Body simulator: a 3D figure from your height, weight and training plan
An online body simulator — a body visualizer where the figure is moved by a training plan and nutrition, not by sliders. Enter height, weight and body fat, add exercises with their sets, and the model computes muscle group by group, fat and girths, then shows a 3D figure of today next to the forecast for 3, 6 or 12 months. Every projection is a range, and the whole model with its sources is laid out on the page — useful for sizing up a muscle gain, a cut or a body recomposition.
How to use it
4 stepsSex, age, height and weight are enough: the tool estimates body fat itself, and six figures let you refine it. From age three the figure and the numbers become a child’s, and the forecast shows growth as well.
Where you train and how many days a week, whether you walk or run, whether you eat as now, less or more. That is enough to see the result.
The Detailed level has the program exercise by exercise, calories and protein as numbers, tape measurements, and a goal that calories are solved for.
Drag the timeline or press play, turn the figure, read the findings — then save a PDF report, a before-and-after picture or a link to the scenario.
Build a training plan and watch what it does to the body over months
How to use itWhat the body simulator does
About you
Body fat: an estimate
Not sure of your body fat? Pick the figure that looks most like you — same height and weight, only the fat differs.Pick the figure that looks like you.
Tape measurements: a better body-fat and frame estimate
Neck, waist and hip give a body-fat estimate that beats anything computed from BMI. Wrist and ankle set your frame, which is what decides how much muscle you can carry — it is the one input you cannot train.
What I will do
Under 6 the tool builds no exercise program: a child this age needs active play. The WHO recommendation for ages 3–4 is at least three hours of movement a day.
Under 18, eating less than usual is a decision for a doctor.
Under 6 the tool builds no exercise program: a child this age needs active play. The WHO recommendation for ages 3–4 is at least three hours of movement a day.
Start from a template or build your own. Sets are working sets taken close to failure; anything easier counts for less than the model assumes.
Cardio and sport
Calories decide what happens to fat. Training decides what happens to muscle. Neither substitutes for the other, and the model will not let one pretend to be the other.
Goal by the end of the period
Drag to turn. Tap a muscle to see its workload.
Height and weight follow the place the child holds today on the WHO growth curves. Every child has their own tempo, most of all through puberty: a guide, not a promise.
Girths
Weekly volume, muscle by muscle
untrained below the minimum productive high past recovery
Sets are counted fractionally: a bench press is one full chest set, half a triceps set and half a front-delt set. Tap a row to find that muscle on the figure.
Chart
What the plan does
Not medical advice. This is a planning model, not a diagnosis, a prescription or a promise. Talk to a doctor before starting to train if you are pregnant, recovering from surgery or illness, or living with heart, joint or metabolic conditions. If food, weight or your reflection are a source of distress, this is not the right tool for you — a clinician is.
What this can and cannot tell you. The model works from published averages, and you are one person, not an average: identical training produces anywhere from no measurable growth to 60 % quadriceps gain across individuals. That is why every projection here is shown as a range rather than a number. Treat the direction and the relative sizes as the useful output — which muscles your plan actually trains, what happens if you add sets, where the fat comes off first — and treat the exact centimeters as an illustration.
How this is calculated — the whole model, in the open
1. Energy
Resting metabolic rate comes from the Katch-McArdle equation (370 + 21.6 × lean kg), the only common formula that responds to you getting more muscular. It is multiplied by an activity factor and added to the measured cost of your own plan: every exercise carries a MET value from the Compendium of Physical Activities, and every set is costed as 40 seconds of work plus its rest. Sustained deficits meet metabolic adaptation of up to −13 %, and sustained surpluses meet a +8 % rise driven by non-exercise activity — modeling only the downward half is what makes naive calculators show bulking as cheaper than it is.
2. Where a kilogram comes from
Weight change is split between fat and lean tissue by Forbes's rule in Hall's energy-conserving form: the leaner you are, the more of each kilogram lost is lean mass. Resistance training and protein then protect lean tissue, and — this is the part most calculators drop — the energy that protection saves is taken out of fat instead, so the books always balance.
