Mifflin-St Jeor or Harris-Benedict — which should I trust?

Mifflin-St Jeor (1990). The Frankenfield 2005 meta-analysis (J Am Diet Assoc) found Mifflin-St Jeor predicted measured resting metabolic rate within ±10% in 82% of non-obese adults and 70% of obese adults — beating Harris-Benedict in every subgroup. Harris-Benedict (1919, revised by Roza-Shizgal 1984) typically overestimates BMR by 5–10%, especially in overweight people. We default to Mifflin unless you give us body-fat %.

Why is Katch-McArdle more accurate if I know my body fat?

Mifflin and Harris use total body weight as a proxy for lean mass — and that proxy is wrong for very lean, very muscular, or very fat bodies. Katch-McArdle (BMR = 370 + 21.6 × lean body mass in kg) skips the proxy and uses lean body mass directly. For someone at 12% body fat, Katch typically reads 100–300 kcal higher than Mifflin and closer to indirect calorimetry. The catch is you need an honest body-fat number (DEXA, BodPod, or a 7-site caliper) — guessing makes it worse.

How accurate is BMR for athletes or very muscular people?

Without body-fat data, all general formulas underestimate athletes by 100–400 kcal because muscle burns ~13 kcal/kg/day at rest vs ~4 kcal/kg/day for fat — Mifflin doesn't see the difference. Always use Katch-McArdle if you have a body-fat reading. Without one, take Mifflin and add 5–10% as a rough adjustment.

Why does BMR drop as you get older?

Roughly 1–2% per decade after age 20. The biggest driver is sarcopenia — gradual muscle loss — which the formulas capture through the −5 × age (Mifflin) or −5.677 × age (Harris) term. Resistance training slows this dramatically; the formulas can't see your training so the prediction is conservative for lifters.

What's the difference between BMR and TDEE?

BMR is the calories you'd burn lying still in bed for 24 hours — life-support energy only. TDEE (total daily energy expenditure) is BMR multiplied by an activity factor (1.2 sedentary → 1.9 extra active) plus the thermic effect of food. BMR is the floor; TDEE is what you actually burn. This tool computes BMR only — for TDEE and macro targets use our calorie calculator.

Are these formulas valid for children or pregnant women?

No. Mifflin-St Jeor and Harris-Benedict were derived on non-pregnant adults aged 19–78. Children need age-specific equations (Schofield or WHO/FAO/UNU). Pregnancy and lactation add 300–500 kcal/day above maintenance and are outside any general BMR formula.

Setting a cutting deficit without nuking your metabolism

You weigh 82 kg, 178 cm, male, 34. Mifflin reads about 1790 kcal BMR; your sedentary-office TDEE lands near 2150. A 20% cut means eating roughly 1720 kcal, not the 1200 a generic app suggested. Anchoring the deficit to BMR keeps you above the floor so you lose fat without the crash, hair shedding, and stalled lifts that come from undereating.

A 12% body-fat lifter who keeps getting under-fed by apps

At 75 kg and 12% body fat, your lean mass is 66 kg, so Katch-McArdle reads about 1795 kcal while Mifflin says 1690. That 100 kcal gap is real muscle metabolism the weight-only formulas miss. Feed Katch your DEXA number and the calculator finally stops treating your quads like fat, so your maintenance estimate matches what the scale actually does.

Explaining the post-40 slowdown to a coaching client

A 48-year-old client insists her metabolism "broke." Run her at 45 then at 25 with the same weight: BMR drops roughly 100 kcal, about 1–2% per decade from muscle loss. Showing the age term side by side reframes it from a defect into normal sarcopenia she can fight with lifting, which lands better than telling her to just eat less.

Sanity-checking a wildly different number from another site

One site gave your friend 1450 kcal, another 1700. Enter the same stats here and you see both formulas at once: Harris-Benedict runs 5–10% higher than Mifflin, which explains the spread. Now they know the 1700 was Harris overestimating and the 1450 Mifflin, so they can pick the realistic floor instead of trusting one black-box result.

BMR Calculator — Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle

BMR calculator — basal metabolic rate by Mifflin-St Jeor and Harris-Benedict (compared side-by-side).

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Sex

Age

Height

Weight

Body fat % (optional)

Unlocks Katch-McArdle — the most accurate formula when you know your %.

Formulas — side by side

Mifflin-St Jeor (1990)

1699

kcal / day

Modern default — most accurate for the general adult population.

