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If you've ever used two different calorie calculators and gotten two different answers — sometimes off by 200+ calories — you're not alone. It's one of the most common questions on r/loseit and r/Fitness. The difference comes down to the formula behind the number, and the activity multiplier on top of it.

The three main BMR formulas

Every calorie calculator starts with BMR (Basal Metabolic Rate) — the energy your body burns at complete rest, just to stay alive. Then it multiplies by an activity factor to get TDEE (Total Daily Energy Expenditure). The formula used for BMR is where the differences start.

1. Mifflin-St Jeor (most modern, most recommended)

Published in 1990, this is the formula the Academy of Nutrition and Dietetics recommends. It's the one DigiCalc uses. For a 35-year-old woman, 5'6" (168 cm), 150 lb (68 kg):

BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
= 10(68) + 6.25(168) − 5(35) − 161 = 1,394 calories

2. Harris-Benedict (original, revised)

The original 1919 formula is still used by many calculators. It tends to overestimate BMR by 5% or more compared to Mifflin-St Jeor. The revised version (1984) is closer but still runs higher. For the same woman:

BMR ≈ 1,447 calories (revised) — about 50 calories higher than Mifflin

3. Katch-McArdle (best if you know body fat %)

Uses lean body mass instead of total weight, so it's more accurate for muscular or lean people. But it requires knowing your body fat percentage:

BMR = 370 + 9.82 × lean mass(kg)

For our example woman at 25% body fat (51 kg lean mass): BMR ≈ 871 calories. Wait — that's much lower. That's because Katch-McArdle accounts for the fact that fat tissue burns very little energy. The other formulas estimate this implicitly, which is why they give higher numbers. If you don't know your body fat %, Katch-McArdle isn't usable.

The activity multiplier is the bigger source of error

After BMR, calculators multiply by an activity factor:

Level	Multiplier	What it means
Sedentary	1.2	Desk job, little to no exercise
Lightly active	1.375	1–3 workouts/week
Moderately active	1.55	3–5 workouts/week
Very active	1.725	6–7 workouts/week or physical job
Extra active	1.9	Athlete or very physical job + training

Here's where it goes wrong: almost everyone overestimates their activity level. A 1.55 multiplier (moderately active) sounds right for "I go to the gym 3–4 times a week" — but the multiplier assumes activity throughout the day, not just during the workout. If you sit at a desk for 8 hours, drive everywhere, and do a 45-minute gym session, you're probably closer to 1.375 (lightly active) than 1.55.

The gap matters: on a BMR of 1,394, the difference between 1.375 and 1.55 is:

• 1.375 × 1,394 = 1,917 calories
• 1.55 × 1,394 = 2,161 calories

That's 244 calories/day — the difference between losing 0.5 lb/week and barely losing anything. Over a month, that's 7,300 calories (~2 lb) of unexpected plateau.

Which calculator should you trust?

Use a calculator that:

1. Uses Mifflin-St Jeor as the BMR formula (or Katch-McArdle if you know your body fat %).
2. Shows you the BMR and the multiplier separately, so you can sanity-check the activity level.
3. Lets you adjust for a goal (lose/maintain/gain) rather than just showing one number.
4. Doesn't hide the assumptions — you should be able to see exactly what formula is used.

Why your calorie target might be wrong (even with a good calculator)

Even with a perfect formula, the number is an estimate with a ±10–15% margin of error. On a 2,000-calorie target, that's ±200–300 calories. The main reasons:

• Metabolic variation — some people naturally burn 5–10% more or less than the formula predicts.
• Under-reporting food — people underestimate intake by 20–40% on average, especially with cooking oils, snacks, and restaurant portions.
• Adaptive thermogenesis — as you lose weight, your BMR drops slightly more than the formula predicts (your body fights back). This is why weight loss slows after the first few weeks.
• NEAT (Non-Exercise Activity Thermogenesis) — fidgeting, walking, standing. It varies by 200–400 calories between people of the same size, and it drops when you diet (you move less without noticing).

The bottom line

Calorie calculators disagree because they use different BMR formulas and activity multipliers — not because one is "right" and the others are wrong. The best approach: use a calculator with Mifflin-St Jeor (the modern standard), pick a conservative activity level, treat the result as a starting estimate, and then validate it against real-world weight data over 2–3 weeks. Your body is the only truly accurate calorie calculator — the online ones just get you close enough to start.