3. Sets into growth
Sets are counted fractionally per muscle, the convention the volume literature uses. Weekly fractional sets map to a saturating stimulus: real returns all the way up, shrinking with every added set. It is deliberately monotonic — the 2025 meta-regression of the resistance-training dose response found a 100 % posterior probability that more volume means more growth and no evidence for an inverted U at the muscle. If you have read that "junk volume" makes muscles shrink, the evidence does not support it. What too much volume actually costs you is in the next section.
4. Recovery, which is where too much hurts
Total work is measured in fatigue units — one moderate isolation set is one unit, a squat is close to two — because a squat is one set to recover from even though it counts across six muscles. Your capacity comes from training age, sleep, energy availability and age. Exceed it and every muscle scales down, including well-programmed ones. That is the honest answer to "what if I do 50 sets of sit-ups": the abs barely notice the sets past the twenties, and the rest of your body grows slower.
5. Ceilings
Rate of gain halves for every year of training, from about 10 kg of lean mass in a well-run first year for men and half that for women, down to a floor of 1–2 kg a year that never quite reaches zero. On top of that sits your frame: Casey Butt's equation from ~300 drug-free champions turns height, wrist and ankle into a maximum lean mass, cross-checked against a normalized FFMI cap of 25 for men and 21.5 for women. Both are the top of the observed distribution among people who competed — a ceiling, not a forecast.
6. From kilograms to a shape
Each measurement site is built as a cross-section: bone and organs, plus the muscle groups crossing it at their real volume, plus a subcutaneous fat layer whose thickness follows site-specific deposition — which is why a man's waist moves roughly six times as fast as his calf when he loses fat, and a woman keeps far more of her hip while her waist shrinks. The perimeter of that cross-section is the circumference, and the silhouette is drawn from the circumferences. Nothing is interpolated between stock body shapes.
7. Why the abs sometimes will not appear
Muscle relief is drawn at an opacity taken from the fat layer over it in millimeters, not from how many sets you did. Around 6–8 mm of abdominal fat and the separations show; above 15 mm they do not, whatever is underneath. The figure will refuse to draw you a six-pack the model does not believe in.
8. Why you get a range and not a number
The same program run by different people produces wildly different results — the classic controlled study found individual quadriceps growth ranging from zero to 60 %. Every projection is therefore run three times, at 0.65, 1.0 and 1.3 of the modeled rate. If a tool in this category shows you one confident number for your body a year from now, that number is the least true thing on its page.
9. A growing body
Under 18 the model is a different one, taken from measurements rather than scaled down from the adult. Typical height and weight are WHO medians: the 2006 growth standards at three and four, the 2007 reference from five. Proportions: the head is 18 % of stature at three and 16 % at six against 13 % in an adult, and leg length follows the ratio of sitting height to stature. Girths are fitted to the median measurements of about 3,900 children: a child has a relatively thicker neck and a rounder belly, a narrower ribcage and pelvis, a smaller share of muscle in the upper arm, and more of their subcutaneous fat on the arms and legs than an adult — that last from measured skinfolds. Body fat is estimated by Deurenberg’s equation for children and resting energy by Schofield’s. Before puberty strength training builds strength with almost no gain in muscle size, and the model counts it that way. Growth, and the weight that comes with it, follow the place the child holds today on the WHO curves — the same distance of height and of body-mass index from the median; the band around the forecast allows for puberty arriving a year earlier or later than average. Under six no exercise program is built.
What this model does not know
- Your genetics beyond frame size — hormone levels, fiber type, how you respond to volume. This is the single biggest source of error and no calculator can see it.
- Where your existing muscle actually sits. If you already train, the tool spreads your lean mass evenly across groups, because a form cannot know that your back is ahead of your chest.
- Technique, effort and how close to failure your sets really are. A set logged is a set assumed hard.
- Medication, illness, injury, menstrual cycle, menopause and thyroid function — all of which move these numbers more than the training variables do.
