,000 balance at 6 percent, one day of interest is about $15,000 × 0.06 ÷ 365 ≈ $2.47. Paying a week early on a simple interest loan saves roughly $17 in interest.

Why Monthly Interest Changes With The Calendar

On a loan that accrues interest daily, the interest for a month depends on how many days it has. A 31-day month costs a bit more than a 28-day month, even at the same balance and rate.

On a $15,000 balance at 6 percent, daily interest is about $2.47. A 31-day month adds about $76.44 in interest, while February adds about $69.04. That is why the interest portion of a simple interest loan statement can move up and down slightly from month to month, even though the payment never changes.

Common Simple Interest Mistakes

Most errors come from unit mix-ups, not the formula itself.

Summary

• Simple interest is I = P × r × t, and the total is A = P + I.
• Turn the rate into a decimal and the time into years before you multiply.
• Rearrange the formula to find rate, time or principal.
• Simple interest grows in a straight line. Compound interest grows faster over time.
• On loans, simple interest on the declining balance costs much less than add-on interest.

Next step: enter your principal, rate and time in the simple interest calculator at the top of this page, or explore more tools on the Calc Solver homepage.

Frequently Asked Questions