Simple Interest Calculator

Calculate simple interest and total amount using principal, annual rate, and time period. Free, instant, no signup.

%
years
Formula: I = P × r × t
  • I = interest
  • P = principal
  • r = annual rate (as decimal)
  • t = time in years

How to use the Simple Interest Calculator

  1. Enter your values. Fill in the fields with your numbers.
  2. Calculate. Press Calculate to run the simple interest calculator.
  3. Use the result. Copy the result or try a related tool next.

Why use our Simple Interest Calculator

Instant results. Enter your figures and the simple interest calculator returns an answer in seconds.
Free & private. Runs in your browser — no signup, and nothing is sent to a server.
Accurate. Uses standard formulas so you can rely on the numbers.

Free to use — premium coming soon

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About the Simple Interest Calculator

The Simple Interest Calculator works out the interest charged or earned on a fixed principal that never compounds. You enter three numbers: the principal (the starting amount), the annual interest rate, and the time period, and it returns the interest and the total amount you will repay or receive. Because the interest is always applied to the original principal and never to interest already accrued, the result grows in a perfectly straight line. That predictability is exactly why simple interest is used for products like short-term personal loans, many car loans, gold loans, and promissory notes where the lender wants a transparent, easy-to-verify figure.

Reach for this tool whenever you need a clean, no-frills answer rather than the snowballing growth of compounding. Typical moments: estimating the flat interest on a car or personal loan before you sign, checking the payout on a short fixed deposit or Treasury bill, working out what a friendly loan between people should cost, or doing homework where the textbook formula is SI = P x R x T. It is also handy as a benchmark: by comparing the simple-interest figure against a compound figure for the same principal and rate, you can see precisely how much extra compounding adds over the term.

Mechanically the calculator applies the formula SI = (P x R x T) / 100 when the rate is entered as a percentage, then adds the principal back to give the accrued total A = P(1 + rt). Time can be expressed in years, months, or days; months are divided by 12 and days by the year basis you choose. Day-based interest depends on convention: most consumer products use Actual/365, while commercial lenders often use a 360-day banker's year, which produces a slightly higher daily charge because the same rate is spread over fewer days. Picking the basis your lender actually uses is what makes the estimate match the real contract.

Every calculation runs entirely in your browser, so the principal, rate, and term you type are never uploaded, stored, or shared, you can use it for sensitive loan numbers with no privacy worry. The math itself is exact for genuine simple-interest products, but treat any figure as an estimate for amortized loans: most real car and personal loans charge simple interest on a declining balance with monthly payments, so the flat total here will run higher than an amortized schedule. Always confirm the rate type, the day-count basis, and any fees against your loan agreement before relying on a number.

Frequently asked questions

What is the formula this calculator uses?

It uses SI = (P x R x T) / 100, where P is the principal, R is the annual rate as a percentage, and T is the time in years. To get the total you owe or earn, it adds the principal back: A = P(1 + rt).

How do I enter a term that is in months or days?

Convert it to years for the formula: divide months by 12 and days by your chosen year basis (365 or 360). The calculator handles this for you when you select months or days as the time unit.

What is the difference between simple and compound interest?

Simple interest is always charged on the original principal only, so it grows in a straight line. Compound interest adds earned interest back to the balance and charges interest on that too, so it grows faster over time. For the same rate and term, the compound total is always equal to or larger than the simple total.

Do car loans really use simple interest?

Most auto loans use simple interest, but on a declining balance with monthly payments, so interest is recalculated each period on what you still owe. This calculator gives the flat simple-interest total on a fixed principal, which is a useful estimate but will be higher than an amortized schedule where the principal shrinks each month.

Why should I pick a 360-day or 365-day basis?

The basis decides how a daily rate is worked out. Actual/365 is standard for most consumer savings and loans, while a 360-day banker's year is common in commercial lending and charges slightly more interest per year. Match the basis to whatever your lender states in the contract for an accurate figure.

From our blog

How to Find Your Body Shape Accurately (and Why the Ratios Matter)

By the Super Simple Digital Tools Team · Updated June 2026

Most people guess their body shape from the mirror, but the mirror is a poor judge of proportion. A body type calculator removes the guesswork by turning three measurements into a clear ratio-based answer. The catch is simple: garbage in, garbage out. If your measurements are sloppy, the shape you get back will be wrong, sometimes by a whole category. Getting the inputs right is the single biggest factor in a useful result, so it is worth slowing down before you type anything in.

Start with the right tool, a flexible cloth or fibreglass tape rather than a stiff carpenter's tape. Stand relaxed in front of a mirror with your weight even on both feet. For the bust, wrap the tape around the fullest part while wearing a normal, well-fitted bra. For the waist, find the natural crease where your torso bends, usually just above the belly button, and avoid sucking in. For the hips, measure the widest point around the buttocks. Keep the tape level all the way around for each measurement.

Once you enter the three numbers, the calculator does straightforward arithmetic. It subtracts waist from bust, waist from hips, and bust from hips, then checks those gaps against fixed cut-offs. An hourglass shows a large waist gap with balanced bust and hips. A pear shows hips clearly winning. An apple, or inverted triangle, shows the bust winning. A rectangle shows all three close together. Because the system uses hard thresholds, a measurement that lands near a boundary can tip you from one shape into the next.

That sensitivity is exactly why you should measure twice and trust the average. If your first and second readings disagree by more than half an inch, take a third. Many people sit right between two shapes, often called a combination figure, and that is completely normal. If the calculator flips between, say, hourglass and pear on repeat tries, you simply carry traits of both, and you can borrow styling ideas from each category rather than forcing yourself into one.

Finally, treat the result as a map, not a verdict. Knowing your shape helps you predict how a cut will hang and gives you a vocabulary when talking to a tailor or filtering an online store. But the goal is clothes that make you feel good, not obedience to a chart. Use the proportions to balance a silhouette when you want to, and to deliberately break the rules when you would rather. The calculator gives you information; what you do with it is entirely your call.

  • Measure over thin clothing or bare skin, never over a bulky sweater, or your bust and hip numbers will read too large.
  • Take each measurement twice and average them, since a half-inch error near a threshold can change your shape category.
  • Don't hold your breath or pull the tape tight at the waist, measure relaxed for the number you actually live in.
  • If two shapes keep coming up, you have a combination figure, so pull styling ideas from both rather than picking one.

Read the full guide →

Tool by the Super Simple Digital Tools Team. Reviewed by our editorial team. Free to use, no signup required.

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