Weight Gain Calculator

Gaining weight is its own problem

Almost everything written about weight assumes you want less of it. If you want more, you have probably noticed that the advice thins out fast, and that the advice you do get tends to be some version of "just eat more", delivered by someone who has never had to.

It is not that simple, and this page is going to take that seriously. You gave the tool your details, a goal weight and a pace, and it returned what to eat and roughly how long the journey should take. Those numbers are honest. What they cannot show you is the thing that makes gaining weight genuinely difficult for the people who find it difficult, which is that your body is not a passive container. It has opinions, and this page is mostly about them.

The formula this calculator uses

First your resting burn, from the Mifflin-St Jeor equation:

Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

That is multiplied by an activity factor from 1.2 to 1.9 to give the calories that maintain your current weight. Your chosen pace then adds a surplus on top: 10 percent for typical, 15 for moderate, 20 for aggressive. Finally, the gap between your current and goal weight is divided by the weekly gain that surplus should produce, which gives the estimated time.

Height goes in as centimetres or inches, weight and goal weight as kilograms or pounds, and the goal has to be above your current weight.

A worked example

Take a 30 year old woman, 165 cm tall, 50 kg, doing light exercise 1 to 3 times a week, who wants to reach 56 kg at a moderate pace. Her resting burn:

BMR = (10 × 50) + (6.25 × 165) − (5 × 30) − 161
BMR ≈ 1220 Calories/day

With the light activity factor of 1.375, her maintenance comes to about 1,678 calories a day. A 15 percent surplus adds roughly 252 calories, putting her goal intake near 1,930 a day. The tool estimates that should add about 0.23 kg a week, so her 6 kg would take somewhere around 26 weeks.

Half a year for six kilograms, on an extra 252 calories a day. If that sounds slow, hold that thought, because the next section is the reason it is slower still for some people.

Why your body fights the gain

Here is the study that should be handed to every person who has ever been told they must be lying about how much they eat.

Researchers at the Mayo Clinic took 16 volunteers and deliberately overfed them by 1,000 calories a day, every day, for eight weeks. Not "encouraged them to eat more". Fed them, measured, controlled. Same excess for everyone.

On the arithmetic that underlies every calculator including this one, they should all have gained roughly the same amount of fat. They did not. The fat they gained varied by a factor of ten. Same surplus, same duration, and one person put on ten times what another did.

So where did the calories go? Not into extra resting burn, and not into workouts. Two-thirds of the increase in their daily energy expenditure came from what the researchers call non-exercise activity thermogenesis: fidgeting, shifting about, standing rather than sitting, small movements nobody notices and nobody chooses. The people who barely gained were the ones whose bodies quietly turned up the fidgeting to burn off the surplus. The correlation between that response and resistance to fat gain was strong, and the researchers concluded that overfeeding activates this involuntary movement to dissipate excess energy and preserve leanness.

Read that again if you have ever struggled to gain. Your body may genuinely be spending a chunk of every surplus you create, automatically, without asking you, in ways you cannot feel and no calculator can see. You are not imagining it, and you are not doing it wrong.

What follows practically is this. If you are naturally lean and the tool's surplus does nothing after a few honest weeks, the answer is not that the maths is broken; it is that your maintenance is higher than estimated and your body is defending it. Add another hundred or two and try again. Meanwhile, do the unglamorous things that work: eat on a schedule rather than by appetite, since appetite is exactly what is being regulated against you; lean on calorie-dense foods like nuts, nut butters, oats, olive oil, whole milk and dried fruit, because volume, not willpower, is usually the real barrier; drink some of your calories, since liquids do not fill you up the way solids do; and lift weights, so that what you gain is worth having.

Picking a goal weight worth reaching

One honest note about the goal weight box, because the tool will not raise it with you. It accepts any target above your current weight and plans a route to it without judgement. It does not know whether the number you typed is sensible.

So the judgement is yours, and it is worth making deliberately rather than by picking a round number that sounds good. If you are underweight and trying to get back to a healthy range, our Healthy Weight Calculator will show you what that range actually is for your height, and the BMI Calculator will tell you where you currently sit. If you are already in a healthy range and gaining for sport or physique, then the number on the scale matters less than what it is made of, and the goal should really be muscle rather than mass.

And if you are underweight without meaning to be, or losing weight you did not intend to lose, that is worth a conversation with a doctor before it is worth a calculator. Unintentional weight loss can have causes that no amount of extra porridge will fix, and getting that checked is more useful than any number this page can give you.

Where the estimate can be off

A few honest limits:

• The timeline assumes you are gaining fat. It divides by the energy stored in a kilogram of body fat. Muscle is mostly water and costs far less per kilogram, so if your gain is muscle the real timeline differs, as our Calorie Surplus Calculator explains in detail.
• Your maintenance rises as you gain. A bigger body costs more to run, so the surplus shrinks as you go. Recalculate every few kilograms.
• Appetite is the real limiter. The number is easy; eating it consistently, day after day, when you are not hungry, is the actual work.
• Health conditions change everything. Thyroid problems, digestive conditions, medications and illness all affect weight in ways no formula can see. If gaining is unexpectedly hard, that is worth investigating rather than pushing through.

Questions people ask

Why can I eat loads and not gain anything?

Two reasons, usually. Your body ramps up involuntary movement to burn off surpluses, as the overfeeding study above showed, and people who feel like they eat loads often eat far less consistently than they think. Track honestly for a week and eat on a schedule rather than by hunger.

Can I gain faster than this?

You can gain weight faster with a bigger surplus, but past a point the extra arrives as fat rather than muscle, because your muscle-building rate is capped by training and recovery rather than by food.

How do I make sure I gain muscle and not fat?

Lift weights properly and eat enough protein. The surplus supplies the material; resistance training is what tells your body where to put it. Without the training, a surplus has only one destination.

Is this the same as the Calorie Surplus Calculator?

The maths behind them is the same. That page focuses on sizing the surplus and what it becomes; this one focuses on why gaining is hard and how to make it stick.

References

Where the figures come from. The resting equation this tool runs is Mifflin-St Jeor, published in the American Journal of Clinical Nutrition. The overfeeding study described above, including the ten-fold spread in fat gain and the role of involuntary movement, is Levine, Eberhardt and Jensen's work at the Mayo Clinic, published in Science. The point that it is the most accurate of the common resting equations comes from a systematic review by the Academy of Nutrition and Dietetics.

1. Levine JA, Eberhardt NL, Jensen MD. Role of nonexercise activity thermogenesis in resistance to fat gain in humans. Science. 1999;283(5399):212-214. https://pubmed.ncbi.nlm.nih.gov/9880251/
2. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition. 1990;51(2):241-247. https://doi.org/10.1093/ajcn/51.2.241
3. Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association. 2005;105(5):775-789. https://doi.org/10.1016/j.jada.2005.02.005