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Simple Interest Calculator

Calculate simple interest from principal, annual rate and an explicit days, months or years duration with a selected day-count basis.

Use this result well

Inputs that matter
Principal, annual simple rate, positive duration, explicit days/months/years unit and 360- or 365-day basis
Output to expect
Converted time in years, simple interest and principal-plus-interest arithmetic
How it works
Converts days by the selected basis, months by 12 or uses entered years, then applies I = P × r × t exactly once with no compounding
  • Verify the agreement actually uses simple interest and the selected day-count/accrual convention.
  • Payments, APR, fees, compounding, changing rates, taxes and payoff rules are outside this estimate.

Choose your path

Built around the job you need to finish

Calculate principal-times-annual-rate-times-explicit-time simple interest with a stated duration unit and day-count basis, without implying payments, compounding or product costs.

Borrower checking a short-term example

Convert exact days to a documented year fraction.

Enter principal, annual simple rate, days and the agreement day basis.

Can reproduce interest and principal-plus-interest without mistaking it for a payment schedule.

Student learning the formula

See how months or years become t.

Choose the duration unit and read each formula step.

Distinguishes annual rate, time in years, interest and ending balance.

Reviewer comparing an actual financial product

Avoid substituting a simplified worksheet for disclosures.

Use the arithmetic only if the agreement uses simple interest, then verify fees, APR, compounding and accrual rules externally.

Does not use the estimate as a quote or payoff statement.

Authoritative checks for this tool

Outputs and checklists are planning aids. Review the linked current authorities and the records, terms, instructions, and requirements that apply to your exact situation before a consequential decision.

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Reference & details

How it works

Explicit time conversion

Days use the selected day-count denominator, whole months use 12 and years use the entered value directly.

t = days ÷ basis, months ÷ 12, or entered years

Simple interest only

The annual rate is divided by 100 and multiplied by principal and time exactly once. Interest is not added back into a compounding balance.

I = P × (annual rate ÷ 100) × t

Scope of the ending balance

Principal plus calculated interest is displayed as arithmetic only. Payments, fees, APR, changing rates, taxes, compounding and lender-specific accrual or payoff rules are excluded.

ending balance = P + I

Updated: August 2026

Example Scenarios

$10,000 at a 5% annual simple rate for 90 days converts to 90/365 years before applying I = P × r × t.

Principal: $10,000Rate: 5%Duration: 90 daysBasis: 365 days

→ Simple interest: $123.29

A reviewer selects 360 only because the agreement specifies that convention and records the visible difference from a 365-day calculation.

A borrower uses the result as a formula check, then reads the actual APR, fees, payment schedule and payoff rules rather than treating the worksheet as a quote.

Common Mistakes to Avoid

Entering 90 days as 90 years or 90 months

Choose the duration unit explicitly and inspect the converted time-in-years step.

Treating principal plus interest as a monthly payment or compound projection

Use a payment/amortization or compound-interest tool only when those product mechanics actually apply.

FAQ

It uses I = P × r × t once, where r is the annual rate as a decimal and t is time in years. It does not compound.

Days are divided by the selected 365- or 360-day basis. CFPB’s cited educational example uses 365 days for periods under one year, but an actual agreement may specify another convention.

Whole months are divided by 12. The day-count selection is ignored when months or years are selected.

No. Principal plus interest is not a payment schedule. Payment timing, amortization, fees and other product terms are outside this simple calculation.

Not necessarily. APR can reflect additional costs and disclosure rules. Enter only the simple annual rate that belongs in the formula and verify the product documents.

About Simple Interest Calculator

Enter principal and an annual simple rate, then state the exact duration unit. For days, choose the day-count basis used by the agreement. The result is arithmetic, not a quote, payment schedule or payoff statement.