Simple Interest Calculator

Calculate simple interest on a principal amount and compare it with compound interest at the same rate.

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Principal Amount

Annual Interest Rate (%)

Time

Time Unit

Simple Interest

Total Amount

Principal + interest

Daily Interest

Per day

Monthly Interest

Per month

Annual Interest

Per year

Simple vs Compound Interest Comparison

Simple Interest

Compound Interest

Simple Total

Compound Total

Period	Simple Balance	Simple Interest	Compound Balance	Compound Interest

Work out simple interest on a principal amount over any length of time, and see side-by-side how much more the same money would earn under compound interest instead.

How It Works

How Simple Interest Calculator Works

Simple interest is calculated with the formula SI = P × r × t, where P is the principal, r is the annual interest rate (as a decimal), and t is the time expressed in years. If you enter time in months or days, the calculator first converts it to years by dividing by 12 or 365 before applying the formula, so a rate entered as an annual percentage always lines up with the time period you chose.

From that single result, the calculator derives a daily figure (principal × rate ÷ 365), a monthly figure (principal × rate ÷ 12), and a full annual figure (principal × rate), so you can see the same interest rate expressed at different paces regardless of how long you're actually holding the money for.

To put the number in context, a second column recalculates the same principal and rate using annual compound interest — balance × (1 + r)^t minus the principal — and lines it up year by year (or month by month for periods under a year) against the simple-interest balance, showing exactly how compounding pulls ahead over time.

Worked Example

See It In Action

A $10,000 principal at 5% annual interest over 3 years earns simple interest of 10,000 × 0.05 × 3 = $1,500, bringing the total to $11,500. That works out to about $1.37/day, $41.67/month, or $500/year. Over the same 3 years, compound interest (compounded annually) on the same $10,000 at 5% grows to 10,000 × 1.05³ = $11,576.25 — $76.25 more than simple interest earns, simply because each year's interest starts earning its own interest.

Real-World Use Cases

Who Uses Simple Interest Calculator and Why

Working out interest owed on a short-term promissory note or personal loan that\'s explicitly written as simple interest rather than compound.
Estimating interest on a short-term certificate or savings promotion quoted as a flat annual rate over a fixed period.
Checking whether a lender\'s quoted "simple interest" auto loan figure matches your own calculation before signing.
Comparing a simple-interest scenario against compound interest to see how much is actually lost by not letting interest compound.

Common Mistakes

Mistakes to Avoid

Assuming a savings account or CD advertised with an annual rate uses simple interest — most deposit accounts actually compound, so plugging their rate into this tool will understate what you\'d really earn; use the CD Calculator or a compound-interest tool for those instead.
Entering time in months or days but forgetting the rate you entered is still read as an annual rate — the calculator converts the time to years internally, but if you mentally expect the raw rate x raw time period, the two won\'t match up.
Confusing the daily/monthly/annual breakdown figures for separate compounding periods — they\'re just the same total simple-interest rate re-expressed at different paces, not a sign that interest is compounding.

Pro Tips

Tips for Best Results

If you\'re not sure whether your loan or account is simple or compound, run the same numbers through both this calculator and a compound-interest one — the gap between the two results is a quick sanity check on which method actually applies.
For a short time frame (a few months or less), the difference between simple and compound interest is usually small in dollar terms; the gap widens mainly with a long holding period or a high rate.

Troubleshooting

Fixing Common Problems

My result doesn\'t match what my lender quoted. — Confirm your lender is actually using simple interest (P × r × t) rather than an amortizing or compound method — many "simple interest" auto loans still compound daily on the remaining balance, which produces a different total than this formula.

Glossary

Terms Explained

Principal: The original amount of money the interest is calculated on — it never changes under simple interest, unlike compound interest where interest is added back into the base.

Compound interest: Interest calculated on both the original principal and any interest already earned, which is why it grows faster than simple interest over time.

FAQ

Frequently Asked Questions

What is the actual difference between simple and compound interest?

Simple interest is always calculated on the original principal only, so it grows by the same dollar amount every period. Compound interest is calculated on the principal plus any interest already earned, so the dollar amount grows a little more each period — the gap widens the longer the money is invested.

Why does the calculator show a compound interest comparison if this is a simple interest tool?

Most real-world savings accounts, CDs, and loans actually use compound interest, so the side-by-side comparison helps you see how much you'd be leaving on the table (or paying extra, for debt) if your rate were compounded instead of simple.

How does entering time in months or days change the calculation?

The interest rate is always annual, so the calculator converts your chosen time period into years first (dividing months by 12 or days by 365) before multiplying by principal and rate — the underlying formula never changes, only the time value plugged into it.

Where is simple interest actually used?

Simple interest shows up in short-term loans, some auto loans, promissory notes, and certain bonds. Most credit cards, mortgages, and savings accounts use compound interest instead, which is why comparing both matters when estimating real returns or borrowing costs.

Does a higher rate always mean more interest paid over time?

Not necessarily on its own — the total interest depends on principal, rate, and time together. A lower rate held for much longer can produce more total interest than a higher rate held briefly, which is why all three inputs matter.