Weight Gain Calculator (UK): Your Daily Calorie Target

Written by James Frost, founder of Flaming Phoenix. Updated October 2026.

Quick answer: This weight gain calculator works out your daily calorie target: your maintenance calories from the Mifflin-St Jeor equation, plus 300 or 500 a day for steady gain. That surplus matches the NHS range for adults and adds roughly 0.25 to 0.5kg a week. Enter your details below, in kilograms or stones.

You get your maintenance estimate, your target as a range, and a link to a full day of meals at roughly that number. The formula and a worked example are written out underneath, so you can check the result by hand.

How the weight gain formula works

The calculator works in three steps, and you can check any result with a pen and paper.

• Resting calories, using the Mifflin-St Jeor equation (1990): 10 × your weight in kg, plus 6.25 × your height in cm, minus 5 × your age in years, then plus 5 for men or minus 161 for women.
• Maintenance: resting calories × an activity factor of 1.2 (mostly sitting), 1.375 (lightly active), 1.55 (training three to five times a week), 1.725 (very active or a physical job) or 1.9 (physical job plus hard training), rounded to the nearest 50.
• Target: maintenance plus 300 for the steady pace or plus 500 for the standard pace, shown as a range of 100 either side because no equation is exact.

Stones and pounds are converted at 6.35kg a stone and 0.45kg a pound, feet and inches at 30.48cm and 2.54cm. The two paces rest on the usual rule of thumb that roughly 7,700 extra calories add a kilogram, so 300 a day is about 0.27kg a week and 500 a day about 0.45kg. Mifflin-St Jeor is used because a 2005 systematic review found it estimated resting calories within 10% of the measured figure in more people than the other common equations. The full explanation of these numbers, and why they vary from person to person, is in my guide to how many calories you need to gain weight.

A worked example

Take a 28-year-old man who is 178cm tall, weighs 62kg (9st 11lb) and trains three to five times a week.

• Resting calories: 10 × 62 = 620, plus 6.25 × 178 = 1,112.5, minus 5 × 28 = 140, plus 5. Total: 1,597.5.
• Maintenance: 1,597.5 × 1.55 = 2,476, which rounds to 2,500 calories a day.
• Steady target: 2,500 + 300 = 2,800, shown as 2,700 to 2,900 calories a day.
• Standard target: 2,500 + 500 = 3,000, shown as 2,900 to 3,100.

On the steady setting he can expect about 0.25kg a week, or roughly 1kg a month. The calculator describes 2,800 as about three 1,000-calorie meals plus snacks and points him to the 3,000 calorie meal plan to see how a day at that level fits together.

How to use your number

Treat it as a starting estimate, not a verdict. Eat within the range for two to three weeks, weigh yourself once a week under the same conditions, and let the scale referee: if it has not moved, add around 200 calories a day; if you are gaining faster than about half a kilo a week, ease back by the same. Re-run the calculator after every 2 to 3kg, because your maintenance rises as you gain. Being consistent counts for far more than being precise, and how to eat in a calorie surplus without obsessing is the guide to holding the habit without letting the numbers run your life.

From there it's execution: 1,000 calorie meals for individual plates, 2,000 calorie meals when a single sitting has to carry more, and daily plans such as the 5,000 calorie meal plan for the full structure. For the everyday side, what to eat and how to fit it into a normal day, my guide to how to gain weight has two worked days. Between meals, weight gain bars are a simple way to top up without cooking. If you're training for muscle, the surplus works alongside the lifting rather than replacing it, and the bulking guide covers that pairing.

Why the calculator only offers 300 or 500 extra calories

Because 300 to 500 a day is the range the NHS suggests for adults gaining weight, and it is a pace most people can keep up for months. At that level, with strength training, a fair share of what you gain can be muscle. Push the surplus well past it and a bigger share is stored as fat, while the eating itself gets much harder. If you do want a faster pace, my guide to gaining weight fast sets out the trade-offs.

When this calculator is the wrong tool

There are three cases. Under 18, your calorie needs include growth, and no adult formula handles that; your GP or a dietitian is the right guide. During pregnancy or breastfeeding, weight change belongs with your midwife or GP, not a formula on a food brand's website. And if numbers around food feel harmful to you right now, skip the calculator entirely; the gentler starting points are what to eat when your appetite is low and how to increase your appetite. If you have been losing weight without trying, see your GP before any of this.

When the number is bigger than your appetite

For many people the hardest part of the result is the gap between the target and what they can face eating. Two things help: food that carries a lot of calories in a small volume, and calories you can drink. My guides to high calorie low volume foods and homemade gainer shakes cover both.

A bar is another option. Phoenix Bars, which I make, have up to 557 calories in only 120g, with up to 66g of carbohydrate and 19g of protein, and they are vegan and gluten-free. One covers either surplus setting without any cooking. If you would like to compare other bars, I've ranked the best energy bars for weight gain.

Frequently asked questions

How many calories do I need to gain weight? Your maintenance plus 300 to 500 a day for most adults, which typically lands between about 2,300 and 3,500 depending on size, sex and activity. The calculator above gives your personal estimate.

How many calories should a woman eat to gain weight? About 2,300 to 2,500 a day for an average woman, based on the NHS maintenance figure of 2,000 plus a 300 to 500 surplus. Smaller or less active women often need less: a 25-year-old who is 163cm, weighs 50kg and is lightly active gets a steady target of about 2,000 from this calculator.

How many calories does it take to gain 1kg? Roughly 7,700 extra calories in total. At the calculator's 500-a-day setting that takes about two weeks; at 300 a day, nearly four. The rule is approximate, and my guide to how many calories to gain weight explains why.

How accurate is a weight gain calculator? Good enough to start from, not exact. Mifflin-St Jeor lands within 10% of measured resting calories more often than other common equations, which is why results here show as a range. Your scale over two to three weeks is the real answer; adjust by about 200 calories based on what it says.

Why am I not gaining on my calculated number? Usually one of three things: the estimate ran low for your body, you're burning more than the activity setting assumes, or intake is falling short of the target on busy days. Add 200 and hold it for two more weeks before concluding anything. If busy days are the gap, I've compared the best meal replacement bars in the UK for when cooking won't happen.

Do I need a protein target too? If you're training for muscle, the International Society of Sports Nutrition suggests 1.4 to 2.0g of protein per kilogram of body weight a day, spread across meals. For weight gain generally, the calorie target leads and protein supports. The protein calculator for weight gain works out your figure.

This tool gives general estimates for healthy adults, not medical advice. If you have a question about your result, email me at jfrost@flaming-phoenix.co.uk.

Sources: Mifflin and others, American Journal of Clinical Nutrition, 1990; Frankenfield and others, Journal of the American Dietetic Association, 2005; NHS, Healthy ways to gain weight; NHS, Understanding calories; International Society of Sports Nutrition position stand on protein, 2017.