Weight Gain Calculator

Estimate the calories you need to reach your weight and muscle‑gain goals. Use this calculator to convert your current weight, height, age and activity level into a daily calorie target and a realistic rate of gain.

A weight gain calculator is a calorie calculator that estimates how many calories you should eat to reach a target body weight. It uses your current weight, height, age and activity level to estimate your basal metabolic rate (BMR), adjusts that for your daily energy expenditure (activity/TDEE) and then adds a recommended calorie surplus to support steady weight gain.

How the calculator works (brief)

  • Step 1 — BMR: the tool estimates your BMR (the calories your body needs at rest) using standard formulas such as Mifflin–St Jeor.
  • Step 2 — Activity multiplier: it multiplies BMR by an activity factor based on your reported activity level to get total daily energy expenditure (TDEE).
  • Step 3 — Calorie surplus: the calculator adds a sensible surplus to TDEE (for example ~250–500 kcal/day) to estimate the calorie intake required to gain weight at a chosen rate (calories per day and calories per week).
  • Step 4 — Results and planning: the tool converts the calorie difference into an estimated rate of weight gain (for example ~0.25–0.5 kg/week depending on surplus and individual factors) and suggests targets for calorie intake, meals and macronutrient priorities.

Example: a 25‑year‑old, 70 kg, 175 cm person with moderate activity — the calculator estimates BMR, multiplies by a moderate activity level, then adding ~300 kcal/day may give a daily calorie target that should support gradual weight and muscle gain over weeks; individual results vary.

Note: these outputs are estimates. The calculator gives a useful starting point for your nutrition plan, but personal needs (medical conditions, medications, body fat percentage, training status) will affect the exact calorie and macronutrient needs — see the disclaimer and consult a specialist for personalised advice.

Risks associated with being underweight

Being underweight can have negative consequences for health as well as being overweight. Low body weight and low body fat can affect energy, immunity, muscle mass and long‑term wellbeing. Some of the common risks linked with being underweight are listed below, with brief explanations of the likely mechanisms.

  • Malnutrition: Consuming too few calories or poor‑quality food can cause deficiencies in vitamins and minerals (for example iron, vitamin D and B vitamins), which undermines overall health and energy levels.
  • Weakened immune system: Insufficient calorie and protein intake impairs immune function, making infections more likely and slowing recovery.
  • Decreased muscle mass: If energy intake is too low the body may break down muscle for fuel. Reduced muscle mass lowers strength, reduces metabolic rate and can limit exercise capacity.
  • Lower bone density and osteoporosis risk: Chronically low nutrition, especially inadequate calcium and vitamin D, is associated with reduced bone mineral density and a higher long‑term fracture risk.
  • Fertility and hormonal effects: Very low body fat and energy availability can disrupt hormonal balance — in women this may cause irregular periods and difficulty conceiving; in men it can reduce testosterone and sperm quality.
  • Delayed growth and development: Children and adolescents who do not meet energy and nutrient needs may experience slower growth and delayed development, with lasting consequences.
  • Mental health and wellbeing: Chronic under‑eating is linked with low mood, anxiety and reduced self‑esteem; it can also be associated with eating disorders that need specialist support.

How to gain weight in a healthy way?

Below are evidence‑based, practical steps to increase calorie intake and gain weight while prioritising muscle, nutrition and long‑term health.

  • Eat a calorie surplus consistently: To gain weight you must consume more calories than your daily energy expenditure. Aim for a moderate surplus (about +250–500 kcal/day) to support steady gain of around 0.25–0.5 kg per week; larger surpluses lead to faster gain but more fat.
  • Prioritise nutrient‑dense foods: Add energy‑dense, nutritious items rather than relying on ultra‑processed foods. Good choices include whole grains, dairy or fortified alternatives, oily fish, nuts, seeds, avocados and starchy vegetables to raise calorie intake without sacrificing vitamins and minerals.
  • Increase protein for muscle gain: Aim for roughly 1.6–2.2 g protein per kg body weight per day (from chicken, fish, eggs, dairy, beans, lentils or plant protein blends). Higher protein intake helps convert extra calories into muscle when combined with resistance training.
  • Include healthy fats to boost calories: Fats provide many calories in small portions — add nuts, nut butters, seeds, olive oil and oily fish to meals and snacks to increase calorie intake without overly large meal volumes.
  • Structure meals and snacks: Aim for 3 main meals plus 1–3 calorie‑dense snacks (e.g., yoghurt with granola, smoothies made with milk/plant milk, oats and nut butter). Tracking number of meals and overall calorie intake for a few weeks helps you hit targets consistently.
  • Strength‑train regularly: Focus on resistance exercise 3–4 times per week, progressive overload (3–5 sets of 6–12 reps for major lifts) to maximise muscle gain rather than fat. Track performance — increasing lifts usually indicates lean mass gain.
  • Use the calculator and monitor results: Use the calorie calculator to set an initial calorie intake and expected rate of gain, then track body weight and body fat percentage weekly. If gain is too slow, increase intake by 100–200 kcal/day; if too fast and mostly fat, reduce the surplus.
  • Tailor your plan to goals and health: If your priority is muscle, prefer a smaller surplus with structured training; if rapid weight restoration is needed for health reasons, a larger, supervised increase may be appropriate. Consider special diets (vegetarian/vegan) and allergies when choosing foods.
  • Seek professional support: If you struggle to gain, have underlying health issues or suspect an eating disorder, consult a registered dietitian, GP or qualified trainer to create a customised plan and monitor progress safely.

Worked example (quick)

Example: a 70 kg person with moderate activity has a TDEE of ~2,500 kcal. Adding a 300 kcal surplus gives a target intake of ~2,800 kcal/day — about +2,100 kcal/week — which typically supports a gradual gain of roughly 0.25–0.5 kg per week depending on training and individual response. Use the calorie calculator to get personalised numbers and then adjust based on weekly results.

If you’d like, download a simple 7‑day sample meal plan or try the calculator now to get a tailored daily calorie target and meal suggestions to meet your weight and muscle‑building goals.

Disclaimer: The figures produced by this weight gain calculator are estimates intended to help you plan calorie intake and progress towards your weight and muscle‑building goals. They are based on standard equations (BMR → activity multiplier → calorie surplus) and general population averages, so individual needs will vary according to medical conditions, medications, body fat percentage, age, height and training status.

Before starting any major change to your diet or exercise routine, consult a registered dietitian, GP or other qualified healthcare professional for personalised advice. If you have a medical condition, are pregnant, breastfeeding or suspect an eating disorder, seek specialist support.

What to monitor

  • Body weight — record once weekly at the same time of day.
  • Body composition — measure body fat percentage every 2–4 weeks if possible to check muscle vs fat gain.
  • Strength and performance — track lifts and exercise progress to confirm muscle improvements.
  • Symptoms and wellbeing — note energy, sleep, mood and any digestive or health changes.

If you want a tailored plan, consider downloading a sample 7‑day meal plan or booking time with a dietitian who can convert the calculator results into a customised diet and training programme. For privacy and data handling details, see our privacy policy.