investment calculator

Simple Interest Calculator

The Simple Interest Calculator estimates interest earned or charged when interest is based only on the original principal. Enter the principal, fixed annual rate, and years to see the simple interest amount and the final value.

Result

Interest earned: $1,500.00
Final value: $11,500.00

Scenario nameNotes

Save this result, then use saved scenarios to switch between assumptions.

Before you calculate

Use this calculator for a note, classroom example, deposit, or borrowing-cost scenario where interest does not compound and no payments, withdrawals, or new deposits happen during the term. If the balance compounds, receives regular deposits, or amortizes through payments, use a more specific calculator instead.

Formula summary

Simple interest is Interest = P * (r / 100) * t. P is the Initial principal, r is the Annual rate, and t is Years. Final value equals principal plus interest. The key limit is that each year calculates interest from the same original principal, not from a growing balance.

Worked example

Example: $10,000 at 5 percent for 3 years

With a $10,000 principal, a 5 percent annual rate, and 3 years, simple interest is $10,000 * 0.05 * 3, or $1,500. The final value is $11,500 before any fees, tax, inflation, repayments, or product rules.

The result is useful when interest stays tied to the original principal. If interest is credited back to the balance, or if payments and deposits occur during the term, a compound, savings, loan-payment, or flexible interest calculator is a better match.

Assumptions

The annual rate stays fixed and is applied as simple, non-compounding interest for the entered number of years.
There are no deposits, withdrawals, repayments, fees, taxes, inflation adjustments, day-count conventions, changing rates, or product-specific rules.
Results use the numbers entered on the page, round currency outputs to two decimals, and are educational estimates rather than financial, legal, tax, lending, investment, or regulated advice.

Common mistakes to avoid

Using simple interest for an account, investment, card balance, or loan where interest compounds or the balance changes during the term.
Treating the final value as a lender quote, payoff amount, account statement, or promise instead of a mechanical estimate from the entered assumptions.
Entering a monthly rate or monthly term as if the calculator accepted monthly fields. Convert the rate to an annual rate and use decimal years, such as 0.5 for about six months.

Methodology and sources

This calculator is an original implementation based on documented formulas, app-specific assumptions, deterministic fixtures, edge cases, rounding policy tests, and internal validation. It is not copied from a single source.

Outputs are checked with deterministic fixtures, edge cases, rounding policy tests, and internal independent comparator checks where overlapping outputs are available. The result remains an educational estimate, not a quote, approval, tax answer, or personalised advice.

Formula version 2026.05.20. The version marks the calculation logic and validation fixture set used for this estimate.

This calculator uses generic financial math, so there is no single official source for the formula.

Estimate only

The result is educational and is not financial, tax, legal, lending, investment, or regulated advice. Real provider terms, fees, rates, taxes, and personal circumstances can change the final answer.

Formula and help: read the full simple interest calculator methodology notes.