You calculate your TDEE on one site, get 2,650. Try another, get 2,200. A fitness app says 2,900. Nothing’s broken — this is normal, and it happens for two specific, explainable reasons: different calculators use different formulas, and even the best formula carries roughly ±10% real-world variance between individuals. Neither means your metabolism is unusual or that the tools are useless — it means a single number was never going to be exact, and knowing why helps you use it correctly.
Why Different Calculators Give Different Numbers
There isn’t one universal TDEE formula — there are several, and they don’t all ask for the same inputs. Here’s how the three most common ones compare, using Maya (38, 160cm, 80kg) as an example:
| Formula | What it needs | Maya’s BMR | Best for |
|---|---|---|---|
| Mifflin-St Jeor | Age, sex, height, weight | 1,485 kcal | General population — the current standard |
| Harris-Benedict (revised) | Age, sex, height, weight | 1,518 kcal | Older formula, tends to run slightly higher |
| Katch-McArdle | Body fat % + weight | 1,493 kcal | Lean or muscular individuals, since it accounts for body composition instead of total weight alone |
Notice the spread is real but modest — about 30 calories between the three, on a base of roughly 1,500. Our calculators use Mifflin-St Jeor because a 2005 systematic review in the Journal of the American Dietetic Association found it was more likely than the alternatives to estimate resting metabolic rate within 10% of a person’s measured value. It’s the closest thing this field has to a standard — but “closest to standard” still isn’t “exact for you.”
The Bigger Source of Error: Activity Level
Formula choice accounts for a small gap. Activity level selection usually accounts for a much bigger one. Multiplying the same BMR by “lightly active” (×1.375) versus “very active” (×1.725) shifts the result by hundreds of calories — and it’s genuinely easy to pick the wrong tier. A desk job with three gym sessions a week is “lightly” or “moderately” active for most people, not “very active,” even though it can feel more intense in the moment than it registers as over a full week. If two calculators are giving you wildly different numbers, check what activity level you selected on each before blaming the tool itself.
Why Even the “Right” Number Won’t Match Perfectly
Here’s the part most calculators don’t tell you: even with the correct formula and an honest activity level, individual metabolism varies enough that formula-based TDEE is typically accurate within about ±10%, not to the exact calorie. For Maya, a TDEE of 2,042 realistically means somewhere between roughly 1,838 and 2,246 — a range, not a single fixed target.
This is also why the “3,500 calories = 1 pound” rule can feel wrong week to week even when the underlying math is sound. Water retention, sodium intake, glycogen levels, and hormonal cycles can swing the scale by a kilogram or more in either direction, completely independent of actual fat loss. A calculator being “off” by a pound on the scale this week usually isn’t the calculator’s fault at all.
What to Actually Do About It
Use your calculated TDEE as a starting estimate, not a verdict. Track your weight and food intake consistently for 2–3 weeks, then compare what actually happened to what the number predicted. If you’re losing weight faster or slower than expected at a given calorie level, that’s real, personalized data — more accurate for you specifically than any formula could be on its own. This is the idea behind recalculating your TDEE from your own results over time, rather than trusting a single calculation forever.
Frequently Asked Questions
Why do different TDEE calculators give me different numbers?
Mainly two reasons: they may use different underlying formulas (Mifflin-St Jeor, Harris-Benedict, or Katch-McArdle), and the activity level you select has a much bigger effect on the result than which formula is used. Individual metabolic variance of roughly ±10% adds a further, unavoidable gap between any formula and your exact number.
Is my metabolism actually slow if the calculator seems wrong?
Usually not. A mismatch between a calculator’s prediction and your real-world results is far more often explained by an inaccurate activity level selection, inconsistent tracking, or normal water-weight fluctuation than by an unusually slow metabolism.
How accurate is Mifflin-St Jeor specifically?
Research comparing predictive equations found it was more likely than other common formulas to fall within 10% of a person’s actual measured resting metabolic rate — the best available estimate without lab equipment, though still an estimate rather than an exact measurement.
Should I recalculate my TDEE if I don’t see results?
Give any calorie target at least 2–3 weeks before adjusting — short-term fluctuation is normal. If your real trend genuinely diverges from the prediction after that window, adjusting your target based on your own logged results is more reliable than switching to a different calculator.
Calculate Your Number
Our calculator now shows a realistic range alongside your result, not just a single figure — because a single figure was never the whole picture.
Related Reading
Disclaimer: This article is for informational and educational purposes only and does not constitute medical or nutritional advice. Individual metabolism varies. Always consult a qualified healthcare professional for personalised guidance.
References & Sources
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775-789.
- Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247.