Harris-Benedict (1984 revision)

1763

kcal / day

Older — tends to overestimate BMR by 5–10%.

Katch-McArdle

—

kcal / day

Uses lean body mass — most accurate when you know body fat %.

Difference

Harris − Mifflin = 64 kcal / day

Harris-Benedict gives a higher number than Mifflin in almost every adult — that's the documented 5–10% overestimate. Mifflin is closer to measured RMR (indirect calorimetry) in modern validation studies.

Add body fat % above to compute Katch-McArdle. If your body-fat is under ~15% (men) or ~22% (women) it is the most accurate formula available without a metabolic cart.

Recommended for you

1699

kcal / day

Using Mifflin-St Jeor — without body-fat data this is the most accurate formula.

This is BMR only — the calories you'd burn lying still for 24 h. For total daily needs add activity (TDEE).

What this tool does

Free online BMR (basal metabolic rate) calculator. Enter sex, age, height and weight in metric (cm / kg) or imperial (ft+in / lb) and we compute your BMR with three formulas side-by-side: Mifflin-St Jeor (1990 — the modern default), Harris-Benedict (1984 revision — tends to overestimate by 5–10%) and, if you provide body-fat percentage, Katch-McArdle (uses lean body mass, the most accurate formula for lean / muscular bodies). We show the difference between formulas and explain which one applies to you. Unlike a calorie / TDEE calculator, this focuses purely on BMR — the calories your body burns at rest. 100% client-side — your numbers never leave the browser.

Tool details

Input: Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: Shareable URL state; Key settings are encoded in the URL so another person can reopen the same setup.
Performance budget: Initial JS <= 10 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Content Creator; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How BMR Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Setting a cutting deficit without nuking your metabolism
You weigh 82 kg, 178 cm, male, 34. Mifflin reads about 1790 kcal BMR; your sedentary-office TDEE lands near 2150. A 20% cut means eating roughly 1720 kcal, not the 1200 a generic app suggested. Anchoring the deficit to BMR keeps you above the floor so you lose fat without the crash, hair shedding, and stalled lifts that come from undereating.
A 12% body-fat lifter who keeps getting under-fed by apps
At 75 kg and 12% body fat, your lean mass is 66 kg, so Katch-McArdle reads about 1795 kcal while Mifflin says 1690. That 100 kcal gap is real muscle metabolism the weight-only formulas miss. Feed Katch your DEXA number and the calculator finally stops treating your quads like fat, so your maintenance estimate matches what the scale actually does.
Explaining the post-40 slowdown to a coaching client
A 48-year-old client insists her metabolism "broke." Run her at 45 then at 25 with the same weight: BMR drops roughly 100 kcal, about 1–2% per decade from muscle loss. Showing the age term side by side reframes it from a defect into normal sarcopenia she can fight with lifting, which lands better than telling her to just eat less.
Sanity-checking a wildly different number from another site
One site gave your friend 1450 kcal, another 1700. Enter the same stats here and you see both formulas at once: Harris-Benedict runs 5–10% higher than Mifflin, which explains the spread. Now they know the 1700 was Harris overestimating and the 1450 Mifflin, so they can pick the realistic floor instead of trusting one black-box result.

Common pitfalls

Treating BMR as your daily calorie target. BMR is bed-rest energy only; a 1790 kcal BMR means a sedentary TDEE near 2150, so eat against TDEE, not BMR.
Picking Katch-McArdle with a guessed body-fat number. A 5-point error (say 12% vs 17%) shifts BMR by 80–120 kcal, so use Katch only with a DEXA or caliper reading.
Trusting Harris-Benedict because it gives a higher, friendlier number. It overestimates 5–10%, so an "easier" 1700 over Mifflin's 1600 just hides a deficit that never happens.

Privacy

Every formula runs in your browser with JavaScript. Your sex, age, height, weight, and body-fat percentage are never sent to any server, logged, or stored. If you share a result link, only the numbers needed to redraw the calculation travel in the URL; close the tab and nothing remains.

FAQ

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BMR Calculator — Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How BMR Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Setting a cutting deficit without nuking your metabolism

A 12% body-fat lifter who keeps getting under-fed by apps

Explaining the post-40 slowdown to a coaching client

Sanity-checking a wildly different number from another site

Common pitfalls

Privacy

FAQ

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