Run the Macros

The reasoning

Methods, equations and references

Everything the calculators do, written out. If you disagree with a choice made here, you can see exactly which choice it was and substitute your own.

Resting metabolic rate

Two equations, chosen by whether you supplied a body fat percentage.

Mifflin–St Jeor (1990)
RMR kcal/day = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(y) + s
where s = +5 for males, −161 for females

Cunningham (1980)
RMR kcal/day = 500 + 22 × fat-free mass(kg)
fat-free mass = weight(kg) × (1 − body fat% / 100)

The switch is deliberate. A 2023 systematic review with meta-analysis of prediction equations in athletes found Mifflin–St Jeor significantly underestimated measured resting metabolic rate, while the Cunningham equations, Harris–Benedict, De Lorenzo and Ten Haaf did not differ significantly from measured values. The Ten Haaf equation was the most precise of the group, correctly placing 80.2 percent of participants within ten percent of their measured value against 40.7 to 63.7 percent for the others.[1]

Ten Haaf would arguably be the better default, and it is not used here for a plain reason: the published coefficients are printed as figures rather than text in the source paper and could not be verified to the standard this site holds itself to. Cunningham is the best-supported equation whose coefficients are unambiguous. If that changes, this page changes with it.

Activity multipliers

Applied to resting metabolic rate to estimate total daily energy expenditure. These are the conventional Harris–Benedict style factors, mapped onto the training-load descriptions used by the carbohydrate guidelines so that one selection drives both.

desk job, no structured training → ×1.20  |  carbohydrate 3–5 g/kg
light, skill or technique work → ×1.375  |  carbohydrate 3–5 g/kg
moderate, ~1 h/day → ×1.55  |  carbohydrate 5–7 g/kg
high, 1–3 h/day → ×1.725  |  carbohydrate 6–10 g/kg
very high, 4+ h/day → ×1.90  |  carbohydrate 8–12 g/kg

Activity factors are the crudest step in the whole chain. They compress enormous variation — body size, exercise economy, non-exercise movement, occupation — into five buckets. Treat the resulting maintenance figure as accurate to within a few hundred calories at best.

Macronutrient targets

Protein — 1.6 g/kg maintaining, 1.8 g/kg building, 2.0 g/kg in a deficit. ISSN range 1.4–2.0 g/kg/day.[4]
Per feed — 0.25 g/kg, or 20–40 g absolute, carrying 700–3,000 mg leucine, every 3–4 h.[4]
Fat — starts at 27.5% of intake, the middle of the recommended 20–35% range.[5]
Carbohydrate — whatever calories remain, aiming at the midpoint of the training-load band above.[5]

Order of operations matters and is not arbitrary. Protein is set first because it is doing structural work. Fat is then set at the middle of its recommended range rather than at the floor, because parking every athlete at the bare 20% minimum is poor practice. Carbohydrate takes what is left. If that lands below the band for your training load, fat is trimmed toward its 20% floor to buy carbohydrate back; if it lands above the band, the surplus goes to fat instead, up to the 35% ceiling. When even the floor is not enough to reach the carbohydrate band, the calculator reports the conflict rather than silently producing an unachievable split.

Leucine is estimated at 8.5 percent of protein by mass, a reasonable figure for a mixed diet. Whey runs nearer 10–11 percent and most plant proteins run lower, so the estimate is conservative for animal-protein-heavy diets and optimistic for plant-based ones.

Energy availability

EA = (intake kcal − exercise energy expenditure) / fat-free mass(kg)
exercise energy expenditure = training kcal/kg/day × bodyweight(kg), taken from the training-load band:
light 3  |  moderate 6  |  high 12  |  very high 24 kcal/kg/day

Those per-kilogram figures assume roughly 6 kcal per kilogram of bodyweight per hour of moderate-to-vigorous training, a conventional MET-based approximation, applied to the midpoint of each band's training hours.

Thresholds used: 45 kcal/kg fat-free mass per day is the benchmark for adequate availability, and below 30 is the low-energy-availability range associated with the Relative Energy Deficiency in Sport syndrome.[6] The zone between them is treated here as acceptable rather than alarming, because a great many healthy athletes measure in it and the evidence of harm there is far weaker than it is below 30. A warning appears in that middle zone only when you have selected a deliberate fat-loss phase.

Be clear-eyed about the error. This figure stacks a predicted resting metabolic rate, a bucketed training-burn estimate and a body composition estimate on top of one another, and all three carry error in the same units. Read it as a screening flag worth acting on when it comes back low, not as a measurement.

Hydration

Sweat rate = [(pre-exercise mass − post-exercise mass) + fluid consumed − urine passed] / duration in hours[7]
1 kg of mass lost is treated as 1 L of sweat.

