Three numbers stacked on each other
Every calorie calculator on the web runs the same chain. Resting burn, times an activity factor, plus or minus a goal offset. The sample above, step by step:
- 1
Resting burn: 1,618 kcal
Mifflin-St Jeor for men: 10 × 70 kg, plus 6.25 × 170 cm, minus 5 × 30 years, plus 5. The sum is 1,617.5. For women the last term is minus 161 instead of plus 5.
- 2
Activity: × 1.55 gives 2,507 kcal
The multiplier stands for everything outside a hospital bed. Walking to the train, training sessions, fidgeting, digesting food. Here 889 kcal sits on top of the resting figure.
- 3
Goal: minus 500 gives 2,007 kcal
A 500 kcal daily gap adds up to 3,500 kcal a week, the old rule of thumb for one pound of fat. Expect the real loss to slow down after the first month. The section on the 3,500 rule below explains why.
Step 2 carries the biggest error. Formulas for resting burn land within about 10 percent for most adults. Picking the wrong activity row moves the answer by 280 kcal or more per step for this person.
Pick the activity row one lower than your gut says
People overrate their own week. A one-hour gym session three times a week, with a desk job around the sessions, sits closer to light than moderate. Here is the same 30-year-old man at every row:
| Row | Multiplier | Maintenance | What the week looks like |
|---|---|---|---|
| Desk job | 1.2 | 1,941 | Under 5,000 steps a day, no planned training |
| Light | 1.375 | 2,224 | Office work plus 2 or 3 workouts, or 7,000 to 9,000 steps daily |
| Moderate | 1.55 | 2,507 | 4 or 5 solid sessions a week, or 10,000+ steps with some training |
| Hard | 1.725 | 2,790 | Training most days at real effort, often twice on some days |
| Physical job | 1.9 | 3,073 | Construction, warehouse, farm work, plus training on top |
From desk job to physical job the estimate swings by 1,132 kcal. No formula tweak comes close to the size of this choice. When you sit between two rows, take the lower one. Underestimating maintenance by 150 kcal costs you a slightly slower result. Overestimating by 300 erases a gentle deficit outright.
Mifflin-St Jeor, Harris-Benedict, or Katch-McArdle
The formula menu changes the resting burn only. The activity factor stays the same. Three people, three formulas, maintenance in kcal per day:
| Person | Mifflin | Harris | Katch |
|---|---|---|---|
| Man, 30, 170 cm, 70 kg, 15% fat, moderate | 2,507 | 2,591 | 2,566 |
| Woman, 38, 163 cm, 62 kg, 28% fat, light | 1,771 | 1,872 | 1,835 |
| Man, 52, 183 cm, 96 kg, 30% fat, desk job | 2,219 | 2,349 | 2,186 |
Mifflin-St Jeor
The default here. A 2005 review by the American Dietetic Association found Mifflin the closest of the common equations to measured resting burn in healthy adults, within 10 percent for most people tested. Use this unless you have a reason not to.
Harris-Benedict
Published in 1919, revised in 1984. The revision still reads 80 to 130 kcal higher than Mifflin for most people in the table. Pick this one only to match a number from a coach, an app, or the Harris-Benedict BMR calculator.
Katch-McArdle
Ignores age, sex, plus height. Uses lean mass alone: 370 + 21.6 × lean kg. Best for lean, muscular people whom the other two underestimate. Only as good as your body fat figure. A bathroom-scale reading off by 6 points shifts the resting figure by about 90 kcal.
Do you have a body fat number from a DEXA scan or a careful tape measurement? Katch is worth a look. A guess from the mirror adds error instead of removing some. The body fat calculator gives a US Navy tape estimate to feed in here.
The 3,500 calorie rule falls apart after month one
The goal ladder above uses the classic conversion: 3,500 kcal per pound, about 7,700 kcal per kilogram. Treat the weekly rates as the first few weeks only.
- Lighter body, lower burn. Lose 6 kg. The sample man's maintenance drops by about 90 kcal from body weight alone. A 500 kcal deficit quietly becomes 410.
- Less unplanned movement. Many dieters fidget less, pace less, take the lift more. Studies on metabolic adaptation put the drop near 100 kcal a day, far more for some people.
- Water first. Week one often shows 1 to 2 kg gone, mostly glycogen plus the water bound to glycogen. Weeks three to six often look slower than the ladder promises.
Kevin Hall's team at the NIH built a dynamic model to replace the 3,500 rule. Their rough guide: a 10 kcal daily cut leads to about 0.45 kg lost over three years, half of the loss coming in the first year. Slower than the arithmetic suggests, then a plateau.
Correct the estimate with two weeks of your own data
Any formula guesses for a population. Your scale reports on one person. Run this check:
- Eat the maintenance number, or your chosen target, for 14 days. Log food with a kitchen scale, not by eye.
- Weigh every morning after the bathroom, before food. Average week one. Average week two.
- Compare the two averages. Each 0.1 kg per week of unexplained change is about 110 kcal a day.
Example: the target was 2,007 kcal for a 0.45 kg weekly drop. The averages fell by 0.2 kg instead. The gap is 0.25 kg a week, about 275 kcal a day. Real maintenance sits near 2,230, not 2,507. Drop the target to about 1,730 or accept the slower pace.
Where this calculator is the wrong tool
Pregnancy or breastfeeding
Needs rise by roughly 340 kcal in the second trimester, 450 in the third, about 330 to 400 while breastfeeding. None of those offsets is built in. Ask a midwife or dietitian.
Under 18
Growth adds energy needs the adult formulas ignore. Ages under 15 are blocked here on purpose.
Eating disorder history
A deficit ladder is the last thing to stare at. Skip the loss rows. Work with a clinician on a number.
Medical conditions
Thyroid disorders, kidney disease, cancer treatment, recent surgery, plus some medications change resting burn well outside the 10 percent band.
The loss rows never go under 1,200 kcal for women or 1,500 kcal for men. Those floors are common clinical guidance for unsupervised dieting, not a hard biological line. When a target hits the floor, the ladder says so instead of quietly hiding the row. A smaller deficit, or more activity, is the better fix.
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