Mifflin-St Jeor vs Katch-McArdle - the BMR difference
Your BMR is the calories your body burns at rest. Mifflin-St Jeor works it out from your weight, height, age and sex. Katch-McArdle works it out from your muscle mass. For most people they nearly agree; for lean or muscular people, Katch is more accurate. Here is how each is calculated.
BMR means basal metabolic rate - the calories your body burns just to stay alive, even if you lay still all day. It is the biggest part of your daily burn, usually more than half. Two formulas are used most: Mifflin-St Jeor and Katch-McArdle. They give almost the same answer for most people, but they ask for different numbers and differ a lot for very lean or very muscular bodies. Here is what each one uses and how to pick.
Same person (80 kg, 175 cm, 30 yr) - what each formula gives
One BMR formula is always more accurate.
it dependsIt depends. Mifflin-St Jeor is the safe default - it is accurate for most people and needs no body-fat number. Katch-McArdle is more accurate if you are lean or muscular (men under ~20% body fat, women under ~30%), because it uses your actual muscle mass. But Katch needs your body-fat %, and home scales are rough at measuring that. So: no body-fat data? Use Mifflin. Lean and know your body fat? Use Katch.
A higher BMR formula means I can eat more and still lose weight.
falseFalse, and it is a trap. A higher BMR estimate is not permission to eat more - it is often just a guess that is too generous. If the formula overestimates your real burn, you eat above your true maintenance and gain instead of losing. The safer move is to start with a modest BMR, set a real deficit, and let the scale (averaged over weeks) tell you the truth. Burnie adjusts your target from your real weight trend, not from a formula's promise.
Pick your BMR source in Burnie - then trust the scale
If you do not know your body fat, pick Mifflin-St Jeor - it is the safe, accurate default for most people. If you are lean or athletic and you have a real body-fat number (from a DEXA or skinfold, not a guess), pick Katch-McArdle. Burnie lets you choose your BMR source so the number reflects you, not a fixed guess. Then set a small deficit (about 300-500 kcal/day) and watch your weekly average weight. If it is not dropping after three weeks, your real burn is lower than the formula said - trim the target, not your patience.
Assumptions: For the example 80 kg, 175 cm, 30-yr male, Mifflin gives BMR ~1749 kcal. Total daily burn is BMR x an activity factor (about 1.2 sedentary to 1.55 active). The point is to pick the source that fits your body, then let the weight trend confirm. Delta 0 vs the Mifflin baseline - same person, different formula choice.
Use your BMR number the right way
- BMR is only rest. Multiply it by an activity factor - about 1.2 for desk days, 1.55 for active days - to get your real daily burn.
- Do not pick the formula that gives the highest number. Pick the one that fits your body - a generous guess makes you gain, not lose.
- If you do not know your body fat, use Mifflin. It needs only weight, height, age and sex, and it is accurate for most people.
- Check the trend, not the formula. Three weeks of no weight loss means the burn estimate is too high - trim the target by ~200 kcal.
- Use the same formula every week. Switching formulas mid-diet makes your numbers jump and hides the real trend.
The bottom line
BMR is the calories your body burns at rest, and two formulas rule. Mifflin-St Jeor uses weight, height, age and sex - the safe default for most people. Katch-McArdle uses your muscle mass - better if you are lean or athletic, but it needs your body-fat %. For most bodies they nearly agree; for very muscular or very lean people, Katch is higher and more accurate. Pick the one that fits your body in Burnie, set a small deficit, and let your weekly weight trend - not the formula's number - judge whether it is right. Start logging today.