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Simple Interest Calculator

A simple interest calculator using I = P × r × t. Enter principal, annual rate, and time in years or months to see interest earned or owed and the ending balance. It does not compound and does not build an amortization schedule.

The amount interest is calculated on. Interest is not added back into principal.

5 means 5% a year, used as 0.05. This rate does not compound.

Whole or partial years. Leave blank if you only enter months.

Added as months ÷ 12. 6 months is 0.5 years. 18 months is allowed.

Simple interest

Interest
$100.00
Ending balance
$1,100.00
Principal
$1,000.00
Annual rate
5% (0.05)
Time
2 years
Formula
I = P × r × t
  1. Annual rate 5% is 0.05.
  2. Time is 2 years + 0 months = 2 years.
  3. I = $1,000.00 × 0.05 × 2 = $100.00.
  4. Ending balance = $1,000.00 + $100.00 = $1,100.00.

Simple interest uses I = P × r × t only. It does not compound and it does not build an amortization schedule. The math stays in your browser.

How to use

  1. Enter the principal, the amount the interest is calculated on.
  2. Enter the annual interest rate as a percent. 5 means 5% a year, which the formula uses as 0.05.
  3. Enter time in years, months, or both. Months are divided by 12 and added to the years.
  4. Read the interest and the ending balance. Ending balance is principal plus interest. Nothing compounds.

Formula

I = P × r × t. P is principal, r is the annual rate as a decimal (percent ÷ 100), and t is time in years. Months become years with months ÷ 12, so 6 months is 0.5 years and 1 year plus 6 months is 1.5 years. Ending balance = P + I. Interest is charged or earned only on the original principal.

Example. $1,000 at 5% for 2 years is I = $1,000.00 × 0.05 × 2 = $100.00. The ending balance is $1,000.00 + $100.00 = $1,100.00.

Example. $1,000 at 6% for 6 months uses t = 6 ÷ 12 = 0.5, so I = $1,000.00 × 0.06 × 0.5 = $30.00 and the ending balance is $1,030.00.

Frequently asked questions

What is the simple interest formula?

I = P × r × t. Interest equals principal times annual rate times time in years. $1,000 at 5% for 2 years is $100.

Simple vs compound interest?

Simple interest is charged only on principal. Compound interest earns (or charges) interest on prior interest.

Can I use months instead of years?

Yes. Convert months to years (months / 12) or let the tool do it.

Is this the same as the CD calculator?

No. CDs typically compound. This page is I = Prt only, useful for simple-interest loans or classroom math.

Does this amortize a loan?

No. For a payment schedule use the amortization calculator.

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