How this is worked out
Two steps. First, basal metabolic rate — what you would burn lying still all day — from the Mifflin-St Jeor equation:
men: 10w + 6.25h − 5a + 5
women: 10w + 6.25h − 5a − 161
with weight in kilograms, height in centimetres and age in years. Mifflin-St Jeor is used here rather than Harris-Benedict because it has better measured accuracy in modern populations; Harris-Benedict dates from 1919 and reads high.
Second, that figure is multiplied by an activity factor between 1.2 and 1.9 to give total daily energy expenditure — everything you burn, including digestion, fidgeting and training.
A deficit or surplus is applied as a percentage of maintenance rather than a fixed number of calories, because a 500-calorie deficit is a modest cut for a large man and a severe one for a small woman. Targets are clamped at 1,500 calories for men and 1,200 for women, and the calculator says when it has clamped rather than printing a figure it should not.
A worked example
- Weight / height / age
- 80 kg, 180 cm, 35
- BMR (Mifflin-St Jeor)
- 1,755 kcal
- Activity — moderately active
- × 1.55
- Maintenance
- 2,720 kcal
- 15% deficit
- −408 kcal
- Daily target
- 2,312 kcal
- Expected change
- about 0.37 kg a week
- Protein at 1.6g/kg
- 128 g
Why a percentage, and why there is a floor
The standard advice of "eat 500 calories less" is a fixed number applied to people of wildly different sizes. For a 100kg man burning 3,000 a day it is a 17% cut. For a 50kg woman burning 1,600 it is a 31% cut that lands her at 1,100 calories — below the point where meeting micronutrient requirements from food becomes genuinely difficult.
So the deficit here is a share of your own maintenance, and the result is clamped. If a 25% cut would take you under 1,200 (women) or 1,500 (men), the calculator raises the target to the floor and tells you it did. Those floors are the conventional guidance for the lowest intake to attempt without medical supervision.
This matters because people follow what calculators tell them. A tool that will happily print 900 calories because the arithmetic said so is not neutral — it is giving advice, badly.
Almost everyone picks the wrong activity level
The multipliers look like descriptions of how busy you feel. They are not — they are descriptions of structured exercise. "Moderately active" means three to five real training sessions a week. A demanding job, small children and a lot of walking do not move you up a level, because the equation already assumes a normal amount of daily movement.
The consequence of picking one level too high is a maintenance figure a few hundred calories too generous, which turns an intended deficit into maintenance. This is the single most common reason a calorie plan does not work as predicted, and it gets blamed on metabolism.
The practical test: pick a level, eat at that figure for three or four weeks, and weigh yourself weekly. If the weight is not moving as predicted, adjust the multiplier down rather than cutting calories further. The equation was right; the input was not.
Why weight loss slows, and it is not your fault
The 7,700 calories per kilogram figure used here is a planning approximation, and it degrades over time. As you lose weight there is less of you to maintain, so maintenance falls. Your body also adapts — non-exercise movement drops, and some metabolic processes become more efficient. This is adaptive thermogenesis and it is a normal physiological response, not a failure of willpower.
The practical effect is that a deficit which produced half a kilo a week at the start produces less after a few months at the same intake. Recalculating every 5kg or so keeps the target honest.
It is also why very aggressive deficits underperform their arithmetic: the larger the cut, the larger the adaptation, and the more muscle is lost alongside fat. Slower is not just safer, it usually works better.
Protein is the number that protects the result
In a deficit, the question is not only how much weight comes off but what kind. Without adequate protein and some resistance training, a meaningful share of the loss is muscle — which lowers your maintenance further and makes the weight easier to regain.
Around 1.6g per kilogram of bodyweight is where the benefit plateaus for most people training recreationally. Higher intakes are not harmful for healthy kidneys but stop adding much. The calculator reports this figure alongside the calorie target because the calorie target alone is half an answer.
If you are eating substantially below maintenance, protein and resistance training are the two things that decide whether you finish lighter and stronger, or just lighter.
Assumptions and sources
- Basal metabolic rate
- Mifflin-St Jeor equation. Chosen over Harris-Benedict, which dates from 1919 and reads high in modern populations.
- Activity multipliers
- Conventional factors from 1.2 (sedentary) to 1.9 (extremely active), applied to BMR.
- Calorie floors
- 1,200 kcal for women and 1,500 for men — the conventional guidance for the lowest intake to attempt without medical supervision. checked 2026-08
- Energy per kilogram
- 7,700 kcal ≈ 1 kg as a planning figure. An approximation that ignores adaptive thermogenesis, which is why real loss slows over time.
- Protein
- About 1.6 g per kg of bodyweight, the point at which further intake stops adding much for recreational trainees.