How it works
Our formulas & sources
Every number on RecompCalc comes from published, peer-reviewed equations, not a black box. Here is exactly what we calculate, what makes our weight-loss timeline more realistic than the usual rule of thumb, and, just as important, what these formulas cannot tell you.
Resting metabolism (BMR)
Everything starts with your basal metabolic rate (BMR): the calories your body burns at rest just to stay alive. By default we use the Mifflin-St Jeor equation, the most accurate general-purpose formula in the research:
- Men: 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
- Women: 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
If you know your body fat percentage, we switch to the Katch-McArdle equation instead, which is more precise because it works from your lean mass rather than total weight: 370 + 21.6 × lean body mass(kg), where lean mass is your weight minus your fat. Two people at the same weight can have very different metabolisms if one carries more muscle. This is how we capture that.
From BMR to daily burn (TDEE)
Your total daily energy expenditure (TDEE), your maintenance calories, is your BMR multiplied by an activity factor (PAL) that reflects how much you move:
| Activity level | Roughly | Factor |
|---|---|---|
| Sedentary | Little or no exercise, desk job | 1.2 |
| Light | 1–3 light workouts per week | 1.375 |
| Moderate | 3–5 workouts per week | 1.55 |
| Active | 6–7 workouts per week | 1.725 |
| Very active | Hard daily training or a physical job | 1.9 |
Pick honestly: most people overestimate how active they are. When in doubt, choose the lower band and adjust once you see how your weight actually responds over a few weeks.
Your calorie goal
From maintenance, we set a target for your goal: roughly a 20% deficit for fat loss (a cut), maintenance for recomposition, or about a 10% surplus for a lean bulk. These moderate steps are deliberate: aggressive deficits lose weight faster but cost more muscle and are far harder to sustain.
Protein & macros
Protein is set per kilogram of bodyweight, leaning higher when you are cutting or recomping to protect muscle. Fat covers a healthy minimum and the rest of your calories go to carbs.
| Goal | Protein | Why |
|---|---|---|
| Maintain | 1.6 – 1.8 g/kg | Keep the muscle you have |
| Lean bulk | 1.6 – 2.0 g/kg | Support new muscle growth |
| Cut (fat loss) | 1.8 – 2.2 g/kg | Protect muscle in a deficit |
| Recomp | 1.8 – 2.2 g/kg | Build and preserve at once |
Body fat (US Navy method)
Our body fat calculator uses the U.S. Navy circumference method: it estimates body fat from tape-measure measurements (neck and waist for men, plus hips for women) together with your height. It is free and repeatable at home, and its real strength is consistency: measure the same way each time and it reliably shows whether your body fat is trending down.
The honest part: our weight-loss timeline
This is where most calculators get it wrong. The old rule says “3,500 calories = 1 pound of fat,” so a 500-calorie daily deficit should lose exactly one pound a week forever. It does not. As you get lighter, you burn fewer calories, so the same deficit shrinks and weight loss gradually slows and can plateau.
Instead, we project your timeline with a dynamic, metabolism-aware model inspired by the NIH Body Weight Planner and Kevin Hall's research. Ours is a deliberately reduced version of that approach: it captures the core idea (your metabolism and body composition shift as you lose weight) using a small set of published constants, but it is not the full NIH model, and its metabolic-adaptation factor is our own conservative estimate rather than a published value. That is why our target date is usually later than the naive rule predicts, and closer to what actually happens. It is still pure math with no data to maintain; it just refuses to over-promise.
The first week is mostly water. Early on, much of the scale drop is water and glycogen, not fat, so real life often beats our projection in week one, then settles into the slower, steadier curve the model describes. Judge progress over 2–3 weeks, not day to day.
What we do not model
To stay honest, here is what these numbers deliberately leave out:
- Water and glycogen swings: the fast early drop, and day-to-day scale noise from sodium, carbs, sleep and (for women) the menstrual cycle.
- Your individual metabolism: equations give a solid starting estimate; real expenditure varies from person to person.
- How much muscle you gain or keep: that depends on your training, protein and recovery, which no calculator can read from a few inputs.
- Medical conditions and medications: thyroid issues, PCOS, certain drugs and other factors can shift the picture. This is general education, not a diagnosis.
Treat every result as a well-informed starting point, then let your real weight trend over a few weeks fine-tune it.
Safety floors
Because this touches health, our calculators apply calorie floors: we will not recommend dropping below about 1,500 kcal/day for men or 1,200 kcal/day for women. If your goal would push you lower, we cap the recommendation and show a warning instead. Very low intakes, and any plan during pregnancy, for under-18s, or with a history of disordered eating, belong with a qualified professional, not a calculator.
Evidence
Sources & references
- Mifflin MD, St Jeor ST, et al. (1990) · “A new predictive equation for resting energy expenditure in healthy individuals,” Am J Clin Nutr. The BMR equation we use by default.
- NIH Body Weight Planner · The public tool from the NIH built on Kevin Hall's dynamic model. The inspiration for our metabolism-aware timeline. Ours is a simplified, reduced version, not the full NIH model.
- Hall KD, et al. (2011) · “Quantification of the effect of energy imbalance on bodyweight,” Lancet: why steady weight loss slows and plateaus.
- Forbes GB: body-composition partitioning · Research on how leaner people lose proportionally more muscle, and higher-fat people more fat, for the same deficit.
- U.S. Navy circumference method (Hodgdon & Beckett, 1984) · The tape-measure body fat equations used by our body fat calculator.
- Examine.com · Independent, citation-led summaries of the nutrition research behind protein targets and supplements.