Katch-McArdle Calculator

Free Katch-McArdle Calculator - BMR From Lean Body Mass

Most BMR calculators only ask for your weight, height, age, and gender. This Katch-McArdle calculator does something different — it works out your Basal Metabolic Rate from your lean body mass instead, using your body fat percentage to figure out exactly how much of your weight is muscle, bone, and organs versus stored fat. That one change makes a real difference, because two people who weigh exactly the same but carry different amounts of muscle do not burn the same number of calories at rest.

Type in your weight and body fat percentage and this tool instantly gives you your lean mass, your fat mass, and your BMR using the Katch-McArdle formula. It also lines that number up against the Mifflin-St Jeor and Harris-Benedict equations so you can see the gap for yourself, then carries everything forward into your TDEE, a goal-based calorie target, and a full protein, carb, and fat breakdown — all in one place, with no login, no app download, and no spreadsheet required.

What Is the Katch-McArdle Formula?

The Katch-McArdle formula was developed by exercise physiologists Victor Katch and Frank McArdle and published in their well-known sports nutrition textbook. Unlike older BMR formulas that plug in your total body weight, Katch-McArdle strips fat mass out of the equation entirely and calculates your resting calorie burn from lean body mass alone.

The logic behind it is simple: fat tissue is metabolically quiet, meaning it burns very few calories just sitting there, while muscle and organ tissue are active around the clock and require a steady supply of energy even at rest. By focusing only on lean mass, Katch-McArdle avoids the main blind spot of weight-only formulas, which treat a kilogram of muscle and a kilogram of fat as if they cost the body the same number of calories to maintain — they do not.

The Katch-McArdle Formula, Explained Simply

The math behind this calculator happens in two short steps, and you can see both worked out with your own numbers in the step-by-step section above.

• Step 1 — Lean body mass = total weight × (1 − body fat % ÷ 100)
• Step 2 — BMR = 370 + (21.6 × lean body mass in kg)
• Notice there's no separate version for men and women, and no age term at all — lean mass already accounts for most of that variation, which is exactly why the formula can skip those inputs.

Why You Need Your Body Fat Percentage First

Body fat percentage is the one input this formula cannot work without, since lean body mass is calculated directly from it. If you already have a number from a DEXA scan, a skinfold caliper test, a bioelectrical impedance scale, or the U.S. Navy tape-measure method, plug it straight in for the most accurate result.

If you don't know your body fat percentage yet, use our Body Fat Calculator first to get a solid estimate, then come back here. Guessing a number that's too low or too high will throw off your BMR in the same direction, so it's worth spending a minute getting a reasonably accurate reading before relying on this tool for a serious diet or training plan.

Katch-McArdle vs Mifflin-St Jeor vs Harris-Benedict

This calculator shows all three side by side because no single formula is 'correct' for every person — each one was built with a different set of assumptions. Mifflin-St Jeor, published in 1990, is generally considered the most accurate weight-based formula for the average adult and is what most dietitians default to today. Harris-Benedict is the oldest of the three, first published in 1919 and revised in 1984, and tends to run a little higher than modern measured values.

Katch-McArdle takes a different path entirely by using lean mass instead of total weight, which tends to make it the most accurate choice specifically for people who are lean, muscular, or who have a body composition noticeably different from average — think competitive athletes, bodybuilders, or anyone who has done serious strength training for a few years. For someone at an average body fat percentage, all three formulas usually land within a similar range; the gap widens the leaner or more muscular a person gets.

• Use Katch-McArdle if you know your body fat % and are lean, athletic, or muscular.
• Use Mifflin-St Jeor if you don't know your body fat % — it's the best general-purpose default.
• Use Harris-Benedict mainly for comparison or if a textbook or chart specifically references it.

Lean Body Mass and Fat Mass: What the Split Actually Shows

This calculator breaks your total weight into two pieces: lean body mass and fat mass. Lean body mass covers everything in your body that isn't stored fat — muscle, bone, organs, connective tissue, and body water. Fat mass is simply the portion of your weight made up of stored fat.

Seeing these two numbers separately matters because lean mass is the engine behind your BMR. Two people who weigh the same but have very different lean-mass-to-fat-mass ratios will have noticeably different resting calorie burns, even though a bathroom scale would tell you they're identical. This is the exact blind spot that Katch-McArdle was designed to fix.