- When a child’s growth spurt will begin. The forecast follows the place they hold today on the WHO curves, and an early or a late puberty moves a year of growth by centimeters — which is why the forecast has a band.
- Regional fat loss by exercise. It is not modeled because it does not happen; you cannot choose where fat leaves.
Sources
Volume dose–response: Pelland et al., Sports Medicine, 2025 (meta-regression of resistance-training volume and frequency); Schoenfeld & Krieger, 2017. Volume landmarks: Israetel et al. Body composition: Janssen et al., J Appl Physiol, 2000 (whole-body MRI, n = 468); Lee et al., 2000. Partitioning: Forbes, 1987; Hall, Br J Nutr, 2007. Body fat from tape: Hodgdon & Beckett, 1984; from BMI: Deurenberg et al., 1991. Ceilings: Butt, Your Muscular Potential, 4th ed.; Kouri et al., 1995. Rate of gain: McDonald; Aragon. Protein: Morton et al., BJSM, 2018. Sleep: Nedeltcheva et al., Ann Intern Med, 2010. Interference: Wilson et al., JSCR, 2012. Individual variation: Hubal et al., MSSE, 2005. Segment proportions: Drillis & Contini, 1966; de Leva, 1996. Energy cost of activity: Ainsworth et al., Compendium of Physical Activities, 2011. The growing body: de Onis et al., Bull WHO, 2007 (growth reference, 5–19 years); Snyder et al., 1977 (anthropometry of children, UM-HSRI-77-17); Fredriks et al., 2005; Liu et al., J Cachexia Sarcopenia Muscle, 2019; Schofield, 1985; Behm et al., Appl Physiol Nutr Metab, 2008. Growth by age: WHO Child Growth Standards, 2006; Tanner et al., Arch Dis Child, 1966; Marshall & Tanner, Arch Dis Child, 1969, 1970. Children’s skinfolds: Fryar et al., Vital Health Stat 11(252), 2012.
What it does
What the body simulator does
These frames were taken on this very page — the same panels and buttons you have open above. Anyone thinks to type a height and a weight; what follows is what the tool was built for.
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A body from four numbers
Sex, age, height and weight — and a figure with those proportions stands on the stage. You do not need to know your body fat: the tool estimates it and, in one line, offers to refine it from six figures of your height and weight that differ in fat alone. Turn the figure with a finger or the mouse.
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Three answers — and the forecast stands beside today’s body
Where and how often you train, whether you walk or run, whether you eat as now, less or more. On the right is the answer in words: weight «from → to» and what the difference is made of — fat, muscle, waist. Both figures are drawn at one scale, and under them is the likely range: the same program gives different people different results.
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A map of the muscles the program really loads
The «Workload» mode colors each muscle by its weekly volume, and the «Muscles» tab sets that volume against three landmarks — the minimum, the productive range and the limit of recovery. Tap a muscle on the figure or in the list: it lights up and the figure turns the right way. This is how you see which groups a home program leaves without work.
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«Detailed»: a program of your own, exercise by exercise
A simple choice writes an ordinary program — the «Detailed» level shows all of it and lets you edit it: sets, reps, times a week, about 75 exercises filtered by equipment, and ready templates. On the «Nutrition» tab the calorie balance and the protein are set as numbers, and «Goal by the end of the period» turns a target weight or body-fat level into calories.
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A child: a child’s figure and a forecast of growth
Set an age from 3 to 17 — height and weight become the typical ones from WHO data, and the figure takes a child’s proportions. The forecast follows the place the child holds today on the growth curves: you see them half a year or a year on, and within what range, and a separate line says how much of it training and food will add.
Questions and answers
What happens if I do 50 sets of sit-ups a week?
Try it — the tool has that exact preset. Your abs grow by roughly a tenth of a kilogram over six months, because abs are about 4 % of your total muscle and cannot claim more than their share of your body's growth. Your waist barely moves, because waist size is mostly the fat layer over the muscle, and sit-ups do not remove it. Meanwhile the volume map goes red past the recoverable range and the fatigue eats into everything else you train. The comparison that matters is on the same page: fewer ab sets plus a moderate calorie deficit takes the fat over your abs from about 11 mm to under 4 — and that, not the sets, is what makes them visible.
