Your numbers
Your bulk
Most calculators hand everyone the same 500 extra calories. Yours is capped by what your training can actually turn into muscle, which for an experienced lifter is a lot less than you would guess. You stop paying for fat you did not need to gain.
Pick how fast you want the scale to move and everything else follows: calories, macros, and how much of that weight ends up as muscle. Drag it and watch the trade-off change in front of you.
Bulks fail at the end, not the beginning. You get the week your body fat reaches the usual cut-off point, so you can plan the whole phase instead of drifting until your clothes stop fitting.
Total bodyweight going up tells you nothing on its own. The chart splits your projection into lean mass and fat so you can see the ratio, which is the only thing that makes a bulk good or bad.
You get the ceiling for someone at your training age, in kilograms per month. If the scale is moving faster than that, the extra is not muscle, and the tool says so rather than congratulating you.
No account, no email, no adverts, no protein powder to buy. Everything runs in your browser and nothing you type ever leaves your device.
Age, height, weight and how active you are away from the gym. Metric or imperial, whichever you use.
Beginner, intermediate or advanced. This is the single most important answer on the form, because it sets both your surplus and your realistic rate of gain.
One slider, from a slow lean bulk to a fast one. If you ask for more than your training can use, we cap it and explain why.
Daily calories, protein, carbs and fat, a chart splitting muscle from fat, and the week you should stop and cut.
You have been training a while but never deliberately eaten to grow. This is the phase where you can gain almost purely muscle, and the numbers reflect that rather than treating you like an advanced lifter.
You eat what feels like a lot and the scale never moves. You will see exactly how many calories a day it takes to shift it, which is usually more than expected.
You are lean, hungry, and at real risk of turning a bulk into a rebound. The slider keeps the rate sane and the timeline tells you how long you have before you should stop.
Years of training means your muscle ceiling is low and your fat gain is not. This tool is unusual in reducing your surplus rather than raising it, which is what the research actually supports.
The instinct when you want to grow is to eat a lot more. The research points the other way: the surplus that builds the most muscle is a small one, and it gets smaller the longer you have been training.
The peer-reviewed off-season guidance for lifters is +10 to 20% above maintenance for novice and intermediate trainees, and only +5 to 10% for advanced ones. For a lot of people that works out at 200 to 350 calories a day, a snack, not a second dinner.
That direction is the part worth internalising. It is the opposite of what most people assume: more experience means a smaller surplus, because the extra calories stop being able to buy muscle and start buying fat instead.
Muscle growth has a speed limit set by your training age, not by your calorie intake. Once you are eating enough to support the growth you are capable of, the rest of the surplus has nowhere useful to go.
The clearest illustration comes from two very different studies. Untrained men fed a surplus of roughly 2,000 calories a day gained about 3.4 kg of lean mass and actually lost a small amount of fat, nearly everything went to muscle. Elite athletes given a counselled surplus of about 600 calories, by contrast, gained significantly more fat than a control group eating close to maintenance, while their extra muscle gain was not statistically significant.
Same principle, both times: a beginner has enormous capacity to use extra calories. A trained lifter does not. Eating past your rate cap does not raise the cap.
The most widely used model in coaching puts monthly muscle gain at roughly:
For an 80 kg intermediate lifter that is about 400 to 800 grams of muscle a month. If the scale is climbing considerably faster than that and you are not a beginner, the surplus is landing somewhere other than muscle.
Two honest caveats. This model has never been published in a peer-reviewed journal, it is a coaching heuristic, and it is the best available one. And the convention that women gain at about half the male rate is exactly that, a convention; the research generally shows women making similar gains in percentage terms, with smaller absolute numbers because they start with less muscle.
The standard advice is to stop around 20% body fat for men and 30% for women, usually justified by the claim that high body fat ruins how your body partitions calories between muscle and fat.
