Every TDEE calculator gives a different number because each one is a stack of choices — which BMR equation, which activity multipliers, what to round and when — and no two sites make the same choices. For the same 85 kg man the gap between two reasonable calculators is easily 300 kcal, and neither of them is "right": both are estimating a population average that your body may or may not sit near.
Different BMR equations
The bottom layer of every calculator is a resting-metabolism equation. The common ones give different answers for identical inputs. Our standard example — male, 30 years, 180 cm, 85 kg:
| Equation | BMR | Gap vs Mifflin-St Jeor |
|---|---|---|
| Mifflin-St Jeor (ours) | 1,830 kcal | — |
| Harris-Benedict (1918) | 1,933 kcal | +103 kcal |
The Harris-Benedict gap is not a quirk of this example: when Mifflin and colleagues measured 498 adults, the 1918 equations overestimated their resting expenditure by about 5%1. A calculator built on Harris-Benedict will therefore run a little high for most users, before anything else is added. Lean-mass equations such as Katch-McArdle move the number again, in a direction that depends entirely on the body-fat figure typed in. See which BMR formula is best for the full comparison.
Different multipliers
This is the bigger lever. The BMR is multiplied by an activity factor, and calculators disagree about both the factors and how to ask for them. The most copied table is a five-step ladder from 1.2 to 1.9; ours is built from two questions and tops out at 1.825.
| Label on the dropdown | Common table | Our calculator | TDEE on a 1,830 BMR (common → ours) |
|---|---|---|---|
| Sedentary | 1.2 | 1.15 | 2,196 → 2,105 |
| Lightly active (1–3 days) | 1.375 | 1.325 | 2,516 → 2,425 |
| Moderately active (3–5 days) | 1.55 | 1.55 | 2,837 → 2,837 |
| Very active (6–7 days) | 1.725 | 1.825 | 3,157 → 3,340 |
| Extra active | 1.9 | 1.825 | 3,477 → 3,340 |
Two calculators using the same Mifflin BMR and the same dropdown choice can differ by 91 kcal at "sedentary" and 183 kcal at "very active" purely from the multiplier. Choose a different rung and you move by 300–500 kcal on either table. The multiplier is where most of the disagreement between calculators lives, and it is also the input most people answer optimistically — see which activity level to pick.
Why do ours differ from the common table? Because ours are two numbers added together — a training-sessions base (1.2 / 1.375 / 1.55 / 1.725) and a NEAT adjustment (−0.05 / 0 / +0.10) — and the calculator's five labels map onto fixed pairs of those answers. The onboarding in the app asks the two questions separately, which is more honest than any single dropdown.
Different rounding and inputs
Smaller, but real:
- Age. Some calculators use age in whole years, some compute it from a birth date to the day. Each year is 5 kcal of BMR; at a 1.55 multiplier that is about 8 kcal of TDEE — trivial alone, but it explains one-digit differences.
- Unit conversion.
1 lb = 0.4536 kg,1 in = 2.54 cm. A calculator that rounds 187 lb to 85 kg rather than 84.8 kg shifts BMR by 2 kcal; one that rounds height to the nearest inch can shift it by 8 kcal. - When rounding happens. We round once, at the end:
round(1,830 × 1.55) = 2,837. A calculator that rounds BMR first, then the multiplier product, then the deficit, can drift by a few kcal at each step. - What "maintain" includes. Some sites quietly add an estimated exercise burn on top of a multiplier that already contains it. That double-count can be a few hundred kcal.
What "accurate" can mean for a population formula
It helps to know what the best case looks like. In the 2005 systematic review of validation studies, the best-performing equation, Mifflin-St Jeor, was the one that predicted resting metabolic rate within 10% of measured for the most people, and the authors still warned of noteworthy errors when it is applied to individuals2. Within 10% of a 1,830 kcal BMR is a ±183 kcal band — and that is before the activity multiplier, which is bucketed and self-reported.
As a rule of thumb, then: a BMR equation is typically 100–200 kcal off for an individual and can be 400–500 kcal off; stacking a multiplier on top pushes the total calculator error for a typical person into the low hundreds and past 500 kcal for a sizeable minority. Nobody is being careless. That is the resolution a population formula has when pointed at one person.
In plain terms: the calculators disagree with each other by a hundred or two; each of them can disagree with your body by more than that.
What to do with the number you get
- Pick one calculator and stop comparing. The differences between reasonable calculators are noise relative to the individual error. Ours is at /calculators/calorie/; it shows every step so you can see which choices it made.
- Answer the activity question conservatively. Rounding down on the multiplier and being corrected upward by data is a better experience than the reverse.
- Treat the output as a hypothesis. Log intake and weigh in for two to three weeks. If trend weight holds steady, the estimate was close. If not, the weight change tells you the size of the correction — that is exactly the loop MacroDiet's expenditure estimate runs automatically (see how apps estimate energy expenditure).
This is arithmetic about formulas, not medical advice. No calculator — including ours — measures your expenditure; it estimates it from four numbers and a dropdown.
The short version
Calculators disagree because they stack different BMR equations (≈100 kcal apart), different activity multipliers (up to several hundred kcal apart) and different rounding. A formula that lands within 10% of measured for most people is doing as well as formulas do. Pick one, answer the activity question honestly, and let two to three weeks of intake and weight data settle the real number. Educational overview only — not medical advice.