How to gain weight in a healthy way
Gaining weight can be as hard for some people as losing it is for others. A small appetite, a fast metabolism, stress, illness, a physically demanding job or simply forgetting to eat can all keep weight low. Being underweight (a BMI under 18.5) is linked with low energy, weak immunity, hair loss, irregular periods, fertility problems and thinning bones. The aim is not to gain weight quickly with junk food, but to add healthy muscle and some fat steadily. This free weight gain calculator shows how many calories you need and how long it may take.
How the weight gain calculator works
- Maintenance calories: your BMR (Mifflin-St Jeor equation) × your activity level.
- Calorie surplus: about 7,700 kcal builds 1 kg of body weight, so gaining 0.25 kg a week needs about +275 kcal a day and 0.5 kg a week about +550 kcal a day.
- Realistic timeline: the calculator recalculates your needs day by day — as you gain weight your body burns a little more, so progress slows slightly.
Example: a 25-year-old man, 180 cm, 60 kg, moderately active: maintenance ≈ (600 + 1,125 − 125 + 5) × 1.55 ≈ 2,488 kcal. To gain 0.25 kg a week he would eat about 2,760 kcal a day; reaching 65 kg takes roughly 5 months.
Weight gain pace: slow, healthy and too fast
| Rate of gain | Daily surplus | Level | What to expect |
|---|---|---|---|
| Under 0.25 kg a week | Under 250 kcal | Slow | Mostly lean gain; good for people close to goal or athletes |
| 0.25 – 0.5 kg a week | 250 – 550 kcal | Recommended | Steady, mostly healthy gain with strength training |
| Over 0.5–1 kg a week | Over 550 kcal | Too fast | More of the gain is fat; digestive discomfort; harder to keep up |
Benefits of gaining weight healthily
- More energy and strength for daily life and exercise.
- Stronger immunity and faster recovery from illness.
- Healthier hormones — regular periods and better fertility in women.
- Stronger bones and lower risk of osteoporosis.
- Better mood and concentration.
Health warnings
- Unexplained weight loss or difficulty gaining weight can be caused by thyroid problems, diabetes, coeliac disease, inflammatory bowel disease, infections or other conditions — see a doctor.
- If you have been eating very little for a long time, increase food gradually under medical guidance to avoid refeeding syndrome.
- Avoid “mass gainer” products high in sugar, and do not rely on fast food — it adds fat around the organs rather than healthy weight.
- If worries about weight, food or body shape are affecting your life, please talk to a health professional.
Easy foods for healthy weight gain
Add calories without huge portions: full-fat yoghurt and laban, milk-based smoothies with banana, dates and peanut butter, nuts and seeds, olive oil drizzled on meals, eggs, hummus, avocado, paneer, cheese, dried fruit, and an extra roti or rice portion. Eat every 3 hours and drink fluids after rather than before meals so you have room to eat. Hit your protein target and read our guide to being underweight.
Frequently asked questions
How many calories do I need to gain weight?
Eat about 250–500 kcal a day more than your maintenance calories. This usually leads to a gain of 0.25–0.5 kg a week. The calculator gives your personal number.
How long does it take to gain 5 kg?
At a steady 0.25–0.5 kg a week, gaining 5 kg takes about 10–20 weeks (2.5–5 months), depending on your surplus and activity.
What should I eat to gain weight fast but healthily?
Choose energy-dense, nutritious foods: nuts, nut butters, dates, full-fat dairy, eggs, olive oil, avocado, hummus, smoothies and extra wholegrain portions, with protein at every meal.
Will I gain fat or muscle?
Combining a moderate calorie surplus with strength training 2–3 times a week and enough protein means a larger share of the gain is muscle.
Why can’t I gain weight even when I eat a lot?
Many people overestimate how much they eat, or have a high activity level. Medical causes such as an overactive thyroid or digestive problems are also possible — see a doctor if you are concerned.
Sources
- Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet 2011;378:826–837.
- Mifflin MD, St Jeor ST, et al. Am J Clin Nutr 1990.
- NICE. Nutrition support for adults (CG32) — refeeding syndrome.
- NHS. Underweight adults – healthy ways to gain weight.