Interest Rate Calculator

Reverse-calculate the annual compound interest rate needed to grow an initial amount to a final amount over a given period. Free, instant, no signup.

years
Formula: r = (Final / Principal)^(1/years) − 1
  • r = annual interest rate
  • years = investment period

How to use the Interest Rate Calculator

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

Why use our Interest Rate Calculator

Instant results. Enter your figures and the interest rate 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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  • Instant results
  • No signup
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About the Interest Rate Calculator

The Interest Rate Calculator works backward from the numbers you already know to reveal the one you usually don't: the actual rate on a fixed-payment loan. You enter the amount borrowed, the loan term in years or months, and the fixed monthly payment, and it returns the annual interest rate baked into that deal. It also shows the total of all payments and the total interest you'll pay over the life of the loan, so a string of monthly figures suddenly becomes a single number you can compare against any other offer.

Use it whenever a loan is quoted as a monthly payment instead of a rate. Car dealers, furniture and electronics financing, buy-now-pay-later plans, and some private lenders love to advertise an attractive payment while staying quiet about the rate behind it. By reverse-engineering the rate, you can spot when a 'low monthly payment' is really an expensive loan stretched over extra months. It's equally useful for double-checking a quote you've already received, comparing two financing offers head to head, or sanity-checking a personal loan from a friend or family member.

Behind the scenes, there is no tidy algebra that isolates the rate, because the standard amortization formula, payment equals principal times i times (1 plus i) to the n, divided by (1 plus i) to the n minus 1, can't be rearranged to solve for the monthly rate i directly. Instead the calculator uses iteration: it guesses a rate, computes the payment that rate would produce, compares it to your real payment, and refines the guess (a Newton-Raphson style search) until the two match to a tiny tolerance. The matched monthly rate is multiplied by twelve to give the annual figure you see.

Every calculation runs entirely in your browser, so the loan amounts and payments you type are never uploaded, stored, or shared. The result is the periodic interest rate implied by your inputs, which equals the APR only when the loan carries no separate fees, origination charges, or points; when those exist, the lender's stated APR will be higher. Treat the output as an accurate read on the rate within the payment itself, and round to the nearest hundredth of a percent the way lenders typically quote.

Frequently asked questions

How does this calculator find the interest rate from a monthly payment?

It can't solve for the rate with a single formula, so it iterates: it tries a rate, calculates the payment that rate would generate for your amount and term, then adjusts up or down until that payment matches the one you entered. The matching rate, multiplied by twelve, is your annual interest rate.

What do I need to enter to get a result?

Three things: the loan amount (principal), the loan term, and the fixed monthly payment. With those, the calculator returns the annual interest rate plus your total payments and total interest over the loan.

Is the result the same as APR?

Only if the loan has no extra fees. This tool returns the rate implied by your payment alone, while APR also folds in origination fees, points, and certain closing costs. When those charges exist, the lender's APR will be higher than the rate shown here.

Why would a dealer or lender hide the interest rate?

Quoting only a monthly payment makes a loan feel affordable while obscuring how much it actually costs, often by extending the term. Reverse-engineering the rate lets you compare offers fairly and catch a cheap-sounding payment that hides an expensive rate.

Does it work for simple-interest or interest-only loans?

This calculator assumes a standard amortizing loan with equal fixed payments, which covers most car, personal, and mortgage loans. It is not built for simple-interest payoff math, interest-only loans, or balloon structures, where the payment doesn't fully amortize the balance.

From our blog

Healthy Weight by Height: How to Read and Use Your Range

By the Super Simple Digital Tools Team · Updated June 2026

Most people picture a single 'goal weight', but health authorities actually define a healthy weight as a span. The reason is simple: a person of a given height can carry slightly more or less weight, depending on frame and muscle, and still be perfectly healthy. The Healthy Weight Calculator captures this by giving you a low and a high figure that bracket the recognised healthy BMI zone of 18.5 to 24.9, turning an abstract index into something you can read off a scale.

The arithmetic behind it is the BMI formula run in reverse. BMI equals weight in kilograms divided by height in metres squared. To find the weights instead of the index, you fix BMI at the two boundary values and rearrange: lower weight equals 18.5 times height squared, upper weight equals 24.9 times height squared. A person 1.60 m tall has a height squared of 2.56, so their healthy band is roughly 47 kg to 64 kg. Add a few centimetres of height and the whole window shifts upward, which is exactly why a borrowed weight target from a taller friend rarely fits.

Once you have the range, the useful question is where you want to sit within it. Landing anywhere inside 18.5 to 24.9 is considered healthy, but many people aim for the middle of their band to leave room for the normal daily swings of one to two kilograms caused by water, food and timing. If your current weight is above the top of the range, the gap to the upper bound is a modest, achievable first goal rather than the daunting jump to the lowest figure. Small targets are easier to keep.

It is worth being clear about what the number cannot tell you. BMI was designed for population screening and treats all weight the same, so it cannot see whether your kilograms are muscle or fat. A rugby player and a sedentary person of identical height and weight get the same band, even though their health picture differs. That is why clinicians pair BMI with waist circumference, blood pressure and other checks. The calculator is a fast first filter, not a diagnosis.

To get the most reliable reading, measure your height accurately without shoes and weigh yourself at a consistent time, ideally in the morning. Re-check your range only if your height changes, which for adults is rare, and track your weight against the band over weeks rather than reacting to a single reading. Used this way, the healthy weight range becomes a steady reference point that supports gradual, sustainable change instead of crash targets.

  • Measure height in bare feet and weigh yourself at the same time of day for a like-for-like comparison against your range.
  • Aim for the middle of your band rather than the bottom edge, so normal daily fluctuations of one to two kilograms do not push you out of range.
  • If you are above the range, set the upper bound as your first milestone instead of the lowest figure, then reassess.
  • Pair the result with a waist measurement, since waist size adds information about fat distribution that BMI alone cannot capture.

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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