Simple Interest Calculator

About the Simple Interest Calculator

Simple interest is usually introduced as the easy thing you learn before compound interest, which rather undersells it. It is not a simplification — it is the actual arrangement in a good number of real contracts.

  I = P × r × t

Interest is charged on the original principal only, never on interest already accrued. That is the whole definition, and everything else follows from it.

The formula has four quantities, so any three determine the fourth. This calculator solves in all four directions, and — because the number on its own does not mean much — it also shows how far the answer sits from what compound interest would have produced.

How to Use the Simple Interest Calculator

Pick what you want to find: interest, principal, rate or time. Enter the other three.

Time can be given in years, months, weeks or days. It is converted to years before the formula runs, because the rate is quoted per year. The day convention here is 365 a year; some contracts use 360, which will give a slightly different answer.

Where Simple Interest Actually Applies

It is worth knowing which is which, because using the wrong model can be expensive.

Bond coupons. A bond paying 5% on its face value pays that every period. The coupon does not compound unless you reinvest it, which is a separate decision.

Late payment and overdue charges. Almost always simple, and often on a daily basis.

Bridging and short-term commercial finance. Frequently quoted as a simple monthly rate.

Precomputed instalment loans. The whole interest charge is calculated at the start and fixed. More on why that matters below.

Not savings accounts. Those compound, and over any meaningful horizon the difference is large.

Not most modern instalment loans. A standard amortising loan charges interest on the declining balance, which is compounding in everything but name.

The Gap Against Compound Interest

This is the output that gives the number its meaning, and it changes character completely with the horizon.

£8,000 at 5.5%:

At 25 years the compounding is worth more than the entire simple interest.

Over three years the gap is £74 on £1,320 — real, but not what decides anything. Over a few months it is negligible. This is why arguing about simple versus compound on a short-term loan is usually a waste of energy, and why it is the single most important distinction in long-term saving.

The 90-Day Row Is Not a Typo

Look again at the first row: over 90 days, compounding produces less than simple interest.

That is not an error, and it catches people out. Under a year, no compounding event has occurred yet. On the interval where t < 1, the curve (1 + r)ᵗ sits below the straight line 1 + rt — they touch at t = 0 and again at t = 1, and between those points the straight line is higher.

So for anything under a year, simple interest is fractionally the better deal for a saver and the worse one for a borrower. The moment the first full compounding period completes, it reverses permanently.

Why "Simple" Does Not Mean "Fairer"

There is a common assumption that a simple interest loan is the more borrower-friendly option, because it sounds less aggressive. On a precomputed loan the opposite is true.

When interest is precomputed, the entire charge is worked out on day one from the full principal and the full term, and added to what you owe. You then repay that fixed total.

The consequence: paying off early saves you far less than you would expect. On an amortising loan, clearing the balance in year two stops all the interest for years three onward. On a precomputed loan, that interest was already added — some jurisdictions require a partial rebate, often calculated by a formula weighted heavily toward the lender, and some do not require one at all.

"Simple" describes the arithmetic. It says nothing about who the arrangement favours.

Step-by-Step Example

£8,000 at 5.5% for 3 years.

  I = P × r × t
    = 8,000 × 0.055 × 3
    = 1,320.00

  Total = 8,000 + 1,320 = 9,320.00

The interest is 16.5% of the principal across the whole period — which is simply 5.5% × 3, because simple interest is linear in time. Double the years and you exactly double the interest. Nothing accelerates.

Working backwards from the same figures:

  P = I ÷ (r × t) = 1,320 ÷ (0.055 × 3) = 8,000.00
  r = I ÷ (P × t) = 1,320 ÷ (8,000 × 3) = 5.5%
  t = I ÷ (P × r) = 1,320 ÷ (8,000 × 0.055) = 3 years

All four agree, as they must.

Understanding Your Result

The result is whichever quantity you asked for.

The total is principal plus interest, which is what you would repay or receive.

The rate in context restates the rate alongside what the interest amounts to as a share of the principal — often a more intuitive figure than the rate itself.

If it compounded is the comparison, and it is the line worth reading.

Worth knowing changes with the horizon, because the advice genuinely does.

When Should You Use This Calculator?

Checking a late-payment or overdue charge. These are almost always simple, often daily, and worth verifying.

Working out a bond coupon. The coupon is simple interest on face value.

Pricing bridging or short-term finance. Usually quoted as a simple rate.

Finding a rate you were not told. If you know the amount, the charge and the period, the rate mode recovers it.

Understanding what you are giving up. Running the same figures as compound interest shows the cost of a non-compounding arrangement over a long period.

Common Mistakes

Using simple interest for savings. Savings compound. Using this formula will understate growth, badly over long periods.

Mismatching the rate and the time. A 5.5% annual rate with a time in months gives an answer six times too large unless the months are converted. This calculator converts for you — but check that the rate you were quoted really is annual.

Assuming 360 and 365 are interchangeable. Some contracts use a 360-day year, which makes the daily rate slightly higher. Over short periods on large sums, the difference is real.

Assuming simple interest favours the borrower. On a precomputed loan it usually does not.

Comparing a simple rate to an APR. They are not the same measure. An APR accounts for compounding and compulsory fees; a simple rate accounts for neither.

Forgetting that early repayment may not help. Check whether interest is precomputed before counting on a saving.

Reading this as advice. Every figure here is an estimate for planning, not financial advice, and the conventions in your contract take precedence over the ones assumed here.