Decision tools built for clarity
Simple Interest Calculator
Calculate non-compounding simple interest for years, months, or an explicit day-count basis.
Enter the required values, select the applicable options, and calculate.
This educational estimate excludes fees, compounding, variable rates, and lender-specific conventions.
Methodology
Formula or transformation
Applies interest equals principal times annual rate times time without compounding.
Variables
- Starting principal
- Annual rate
- Time expressed in years
- Simple interest
Units
- Currency
- Percent
- Year
- Day
Rounding policy
Canonical arithmetic retains declared precision before currency display rounding.
Interest equals principal multiplied by the annual rate and time in years; day durations require a visible 365- or 360-day basis.
Calculate non-compounding simple interest for years, months, or an explicit day-count basis.
Reviewed sources
- CFPB — Calculating loan payments worksheet Reviewed on 2026-09-07
Formula review history
-
Initial formula, day-basis, and source review.
Reviewed 2026-09-07 for formula version simple-interest-prt-v1
Assumptions
- Inputs are interpreted using the displayed units and options.
- The calculation follows the methodology and factors listed in the sources.
Limitations
- Display rounding can hide additional decimal places.
- The calculation cannot add accuracy that is absent from the submitted inputs.
- This educational estimate excludes fees, compounding, variable rates, and lender-specific conventions.
Worked example
A principal of 1,000 at 5% for two years produces interest of 100 and a total of 1,100.
Frequently asked questions
How is a duration in days converted?
The days are divided by the basis you choose: 365 for the actual-year basis or 360 for the commercial basis. That is why 365 days at 5% on 10,000 gives 500.00 on one basis and 506.94 on the other.
Does the interest compound?
No. Simple interest is charged on the principal only, so doubling the time doubles the interest; use the compound interest calculator when interest is reinvested.