The results panel gives you four numbers at once. Two of them are useful immediately, one is context, and one is a rough screening tool that gets far more attention than it deserves.
TDEE is your maintenance estimate
Total daily energy expenditure is the number to act on. Eat at it and your weight holds, on average, over weeks. It is a maintenance estimate, so treat the first two weeks as a calibration period rather than a verdict.
If you are cutting, subtract 15%–20%. If you are bulking, add 10%–15%. That is the entire decision tree.
BMR is not a target
Basal metabolic rate is what you would burn lying still all day. It typically lands at 60%–70% of TDEE. It exists to show you the floor, not to be eaten at.
The mistake: eating near BMR to “speed up” fat loss. That is not a mild deficit, it is a crash diet with a scientific-looking number attached. It costs muscle, energy and, eventually, adherence.
BMI is a population screening tool
Body mass index sorts populations, not individuals. It does not know whether your weight is muscle or fat, and it was never designed to.
| Situation | What BMI does |
|---|---|
| Untrained adult, average build | Reasonable first pass |
| Heavily muscled | Reads “overweight” wrongly |
| Older adult with low muscle | Reads “normal” while body fat is high |
| Athlete in season | Nearly meaningless |
Use it as a flag, then look at waist circumference and the trend in your own weight. Do not let a screening cut-off overrule your training log.
The four mistakes
1. Recalculating every time the scale moves. Your TDEE drops as you lose weight, but by maybe 20–30 kcal per kilogram lost. Recalculate monthly, not daily.
2. Inputting an aspirational activity level. The multipliers assume a stable pattern. If you sit for nine hours a day, three gym sessions do not make you “very active”.
3. Treating the number as a promise. A 500 kcal deficit predicts about 0.45 kg per week. Real weeks fluctuate by more than that from water alone. Judge monthly trends.
4. Ignoring the measurement error in the inputs. A weight entered 2 kg off shifts the result by roughly 30 kcal; a body-fat percentage guessed 5 points off can move a Katch-McArdle estimate by well over 100 kcal.
A two-week calibration routine
- Enter your real numbers — morning weight, honest activity level, measured body fat if you have it
- Eat to the estimate for 14 days, logging intake
- Average your weight for week one and week two
- If the average is flat, the estimate is right. If it fell, your true maintenance is higher. If it rose, lower
- Adjust by 100–200 kcal and repeat once
After two cycles you have a personal number that beats any equation, because it was measured on you.
When to stop using a calculator
No estimate replaces clinical assessment if you are pregnant or breastfeeding, under 18, managing thyroid disease, diabetes, kidney disease or an eating disorder, or preparing for a competition. In those cases the number is a conversation starter with a professional, not a plan.