Simple Interest Calculator

Growth that stays on the principal only

Simple interest is the straight line of money maths: every year, the same slice of the original principal gets added, and previous interest never earns more interest. The formula is three multiplications — principal × annual rate × years — with no exponents and no compounding rounds, which is exactly why it teaches amortisation, credit and bond mechanics with clean numbers. Side-by-side the two models: 1,000 at 5% simple for 10 years earns 500; compounded yearly the same terms earn 628.89. The gap between 1,500 and 1,628.89 is the price of "interest on interest".

Straight-line interest is more common than people expect: some car loans, bonds and short-term notes price it explicitly, and tax officials often use it for late-payment charges. This page treats both sides — interest and total repayment — so you can sanity-check a contract line or compare a flat-fee loan offer at a glance. Enter a partial year as a decimal (0.25 for a quarter) when the term is short.

A worked example, step by step

Problem: 2,500 lent out at 4.5% simple annual interest for 3 years.

1. Annual slice: 2,500 × 4.5% = 112.50 per year.
2. Total interest: 112.50 × 3 = 337.50.
3. Total owed: 2,500 + 337.50 = 2,837.50.
4. Half-way check: at 18 months you have earned 168.75, exactly half of 337.50 — linear, as promised.
5. Contrast: compounded yearly at the same rate, 2,500 would close at 2,850.91 — 13.41 more, all from reinvested interest.

Decimals are supported, so a 90-day case enters as 8,000 × 3.25% × 0.25 = 65.00 of interest — maths that still works by hand if the tool is not in front of you. That mental portability is one of simple interest's genuine selling points.

Interest under the hood, at 4.5% on 2,500

In year 1 the two columns agree — simple and compound are the same until reinvested interest starts working — and after that the compound column drifts upward faster every year.

From the number to the decision

The linearity is your audit too: 2,500 at 4.5% must earn identical annual slices (112.50 each year), so a schedule that quotes a wobbling interest line while claiming simple interest deserves questions rather than your signature. Half-period checks work the same way: at 18 months the running total is half of 337.50 (168.75), which makes spreadsheet audit trails almost mechanical.

When you meet a quoted rate with mystery around it, stress it two ways before comparing: run this page once at the quoted figures and once at the alternative — say, a flat front-loaded fee expressed as its simple-interest equivalent — and keep the tool's plain-number output next to the counterparty's paperwork. Duration deserves the same explicitness, because both the interest and the total scale exactly with years: halving the term halves the interest, so the annualised cost is the honest point of comparison between short and long offers. A lender quoting "5% over six months" prices on this page as 10% a year flat — annualise before assuming the deal is gentle, and bring the Loan Payment Calculator into the conversation whenever the product turns out to amortise month by month instead.

Mistakes that misprice the money

• Entering a monthly rate as annual. "1% per month" is 12% a year simple; the tool has no separate unit, so label your rate first.
• Reasoning months as years. 18 months is 1.5 years; typing 18 multiplies the interest twelvefold.
• Assuming every loan is simple. Mortgages and credit cards capitalise monthly; the Loan Payment Calculator prices that schedule and no other.
• Comparing simple vs compound totals at different term lengths. Differences compound themselves; hold rate and term fixed, then compare once.
• Forgetting the fee stack. Origination or service charges change the true cost independent of interest method; add them separately.

Keep exploring

• Compound Interest Calculator — the compounding version, with frequency and years.
• Loan Payment Calculator — the amortised monthly payment model most lenders use.
• Savings Goal Calculator — how much to set aside monthly to hit a target.
• Guide: Understanding Compound Interest.
• Guide: How Loan Payments Work.