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Burn – Body Change Simulator

Build a typical week of food, work and exercise and see how your body could change in 4, 8 and 12 weeks.

Try a preset:

1 About you

Sex

Age

years

Height

cm

Weight

kg

Body fat

%

Muscle mass

kg

Strength training experience

Body type

Belly around the waist

Appearance how your character looks

Face

Haircut

Hair colour

Skin tone

2 Typical diet

Build your day: add food and drinks to each meal and insert snacks between meals. Set the portion and how often – e.g. pizza only once a week, beer on weekends.

3 Work activity

How much you move at work (Mon–Fri). Weekends use a light everyday activity level.

4 Weekly exercise

Add activities and tick the days you do them.

Your 12-week forecast

How you could look front and side view – side view shows belly, chest and glutes best

Body measurements estimated circumferences from your fat and muscle mass

Muscle gain where your muscle change comes from over 12 weeks

Time machine

Drag the slider or press play to watch the change day by day.

Body weight kg, scale weight incl. water

Body fat % of body weight

Muscle mass kg skeletal muscle

Body composition kg

Other lean (organs, bone, water)MuscleFat

Energy in vs out average kcal per day, week 1

EatenResting (BMR)Daily & work activityExerciseDigestion

Energy balance by weekday kcal, eaten minus burned

Deficit (burning fat)Surplus (storing fat)

Coach's notes

How the simulation works

Resting metabolism – average of Mifflin–St Jeor and Katch–McArdle (lean-mass based) equations, recalculated every day as your body changes.
Daily activity – job activity factor on workdays (desk 1.25×, on your feet 1.5×, physical 1.8× BMR), 1.3× on weekends.
Exercise – MET values from the Compendium of Physical Activities: (MET − 1) × body weight × hours.
Digestion (TEF) – protein 25%, carbs 7.5%, fat 2.5%, alcohol 15% of their calories.
Metabolic adaptation – in a long deficit your body burns up to ~8% less; in a surplus slightly more.
Fat vs muscle – energy gap is split between fat (9440 kcal/kg) and lean tissue (1816 kcal/kg) using the Forbes curve. Strength training and protein ≥ 1.6 g/kg protect muscle in a deficit and add muscle (beginners up to ~0.7 kg/month men, ~0.35 kg/month women, less with age).
Muscle gain – natural max rate depends on experience (beginner ≈ 0.9 kg/month men, intermediate ≈ 0.45, advanced ≈ 0.2; women about half) and drops ~1.2%/year after 30. Your plan reaches part of that max depending on strength volume (≈ 4 sessions/week is the most that helps), protein (full effect at 1.6 g/kg) and energy (a big deficit can cut growth by up to 85%). Upper vs lower body growth follows the activity: leg day and running grow legs, upper-body gym and swimming grow arms, chest and back.
Muscle – starts from your measured value or an estimate (≈50% of lean mass in men, 45% in women, less after 40). In a deficit without strength training ~60% of lean loss is muscle; lifting cuts that sharply. Lifting in a surplus or at maintenance builds muscle. After 40, muscle slowly declines (~0.8%/year) unless you train.
Body shape – each body part's circumference is built from its share of fat, muscle and bone, calibrated to average adults (men 178 cm/80 kg/18 % fat: waist 84 cm, hips 98 cm; women 165 cm/62 kg/28 %: waist 71 cm, hips 100 cm). Men store extra fat mostly on the belly, women on hips and thighs; muscle grows arms, shoulders, chest, glutes and legs. The cartoon body is fitted to those measurements.
Body type & belly – the type sets the bone frame (shoulder, chest and hip width) and where extra fat is stored; for men it also changes how fast muscle can be built (ectomorph −15 %, mesomorph +15 %). The belly setting moves fat to the waistline; belly fat shrinks a little faster than other fat when you lose weight. Waist ÷ height above 0.5 is flagged as a health risk.
Muscle build-up – each activity trains specific muscle groups (gym full body, upper, legs, swimming, running…). Muscle built is shared between chest, back, shoulders, arms, core, glutes, thighs and calves by how much each is trained, and the 3D body grows exactly those muscles. The 💪 Muscles button highlights them.
Cartoon character – built from the open-source MakeHuman model (CC0). Its muscle setting comes from your simulated muscle relative to your body fat; its weight setting and per-region adjustments are fitted so the body's own tape-measure loops match the simulated waist, hips, chest, arm, thigh, calf and neck. To make changes easy to see, the character exaggerates how far you are from an average body by 1.5× – the numbers in the tables are the realistic ones. Drag to rotate, tap to say hi.
Animation – breathing changes the body shape (lean bodies breathe more with the chest, softer bodies more with the belly and bounce a little more), plus sway, arm swing, looking around and blinking. The character hops when your plan burns fat or builds muscle.
Water – glycogen and water drop quickly at the start of a diet (that's the fast first-week loss) and rebound in a surplus.
This is an educational estimate. Real results vary ±30% with genetics, sleep, stress, hormones, medication and how accurately food is tracked. Not medical advice.