RMR Calculator

Your resting metabolic rate (RMR) is the number of calories your body burns at complete rest in a day — the energy needed just to keep you alive while you do nothing at all. Breathing, circulating blood, keeping your body warm and repairing cells all run on this quiet background burn, and it accounts for most of what most people burn in a day.

That is why RMR is the base of every calorie target. Before you can know how much to eat to lose, gain or hold your weight, you need to know what your body spends before you even get out of bed. This free rmr calculator gives you that number in seconds using the widely used Mifflin-St Jeor formula. There is nothing to sign up for and no name or email needed — enter your numbers, press Calculate, and your result appears right away.

Use the RMR Calculator

Choose your sex, age, weight and height, then press Calculate. You can enter weight in kilograms or pounds, and height in centimeters or in feet and inches — the calculator converts everything for you.

The Mifflin-St Jeor Formula

The calculator above uses the Mifflin-St Jeor equation, the formula most dietitians and researchers reach for today because it has held up well in studies across ages and body sizes. It needs only four inputs — sex, age, weight and height — and it looks like this:

Men: RMR = 10 × weight in kg + 6.25 × height in cm − 5 × age in years + 5

Women: RMR = 10 × weight in kg + 6.25 × height in cm − 5 × age in years − 161

Here is the worked example, step by step, for a 30-year-old woman who weighs 65 kg and is 165 cm tall: 10 × 65 = 650; 6.25 × 165 = 1,031.25; 5 × 30 = 150; then 650 + 1,031.25 − 150 − 161 = 1,370.25, which rounds to 1,370 kcal per day. Try those same numbers in the calculator above and you will get the same answer. The result is an estimate, not a lab measurement — good enough to plan around, and far better than guessing.

RMR vs BMR: What Is the Difference?

RMR and BMR (basal metabolic rate) are close cousins, and most people use them interchangeably — but they are not quite the same measurement. BMR is measured under strict laboratory conditions: after a full night of sleep, after fasting, lying still in a dark, quiet, temperature-controlled room. RMR is measured under looser, more realistic conditions, so it usually comes out slightly higher — typically by about 0 to 10 percent.

For everyday planning, the difference barely matters. Whichever number you start from, you still multiply it by an activity factor to get your daily target, and that step carries far more uncertainty than the RMR-versus-BMR gap. If you want the full daily number worked out for you, our Maintenance Calories Calculator takes your resting rate and your activity level and turns them into one daily target.

What Affects Your RMR?

Several things move your resting burn up or down, and knowing them explains why two people of the same weight can have different numbers. Muscle mass is the biggest lever you can control: muscle burns more at rest than fat does, so two people at the same weight will differ if one carries more muscle. Age pulls the other way — RMR drifts down as we get older, partly because muscle mass tends to fall. Sex matters because men, on average, carry more muscle at the same weight, which is why the formula gives men a higher constant. Body size matters too: a larger body simply has more tissue to maintain, so it burns more at rest.

Two less obvious factors deserve a mention. An underactive thyroid can lower RMR noticeably, which is worth checking with a doctor if your weight is climbing despite a careful diet. And crash dieting can lower it as well: very low intakes over weeks signal the body to conserve energy, so the same person burns less at rest than before. That is one more reason to favor the steady 300-to-500-calorie deficit over an extreme cut.

How to Use Your RMR Number

Your RMR alone is not a diet plan — it is the starting point for one. The calculator result already shows your estimated maintenance at four activity levels, and the same multipliers work by hand:

To hold your weight, eat around your maintenance figure. To lose weight, subtract 300 to 500 calories from it: our moderately active example at 2,124 would aim for roughly 1,624 to 1,824 a day, a pace of about half a pound to a pound a week. Our guide How Many Calories Should I Eat to Lose Weight? walks through that math with a full target table, and the Maintenance Calories Calculator will pin down your own maintenance number directly. As you lose weight, recalculate every 10 pounds or so — a lighter body burns less, so your targets need to come down with you.

One last honest note: no formula knows your life. This calculator is a strong, evidence-based estimate, and most people land within 10 percent of it. Track your weight for two or three weeks at your chosen intake; if it is not moving the way you expect, adjust by 100 to 200 calories and watch again. Your trend is the final judge.