Simple interest,
calculated instantly.

Field guide

How simple interest works.

Simple interest is the most straightforward method of calculating interest on a loan or investment: you earn or owe — a fixed percentage of the original principal for every year the money is outstanding. Because the base never changes, the interest amount accrued each year is identical, producing a perfectly straight line of growth rather than the exponential curve of compound interest.

The formula

The complete picture requires two equations. First, the total interest earned:

• I: total interest earned (or owed)
• P: principal (the starting amount)
• r: annual interest rate as a decimal (e.g. 5% → 0.05)
• t: time in years

Add the interest back to the original principal to find the total amount repaid or received:

That is the entire math. No exponents, no iteration — just multiplication. It is why simple interest calculations can be verified by hand in seconds.

A worked example

You invest $10,000 at 5% per year for 3 years:

Each of the three years contributes exactly $500 of interest — no more, no less. Compare this to the same inputs under monthly compound interest, which produces a total closer to $11,616, or an extra $116 from the compounding effect. Over short terms and at typical rates, the gap is modest. Over decades it widens dramatically.

When simple interest applies

Simple interest governs a surprising share of everyday finance:

• Auto loans: most US car loans calculate interest on the outstanding principal using the simple-interest (actuarial) method. Making extra or early payments reduces the principal immediately, which directly lowers daily interest accrual.
• Personal loans: many fixed-term personal loans use simple interest so that the amortization schedule is straightforward to audit.
• US Treasury bills: short-term T-bills with maturities under one year price their discount on a simple-interest basis relative to par value.
• Short-term commercial lending: bridge loans, invoice financing, and similar instruments often accrue simple interest over their brief life cycle.
• Student loans: federally guaranteed student loans accrue simple interest during in-school deferment periods, meaning unpaid interest is kept separate from the principal until capitalization events.

Simple vs compound interest

The core difference is what earns interest. Under simple interest, only the original principal earns interest — the same dollar amount every period, forever. Under compound interest, previously earned interest is added back to the principal so that future interest is calculated on an ever-growing base.

For borrowers, simple interest is almost always preferable: extra payments reduce the balance immediately and cut future interest dollar-for-dollar. For savers and investors, compound interest is more powerful because earnings build on themselves. The two formulas converge when the term is very short or the rate is very low, and diverge sharply over multi-decade horizons.

Solving for rate, time, or principal

The I = Prt formula can be rearranged to solve for any unknown variable:

For example, if you paid $900 in interest on a $10,000 loan over 2 years, the implicit rate was r = 900 ÷ (10,000 × 2) = 4.5% per year. This rearrangement is useful for reverse-engineering the effective rate on any fixed-fee loan.

Daily and monthly interest

Many lenders, particularly auto lenders, calculate simple interest on a daily basis. To convert the annual rate into sub-annual periods:

These are exact proportional shares of the annual interest, not approximations. If your auto loan charges 6% annually on a $20,000 balance, you accrue approximately $3.29 per day. Make a payment and the outstanding balance falls, so does the daily accrual rate. This is why making extra payments early in the loan term saves the most money.

Tips for using this calculator

• Enter the original loan amount as the principal (not the amount you have already paid down) to see total interest over the full original term.
• For a loan already in progress, enter the current outstanding balance as the principal and the remaining term in years to see interest still owed.
• Compare the result here with the Compound Interest Calculator at the same inputs to quantify exactly what compounding adds or costs.
• Use the year-by-year schedule to cross-check your lender's amortization statement. Significant divergence may indicate the loan uses a different day-count convention or has capitalized fees.

Disclaimer

This calculator provides estimates for educational and planning purposes. It assumes a constant annual rate applied over whole years and does not account for fees, taxes, irregular payment timing, or day-count conventions (Actual/365, Actual/360, 30/360). Always consult your loan agreement for the precise interest method used.