How we calculate

Last updated 25 October 2026

Every number in the app comes from a published method, not a guess. This page lists them so you, or your coach, can check our working. None of it is medical advice.

Body fat

• US Navy circumference method. Men: waist (at the navel), neck and height. Women: waist, hips, neck and height. The result is kept between 2% and 55%.
• You can type your own figure (from calipers or a scan) and we use that instead.
• Lean mass = bodyweight × (1 − body fat %). BMI = kg ÷ height in metres squared.

Calories burned in a session

• With a heart-rate trace (Apple Watch or a Bluetooth strap): the Keytel equation (Keytel et al., Journal of Sports Sciences, 2005), which estimates kcal per minute from heart rate, bodyweight, age and sex. It is applied to every reading across the session, so rests count at the lower rate they really ran at. A reading is never allowed below resting metabolism.
• Without heart rate: MET values from the Compendium of Physical Activities, by machine and pace for cardio and by how hard your sets felt for weights, times bodyweight, times time.
• If the trace covers only part of the session, heart rate is used where it exists and MET fills the gaps. The summary card says which method produced the figure.
• Heart-rate estimates were validated on steady aerobic work. Lifting raises heart rate a little more than oxygen use, so the weights portion reads slightly generous. Every wrist device shares this bias.

Daily calorie target

• Resting metabolism: Mifflin-St Jeor (10 × kg + 6.25 × cm − 5 × age, +5 for men, −161 for women).
• Maintenance: resting metabolism × an activity factor from 1.2 (sedentary) to 1.9 (very active), plus your planned training time at roughly 5 kcal a minute averaged over the week.
• Cut: maintenance minus 300, 500 or 750 kcal depending on the pace you chose. Bulk: maintenance plus 300. Maintain, strength, endurance: maintenance.
• Safety floors: we never set a target below 1,500 kcal for men or 1,200 kcal for women, and we warn when a deficit passes 750 or 1,000 kcal.

Protein, fat and carbs

• Protein: 2.2 g per kg of bodyweight on a cut (the standard 1 g per lb). Otherwise 1.8 to 2.4 g per kg of lean mass depending on goal.
• Fat: 35% of the calories left after protein, at 9 kcal per gram. Carbs: whatever remains, at 4 kcal per gram.

Fat-loss projection

• One kilogram of body fat is taken as 7,700 kcal.
• The fastest loss we will plan for is 1% of bodyweight a week, capped at 1 kg.
• Expected muscle gain while cutting starts at 0.7, 0.4 or 0.15 kg a month for beginner, intermediate and advanced lifters, then scales down for larger deficits, lower body fat and fewer training days. We never project fat loss below 5% body fat.

Strength

• Estimated one-rep max: Epley formula, weight × (1 + reps ÷ 30), from your heaviest set.
• Personal best: the heaviest weight you have lifted on that exercise.
• Strength score: your best one-rep max in each movement pattern (push, pull, squat, hinge, core) divided by bodyweight, placed on a 0 to 100 scale against standard bodyweight-ratio benchmarks. Benchmarks are scaled for women.

Progression and deloads

• After each session, every exercise is rated by how its sets felt. All or most sets easy: add 2.5 kg next time. Most sets struggled, or you told us it was too heavy: take 2.5 kg off. Anything else: hold.
• If you dropped the weight mid-exercise and the lighter sets felt right, we start you there next time.
• Any pain you note overrides an increase: we hold the weight for a session.
• Deload (planned): every fourth week on a programme, every pre-filled weight is cut by 10%. A break of three weeks or more restarts that clock, so you are never deloaded on your first week back.
• Deload (earned): struggling on the same exercise three sessions in a row also cuts it by 10%.

Heart-rate zones

• Maximum heart rate is estimated as 220 minus age. Zone 1 is anything under 60% of that maximum; zones 2 to 5 start at 60%, 70%, 80% and 90%. If you know your real maximum from a test, the 220 minus age estimate can be out by 10 to 15 beats either way.

Recovery

• Each muscle gets a recovery window of 72, 56 or 44 hours for beginner, intermediate and advanced lifters, lengthened for big muscles (legs, glutes, back) and shortened for small ones (arms, forearms, abs).
• How much of that window a session uses depends on reps, how heavy the sets were relative to your one-rep max, and how they felt. Cardio is tracked separately and can only ever add a little, so a walk never hides a leg day.

Questions

If a number looks wrong, email hello@gym-tracker.co.uk with the session or measurement and we will check the working.