From BMR to TDEE: Adding Your Activity Level

BMR only tells you what your body burns lying completely still for 24 hours, which isn't a number anyone actually eats around. This calculator automatically multiplies your Katch-McArdle BMR by an activity factor — from sedentary all the way up to extremely active — to give you your Total Daily Energy Expenditure, or TDEE. That's the real maintenance-calorie figure that reflects walking, working, training, and everything else you do in a normal day, and it's the number that should anchor any calorie target you set.

Setting a Goal: Cutting, Bulking, or Maintaining

Once TDEE is worked out, pick a goal and the calculator adjusts your maintenance calories into a practical daily target — a percentage deficit for fat loss, a percentage surplus for muscle gain, or no change at all for maintenance. It also estimates the rough weekly rate of weight change that gap would produce, using the standard rule that roughly 7,700 kcal equals about one kilogram of body fat.

This is only an estimate, not a promise. Real-world weight change is affected by water retention, sleep, stress, hormones, and training volume, none of which show up in a formula. Treat the number as a sensible starting point and adjust it after two or three weeks based on what actually happens on the scale.

Macro Split: Turning Calories Into Protein, Carbs, and Fat

A calorie target alone doesn't tell you what to put on your plate. The Macro Split option breaks your goal calories down into grams of protein, carbohydrate, and fat using one of three common styles: a balanced split, a higher-protein split favored by lifters trying to hold onto muscle during a cut, or a lower-carb split. Protein and carbohydrate are counted at 4 calories per gram and fat at 9 calories per gram — the standard conversion figures used throughout nutrition science.

Since Katch-McArdle is already built around lean body mass, pairing it with a higher-protein macro split is a common combination for people trying to preserve or build muscle while managing body fat — protein needs generally scale with lean mass rather than total weight, which lines up naturally with what this calculator is already measuring.

Who Should Use the Katch-McArdle Formula

This calculator is most useful for people who already have a reasonably accurate body fat percentage and want a BMR number that reflects their actual body composition rather than a generic weight-based estimate. That includes lifters and bodybuilders tracking a bulk or cut, athletes with low body fat, coaches setting nutrition targets for muscular clients, and anyone who has noticed that weight-only calculators seem to overestimate or underestimate their real-world calorie needs.

It's not the right tool for someone who has no idea what their body fat percentage is, since a rough guess can throw the result off more than simply using Mifflin-St Jeor. It's also not a substitute for medical advice — pregnant or breastfeeding individuals, people under 18, and anyone with a diagnosed metabolic or thyroid condition should get individualized numbers from a doctor or registered dietitian instead of relying on any standard adult formula.

Common Mistakes People Make With Katch-McArdle

A handful of mix-ups come up again and again with this formula specifically, on top of the usual BMR mistakes.

• Guessing a body fat percentage instead of measuring or estimating it properly — this formula is only as accurate as that one input.
• Using an outdated body fat % after several months of training or dieting — recheck it periodically, since lean mass and fat mass both shift over time.
• Assuming Katch-McArdle is automatically 'more accurate' for everyone — it mainly pulls ahead of weight-based formulas for lean or muscular individuals, not the general population.
• Eating at BMR-level calories thinking it's a normal daily target — it's the bare minimum with zero activity included, not a diet plan.
• Forgetting that BMR is an estimate, not a lab measurement — even a well-calculated formula can be off by roughly 5-10% compared to a real indirect calorimetry test.

How Accurate Is the Katch-McArdle Calculator?

For lean and muscular individuals with an accurately measured body fat percentage, Katch-McArdle tends to track closer to real, lab-measured BMR than weight-only formulas, because it directly accounts for the tissue that actually drives resting energy expenditure. The catch is that its accuracy depends entirely on the quality of the body fat % you enter — a caliper or scale estimate that's off by five or six percentage points will shift the final BMR by a noticeable amount.

As with any formula-based calculator, this tool cannot fully replace a lab test using indirect calorimetry, which measures oxygen consumption and carbon dioxide output directly. For nearly everyone, though, that level of precision isn't necessary — a good body fat estimate plugged into Katch-McArdle gets you close enough to build a real nutrition plan around, especially once you fine-tune it based on how your weight trends over a few weeks.