Fluid replacement guidance follows the National Athletic Trainers' Association position statement: 500–600 mL two to three hours before, 200–300 mL ten to twenty minutes before, 200–300 mL every ten to twenty minutes during, keeping total losses under two percent of bodyweight, and approximately 150 percent of the deficit replaced over the four to six hours afterwards. Sodium chloride at 0.3–0.7 g/L is suggested for rehydration drinks in prolonged exercise, limited meal access, or early heat acclimatisation.[7]

Sweat sodium concentration is taken from your self-assessment, at 20, 40, 60 or 80 millimoles per litre, converted at 23 mg per millimole. Published measurements span roughly 10 to 90 mmol/L between individuals, and self-assessment is a poor proxy for laboratory measurement — treat sodium outputs as order-of-magnitude.[8]

Carbohydrate for events

Loading — 10–12 g/kg/day for 36–48 h (full), ~8 g/kg for 24 h (lighter).[9]
Pre-event — 1–4 g/kg in the 1–4 h before, scaled to hours available.[9]
During — under 45 min: none. 45–75 min: mouth rinse or small amounts. 1.25–2.5 h: 30–60 g/h. Over 2.5 h: 60–90 g/h using multiple transportable carbohydrates.[10]
Recovery — 1.0–1.2 g/kg/h for the first 4 h when the next session is within about 8 h.[9]

Practical conversions used in the output assume 22 g of carbohydrate per standard gel and 30 g per 500 mL of a six percent sports drink. Check your own products — they vary by roughly a factor of two.

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How wrong can these be?

Meaningfully wrong, and it is better to know that up front than to discover it after six frustrating weeks.

Prediction equations for resting metabolic rate are typically judged by what share of people they place within ten percent of their measured value. The best-performing equation in the athlete meta-analysis managed 80 percent; most managed between 41 and 64 percent.[1] In plain terms: for one person in five, even the best equation is off by more than ten percent, and for many equations it is closer to one person in two. On a 1,600 calorie resting estimate, ten percent is 160 calories before you have multiplied by anything.

Then the activity multiplier compounds it. The step from 1.55 to 1.725 changes a 1,600 calorie resting rate by nearly 300 calories, and there is no objective test telling you which bucket you belong in. This is why the maintenance figure is the least trustworthy number the calculator produces, and why the guidance is always to test it rather than trust it.

The guideline ranges are more robust, because they are ranges — the honest output of research that found a spread rather than a point. When your protein target sits at 1.6 g/kg inside a band of 1.4 to 2.0, the message is that anywhere in that band is defensible and the exact figure is not worth agonising over.

How to test the estimate properly

Eat close to the target for fourteen to twenty-one days. Weigh yourself at the same time each morning under the same conditions, and use the weekly average rather than any individual day, because daily readings move two to four pounds on fluid and gut contents alone. At the end of the block, compare the trend to what you intended.

If your weight moved roughly as expected, the estimate was fine. If it did not move at all and you wanted it to, adjust intake by about ten percent and run another two weeks. If it moved much faster than intended, the estimate was too aggressive and the same ten percent correction applies in the other direction. Two or three of these cycles will get you closer to your real numbers than any equation ever could, and after that you can stop calculating entirely.

Track two things alongside weight: how your training sessions feel, and how you are sleeping. Both degrade before the scale shows anything is wrong, and both are more important than hitting a target.

Full reference list

  1. O'Neill JER, Corish CA, Horner K. Accuracy of resting metabolic rate prediction equations in athletes: a systematic review with meta-analysis. Sports Medicine, 2023;53(12):2373–2398. PMID 37632665
  2. Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990;51(2):241–247.
  3. Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. American Journal of Clinical Nutrition, 1980;33(11):2372–2374.
  4. Jäger R, Kerksick CM, Campbell BI, et al. International Society of Sports Nutrition position stand: protein and exercise. Journal of the International Society of Sports Nutrition, 2017;14:20. Full text
  5. Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics, 2016;116(3):501–528. PMID 26891166
  6. Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine, 2023;57(17):1073–1097.
  7. McDermott BP, Anderson SA, Armstrong LE, et al. National Athletic Trainers' Association Position Statement: Fluid Replacement for the Physically Active. Journal of Athletic Training, 2017;52(9):877–895. Full text (PDF)
  8. Baker LB. Sweating rate and sweat sodium concentration in athletes: a review of methodology and intra/interindividual variability. Sports Medicine, 2017;47(Suppl 1):111–128.
  9. Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences, 2011;29(sup1):S17–S27.
  10. Jeukendrup AE. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 2014;44(Suppl 1):S25–S33. Full text
  11. Kerksick CM, Arent S, Schoenfeld BJ, et al. International Society of Sports Nutrition position stand: nutrient timing. JISSN, 2017;14:33.
  12. Trommelen J, van Lieshout GAA, Nyakayiru J, et al. The anabolic response to protein ingestion during recovery from exercise has no upper limit in magnitude and duration in vivo in humans. Cell Reports Medicine, 2023;4(12):101324.
  13. Jeukendrup AE. Training the gut for athletes. Sports Medicine, 2017;47(Suppl 1):101–110.
  14. Hew-Butler T, Loi V, Pani A, Rosner MH. Exercise-associated hyponatremia: 2017 update. Frontiers in Medicine, 2017;4:21.

Maintained by Kristen Spang, MS RDN LDLast reviewed September 2026. Corrections are welcome — see contact.