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Why Your BMR Calculator Results Are All Different (And Which One to Trust)

I wasted three months cutting on the wrong calorie target. Here's what happened.

Last year I started tracking calories seriously. I googled "BMR calculator," typed my stats into the first three results, and got: 1,685, 1,810, and 1,592. That's a 218-calorie spread.

I picked 1,685, subtracted 500, ate 1,185 calories a day. Two weeks later I was exhausted, lifts tanked, binging every fourth day. I was in a 700+ deficit without knowing it.

Three Formulas, Three Answers

Mifflin-St Jeor (1990) — The gold standard. Mine: 1,685.
Harris-Benedict (1919) — Overestimates by 5-15%. Mine: 1,810.
Katch-McArdle — Uses lean body mass. Most accurate if you know BF%. Mine: 1,592.

How to Pick

  • Average BF: Mifflin-St Jeor
  • Higher BF: Mifflin or Katch-McArdle
  • Know your BF%: Katch-McArdle
  • Dieting 3+ months: All overestimate, your BMR may be 5-15% lower

Calculate all three, take the median, test for 2-3 weeks.

I built a free calculator showing all three formulas — no signup, no ads.

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