How is this different from a body visualizer?
A body visualizer maps measurements onto a shape: you tell it your waist, it shows you that waist. This one goes the other way. You describe training and eating, it computes what happens to muscle mass per group and fat mass over time, derives the circumferences from those masses through cross-sectional geometry, and then draws them. That is why exercises are inputs here and not decoration, and why the model can tell you that adding a fourth set of curls buys less than a first set of rows.
How accurate are the centimeters?
Treat the direction and the relative sizes as the useful output, and the exact centimeters as an illustration. The geometry reproduces published average girths for a reference man and woman to within a few per cent, and stays in realistic ranges at the extremes, but it is built on population averages. The dominant error is not the model — it is that individual response to identical training ranges from no measurable growth to 60 % in a controlled study. That is why you get a band rather than a number.
Why does more volume never shrink a muscle here?
Because the best current evidence does not support it. The 2025 meta-regression of the resistance-training dose response reported a 100 % posterior probability that gains increase with volume, with diminishing returns and no inverted U at the muscle level. What excessive volume does cost you is systemic: the model tracks total fatigue against your recovery capacity from sleep, age, calories and training experience, and when you exceed it every muscle scales down — including the ones on sensible volume. So overtraining is punished, just in the place the evidence puts it.
Can I lose fat and gain muscle at the same time?
Sometimes, and the model says exactly when. Recomposition is real for people who are new to training, carrying a lot of fat, or returning after a break — put a novice at 30 % body fat in a 600-kcal deficit with resistance training and 2 g/kg of protein and the projection gains quadriceps while losing more than 10 kg of fat. It is close to impossible for a lean, advanced lifter in a steep deficit, and the model will show that too: the same plan on an eight-year lifter at 12 % body fat produces almost nothing.
What is the Bruce Lee preset?
The routine written on the Hak Keung Gymnasium card dated 27 May 1965 — squats, french press, incline curls, concentration curls, push-ups, two-hand curls, triceps stretch, dumbbell circles, reverse curls, two kinds of wrist curl, sit-ups and calf raises — transcribed as written. Load it and the volume map lights up the forearms at around 50 fractional sets a week while the back sits near zero, with no hip hinge and no vertical pull anywhere on the card. That is not a criticism of Bruce Lee; it is one card from one phase of training a man whose goal was never hypertrophy. It is in the tool because it is the clearest possible demonstration of what a volume audit looks like.
Why does it ask for my wrist and ankle?
Because frame size is the one input you cannot train, and it decides how much muscle you can eventually carry. Casey Butt's equation, built from around 300 drug-free champions, turns height, wrist and ankle circumference into a maximum lean mass. Skip them and the tool estimates your frame from height, which is fine for a first look but blurs the ceiling — and the ceiling is what determines whether your projected gains slow down in year two or year five.
Does it work for women?
Yes, and not by relabelling the male model. Sex changes the muscle-mass distribution, the fat deposition weights at every site, the rate of gain, the body-fat bands including where the essential floor sits, the skeletal proportions and the drug-free ceiling. That is why the same weight loss shrinks a man's waist far faster relative to his hip, and why a glute-focused plan grows the hip circumference on the female figure.
Can I look at a child’s or a teenager’s body?
Yes, from age three. Set the age — height and weight become the typical ones for it from WHO data, and the figure takes a child’s proportions: a larger head for the stature, shorter legs. The forecast shows the child three, six or twelve months on: height and weight follow the place they hold today on the WHO growth curves, training and food are added on top, and a separate line says how much of the change is theirs. It is a guide, not a promise: puberty arrives a year earlier or later than average, and the band around the forecast allows for that. Before puberty training adds almost no muscle size. Under six no exercise program is built, and eating less than usual is not offered under 18: that is a doctor’s decision.
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