Weight Gain Calculator

What this weight gain calculator is for

This page plans a muscle building weight gain, the kind of phase lifters run in an off season or a lean bulk. Enter a current weight, a goal weight and a weekly rate, and the calculator returns the calorie surplus, the daily intake, the protein, fat and carbohydrate targets, the weeks the plan takes and the date you should arrive. It also runs a stall check on logged weigh ins, so it works as a weekly review.

The scope is deliberately different from a pregnancy weight gain calculator, which reads a week by week gain band from pre pregnancy BMI. Here the target is a net gain you choose, and the numbers come from sports nutrition position stands rather than obstetric guidelines. If you are underweight for medical reasons, a clinician should set the plan; the CDC adult BMI categories mark anything below 18.5 as underweight, which is the usual signal to see one first.

Everything here is an estimate built from population formulas. Maintenance calories vary by hundreds of kilocalories at the same body weight, so treat the first four weeks as a calibration run and adjust from the scale, which is what the stall check is for.

From a goal weight to a daily calorie target

The chain runs in three steps. Step one is resting energy: the calculator uses the Mifflin-St Jeor equation, 10 × kg + 6.25 × cm - 5 × age + 5 for men and the same equation ending at -161 for women. Step two multiplies that by an activity factor from 1.2 for a desk bound day to 1.9 for heavy labour, turning resting energy into maintenance. Step three adds the surplus your rate implies, priced at 7,700 kcal per kg, so 0.35 kg a week costs 385 kcal a day.

For the default inputs, a 25 year old man at 175 cm and 70 kg with a moderate activity factor lands on 1,674 kcal at rest and 2,594 kcal of maintenance, so the 385 kcal surplus puts intake at 2,979 kcal, 14.8% above maintenance. That sits inside the 10% to 20% surplus the 2026 ISSN review of hypercaloric strategies describes as a viable way to add skeletal muscle while limiting fat.

The 7,700 kcal per kg figure is a static conversion, the same one behind the 3,500 kcal per pound rule. Real gain runs slower because maintenance climbs as you gain: the NIH Body Weight Planner, built at NIDDK by Kevin Hall and colleagues, models that adaptation, which is why long projections at a fixed rate drift high. Re-run the calculator every four weeks with your new weight.

How fast to gain without adding fat

The rate matters more than the total. Iraki, Fitschen, Espinar and Helms reviewed off season nutrition for natural bodybuilders in the journal Sports in 2019 and recommended a gain of about 0.25% to 0.5% of body weight per week for novice and intermediate lifters, with advanced lifters more conservative. At 70 kg that band is 175 g to 350 g a week, and the calculator's default of 0.35 kg sits on the upper edge of it.

The same review paired the rate with a surplus of 10% to 20% above maintenance for novices and intermediates, and 5% to 10% for advanced trainees. The ISSN's 2017 position stand on diets and body composition reaches the same place from the other side: a sustained surplus drives lean mass gain, with larger surpluses suiting untrained lifters and smaller ones suiting advanced trainees who add fat easily when they overeat.

Faster is not better. Calories beyond what muscle tissue can build do not disappear; they land in fat stores, and removing them later costs weeks of dieting. The band below is why the calculator flags a planned rate above 0.5% of body weight as outside the published range.

Protein, fat and carbohydrate on a gaining phase

Protein carries the widest gap between the minimum and the useful dose. The ISSN position stand on protein and exercise (Jäger and colleagues, 2017) puts exercising adults at 1.4 to 2.0 g per kg of body weight per day, and the 2019 off season review narrows the bulking range to 1.6 to 2.2 g/kg, spread over 3 to 6 meals of 0.40 to 0.55 g/kg each. The calculator uses 1.8 g/kg and prints the range beside it, 112 g to 154 g at 70 kg.

Fat follows the 2019 review too: 0.5 to 1.5 g/kg, with 0.9 g/kg as the midpoint, 63 g at 70 kg. Whatever calories remain after protein and fat go to carbohydrate, and the review asks for at least 3 to 5 g/kg to fuel hard sets. The default inputs land at 477 g, or 6.8 g per kg, comfortably above that floor.

These are targets for a training day, not laws. A single meal short of the protein number matters less than a week short of the calorie number, because the surplus sets the direction of the scale and the macros set what the added weight is made of.

Reading the timeline and the projected date

Time to goal is the net gain divided by the weekly rate: 5 kg at 0.35 kg a week is 14.3 weeks, or 100 days, which the calculator turns into a calendar date, February 2, 2027 from October 25, 2026. The date moves with every input, so halving the rate to 0.175 kg doubles the wait to 28.6 weeks.

Trust the early part of the timeline more than the late part. Because maintenance rises with body weight, a fixed intake buys a smaller surplus each month and the straight line bends: the same 2,979 kcal that adds 0.35 kg a week at 70 kg adds closer to 0.28 kg at 75 kg. The table gives the arithmetic at three rates, and the fix is mechanical: re-run the calculator at each new body weight and raise intake by the surplus it names.

Telling a stall from scale noise

Body weight swings a kilogram or more in a day on water, glycogen and gut content alone, so one flat weigh in means nothing. The calculator compares your average over the last two or more weeks with the rate you planned, on the rule that you judge the trend, not the day. Weigh in daily under the same conditions and read the weekly average.

When the check reads stalled, raise intake and hold the change for two full weeks, because the scale needs about that long to reflect a new intake. When it reads faster than plan, cut a little rather than panic cutting, since the first weeks of a bulk carry water and glycogen that inflate the apparent rate by half a kilogram or so.

What the gain is made of, and when to stop

A kilogram of scale gain is not a kilogram of muscle. The split depends on the size of the surplus, training quality and training age, and no formula predicts it for one person, so the calculator prints three scenarios instead: at 65% lean, a 5 kg gain is 3.25 kg of lean mass and 1.75 kg of fat; at 50% the fat share is 2.5 kg; at 80% it is 1 kg. A tape measure at the navel, read weekly next to the scale, tells you which row you are living in.

Stop or reverse the phase on evidence rather than on a calendar date. The CDC adult BMI categories put healthy weight between a BMI of 18.5 and 25, and the default inputs finish at a BMI of 24.5, near the top of that band. A waist that adds more than about 2 cm over eight weeks, or a rate you cannot bring back inside the 0.25% to 0.5% band by trimming 100 to 200 kcal, both say the surplus has stopped buying muscle. This page is arithmetic, not medical advice; anyone with a health condition should run the plan past a clinician or a registered dietitian.