Finance · Simple Interest · Free

Simple Interest Calculator

Work out flat interest with no compounding. Enter a principal amount and an annual interest rate, pick a period in Days, Weeks, Months, Quarters, or Years, choose from 13 currency options, and get the interest amount and total amount — computed right in your browser.

STEP 1

Choose a currency and enter the principal

Pick a currency from the list — Dollar, Euro, Pound, Rupee, and more (or "- other -") — then type the principal amount, the sum the interest is charged on.

STEP 2

Enter the annual rate and period

Type the annual interest rate in percent — 5 means 5% per year — then a period count and its unit: Days, Weeks, Months, Quarters, or Years.

STEP 3

Calculate, copy, or reset

Press Calculate to read the interest amount and the total amount (principal plus interest). Copy the results to your clipboard or Reset to start over.

How the math works

What formula does this calculator use?: The standard simple interest formula: the interest amount is I = P · r · t and the total amount is A = P + I = P · (1 + r · t), where P is the principal, r the annual rate as a decimal (5% = 0.05), and t the period expressed in years.
How is my period converted to years?: This calculator converts the period to years with the conventional factors: 1 year = 365 days, 52 weeks, 12 months, 4 quarters. So 18 Months at 3% per year is the same as 1.5 years — 4.5% of the principal.
How is simple interest different from compound interest?: Simple interest is charged only on the principal, never on previously earned interest, so it grows in a straight line. For the same rate and term, compound interest always ends at or above it — use a compound interest calculator when interest is reinvested.
Does the currency choice change the result?: No. The currency only labels the amounts shown; the numbers themselves are identical in any currency. "- other -" simply hides the symbol.
Worked example: $5,000 at 3% per year for 18 months: t = 18 ÷ 12 = 1.5 years, so interest = 5,000 × 0.03 × 1.5 = $225.00 and the total amount is $5,225.00.

Privacy: every calculation runs in your browser tab — no value you type is sent anywhere. Copy results uses your clipboard only. This tool is for estimation, not financial advice.