That justification is genuinely disputed. It has been argued back and forth in public between well-regarded researchers for years, one side finding little or even an inverse effect, the other arguing the evidence is too indirect to conclude anything. The main peer-reviewed off-season review gives no body fat ceiling at all, and where it does recommend capping the surplus, the reason has nothing to do with partitioning.
There is a much simpler reason to stop, and it holds regardless of who wins that argument: every kilo of fat you add has to come off again. A long, deep cut is precisely where hard-won muscle gets lost. Bulking to 25% and then dieting for five months is a worse use of a year than two modest bulks with short cuts between them.
The largest meta-analysis on the question found that training-induced gains in fat-free mass plateaued at 1.62 g per kg of bodyweight per day, with the confidence interval reaching 2.2. That distinction gets mangled constantly: 2.2 is the top of the confidence interval, not a separate and superior target. Somewhere around 1.8 g/kg is comfortably past the plateau without pretending precision that is not there.
0.5 to 1.5 g per kg works. Lower leaves more room for carbohydrate, which is what fuels hard training; much lower than 0.5 starts to interfere with hormone production. Pick by preference.
Carbohydrate takes the remainder, and it should not be an afterthought, 3 to 5 g per kg is the range for someone training hard. If your carbs land below about 3 g/kg once protein and fat are covered, either raise the surplus slightly or bring fat down.
Less clearly than you would expect. A review that set out to answer exactly this question concluded that the specific energy surplus required for muscle hypertrophy is, in plain terms, unknown. Beginners demonstrably build muscle at or very close to maintenance. What a surplus does is make growth easier and more reliable, particularly for trained lifters and during heavy training blocks.
Which is another reason not to run a big one. If the required surplus is genuinely uncertain, the sensible response is to err small and adjust from what the scale actually does over three or four weeks.
Nobody fictional signs off on this page. Below is every input the maths actually uses, with the paper it came from, so you can audit it yourself.
| Step | What we use |
|---|---|
| Metabolic rate | Mifflin-St Jeor (1990) by default, switching to Katch-McArdle only when you say your body fat came from a real scan. A guessed body fat in a lean-mass equation makes the answer worse, not better. |
| Surplus size | Iraki et al. (2019), a narrative review of off-season nutrition for bodybuilders: +10 to 20% of maintenance for novice and intermediate lifters, +5 to 10% for advanced, with weekly gain of 0.25 to 0.5% of bodyweight. |
| Rate of muscle gain | The Aragon model, expressed as a percentage of bodyweight per month by training level. A coaching heuristic rather than a peer-reviewed finding, the best available, and labelled as such. |
| Protein | 1.8 g/kg, sitting past the 1.62 g/kg plateau found in Morton et al. (2018), a meta-analysis of 49 studies and 1,863 participants. Per-meal targets of 0.4 to 0.55 g/kg from Iraki et al. |
| Muscle-to-fat ratio | Ranges read off the actual studies, untrained subjects on a large surplus versus elite athletes on a counselled surplus (Garthe et al., 2013). These are illustrative bands, not a calculation: no validated equation exists for predicting how a surplus splits. |
| Stop-bulking point | 20% body fat for men, 30% for women, presented as a practical trigger rather than a metabolic one. See the unresolved debate over whether body fat affects partitioning at all. |
| Body fat estimate | The US Navy tape method (±3 to 4 percentage points), falling back to a BMI-based estimate that cannot distinguish muscle from fat and will read high on a muscular person. |
| Energy per kg | 7,700 kcal per kg for the weight-gain projection. Deliberately conservative: lean tissue is far less energy-dense than fat, and the real cost of building muscle is not established. The clamp to a percentage of maintenance is what keeps the number honest. |
Worth stating plainly: this is arithmetic, not coaching. It knows nothing about your injuries, your training history, or what you can realistically stick to for four months. Anyone under 18, pregnant, managing a medical condition, or on medication that shifts appetite or bodyweight should be working from a professional plan rather than a form on the internet.