Simple Interest Calculator
Interest and total on principal at a flat yearly rate with no compounding.
What it does
Simple interest pays a fixed percent of the original principal for each period, with no interest on past interest. A $5,000 loan at 5% accrues $250 every year, whether it runs one year or ten, which keeps the growth line perfectly straight.
The tool above takes principal, yearly rate, and years. It returns the interest earned, the yearly slice, and the total of principal plus interest, so short notes and flat fee loans read at once.
This math fits short term notes, some auto and personal loan quotes, and classroom problems. Savings accounts, credit cards, and mortgages compound instead: each period charges or pays on a balance that already holds past interest, which bends the curve upward over time.
Investor education pages from the SEC and plain language guides from the CFPB both define the two types the same way: simple on principal alone, compound on principal plus accumulated interest. Knowing which one a quote uses matters more than the rate number itself.
How to use
- Enter the starting principal, or zero to test interest per dollar.
- Enter the yearly rate as a percent, for example 5 for 5%.
- Enter the whole number of years, using fractions in the principal if needed.
- Read interest earned first, then yearly interest and the total.
- Run the compound interest page with the same inputs to see the gap compounding creates.
Formula
- principal
- Starting amount the rate applies to.
- rate
- Yearly percent, used as a decimal in the math.
- years
- Full years the flat rate accrues.
Time must match the rate period: yearly rates pair with years, monthly rates with months. A zero rate returns the principal with zero interest, and a zero principal returns zeros throughout.
Worked example
Example: $5,000 at 5% for 3 years
Principal 5000, rate 5%, span 3 years.
- Yearly interest: 5000 x 0.05 = $250.00.
- Three year interest: 250 x 3 = $750.00.
- Total: 5000 + 750 = $5,750.00.
- Tool shows total $5,750.00, interest $750.00, yearly $250.00 /yr.
- Compound check: 5000 x 1.05^3 = $5,788.13, so compounding adds $38.13 here.
Reference table
| Rate | Interest | Total |
|---|---|---|
| 3% | $450.00 | $5,450.00 |
| 5% | $750.00 | $5,750.00 |
| 8% | $1,200.00 | $6,200.00 |
Limits
- Flat rate only. Compounding accounts need the compound page.
- Ignores fees, taxes, and payment timing within the year.
- Early payoffs and late fees change real loan totals.
- Inflation still erodes the result even when the math is exact.
Common mistakes
- Entering 5 as 0.05 and also dividing by 100, which shrinks interest a hundredfold.
- Mixing monthly rates with yearly spans without converting.
- Assuming bank savings use this formula. They compound.
- Forgetting that each year pays the same under simple rules.
- Comparing a simple quote against a compound quote by rate alone.
Try it yourself
Quick checks. Answers sit tucked under each question.
$2,000 at 4% for 2 years?
Interest 2000 x 0.04 x 2 = $160.00, total $2,160.00.
$10,000 at 6% for 5 years?
Interest 10,000 x 0.06 x 5 = $3,000.00, total $13,000.00.
Yearly interest on $800 at 7%?
800 x 0.07 = $56.00 per year.
Questions
What is the simple interest formula?
Principal times rate times time, with the rate as a decimal. Total adds the principal back.
Simple vs compound interest?
Simple pays on the original principal only. Compound pays on principal plus past interest, so it grows faster.
When is simple interest used?
Short notes, some personal loans, and school problems. Most deposits and credit cards compound.
How do monthly rates fit?
Convert first: divide a yearly rate by 12 for months, or multiply months into years, then apply the same formula.
Can the rate be zero?
Yes. Zero rate means zero interest and the total equals the principal.
Why compare with compounding?
The gap grows with time and rate. At 5% for 3 years on $5,000 it is only $38, but over decades it dominates.