{
  "id": "weight-gain",
  "version": "424fe7c1be9e",
  "status": "published",
  "name": "Weight Gain Calculator",
  "question": "How long will my weight gain take?",
  "summary": "Estimates how long it takes to reach a goal weight eating a fixed number of calories a day above your needs, with daily energy needs (Mifflin-St Jeor or Harris-Benedict × activity) rising as weight rises.",
  "category": "health",
  "subcategory": "nutrition",
  "url": "https://www.acalculator.org/health/weight-gain-calculator",
  "markdown": "https://www.acalculator.org/health/weight-gain-calculator.md",
  "kind": "function",
  "method": "Days to goal = (7,716.2 ÷ s) × ln(S₀ ÷ S_g), where S₀ and S_g are the daily surpluses (intake − TDEE) at the current and goal weights and s is the rise in TDEE per kilogram (activity factor × BMR weight coefficient).",
  "assumptions": [
    "A pound of body weight is 3,500 calories (7,716.2 calories a kilogram).",
    "Daily needs are the BMR (Mifflin-St Jeor or revised Harris-Benedict) × the activity factor, recomputed continuously as weight rises; age, height and activity stay as entered.",
    "The daily intake stays the same until the goal. The goal date rounds the days up to a whole day.",
    "This is an estimate for planning, not medical advice. Talk to a doctor or a registered dietitian if you need to gain weight for your health."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "sex": {
        "title": "Sex",
        "description": "Sex at birth, which the body formulas use.",
        "type": "string",
        "enum": [
          "male",
          "female"
        ]
      },
      "age": {
        "title": "Age",
        "description": "Your age in whole years.",
        "type": "integer",
        "minimum": 18,
        "maximum": 100
      },
      "height": {
        "title": "Height",
        "description": "Your height without shoes.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "length",
        "minimum": 0.9,
        "maximum": 3
      },
      "weight": {
        "title": "Weight",
        "description": "Your body weight.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "mass",
        "minimum": 25,
        "maximum": 700
      },
      "activity": {
        "title": "Activity",
        "description": "How active you are in a usual week, which sets the multiplier on your BMR.",
        "type": "string",
        "enum": [
          "sedentary",
          "light",
          "moderate",
          "very",
          "extra"
        ]
      },
      "goal": {
        "title": "Goal weight",
        "description": "The weight you want to reach.",
        "type": [
          "number",
          "string"
        ],
        "x-quantity": "mass",
        "minimum": 25,
        "maximum": 700
      },
      "intake": {
        "title": "Calories you eat a day",
        "description": "The calories you plan to eat each day until you reach your goal.",
        "type": "number",
        "minimum": 500,
        "maximum": 10000
      },
      "start": {
        "title": "Start date",
        "description": "The day you start. Today, unless you change it.",
        "type": "string",
        "format": "date"
      },
      "formula": {
        "title": "BMR formula",
        "description": "The equation for basal metabolic rate: Mifflin-St Jeor (1990) or the revised Harris-Benedict (1984).",
        "type": "string",
        "enum": [
          "mifflin",
          "harris"
        ]
      }
    }
  },
  "outputs": {
    "date": {
      "label": "You reach your goal on",
      "description": "The date you reach the goal weight: the start date plus the days to the goal, rounded up.",
      "format": "date"
    },
    "weeks": {
      "label": "Weeks to your goal",
      "description": "Time to the goal weight in weeks, with daily needs rising as weight rises.",
      "format": "number"
    },
    "days": {
      "label": "Days to your goal",
      "description": "Time to the goal weight in whole days, rounded up.",
      "format": "integer"
    },
    "tdee": {
      "label": "Calories you burn now (TDEE)",
      "description": "Total daily energy expenditure at your current weight: BMR × activity factor.",
      "format": "number"
    },
    "surplus": {
      "label": "Daily surplus now",
      "description": "The calories you eat minus your TDEE at your current weight.",
      "format": "number"
    },
    "tdeeGoal": {
      "label": "Calories you burn at your goal weight",
      "description": "TDEE at the goal weight, which is higher, so the surplus shrinks as you gain.",
      "format": "number"
    },
    "poundWeek": {
      "label": "Calories for about 1 lb a week now",
      "description": "TDEE now + 500: a 3,500-calorie surplus a week, a pound a week at the start by the 3,500-calorie rule.",
      "format": "number"
    },
    "firstWeek": {
      "label": "Gain in the first week",
      "description": "Weight gained in the first 7 days.",
      "format": "quantity",
      "unit": "lb"
    },
    "simpleWeeks": {
      "label": "Weeks by the 3,500-calorie rule",
      "description": "Weeks to the goal if the surplus stayed at its starting size.",
      "format": "number"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "sex": "female",
      "age": 30,
      "height": 1.651,
      "weight": 54.4310844,
      "activity": "light",
      "goal": 58.9670081,
      "intake": 2200,
      "start": "2026-09-30",
      "formula": "mifflin"
    },
    "outputs": {
      "date": "2026-12-21",
      "weeks": 11.6705550565,
      "days": 82,
      "tdee": 1739.6305355,
      "surplus": 460.3694645,
      "tdeeGoal": 1801.99948638,
      "poundWeek": 2239.6305355,
      "firstWeek": 0.415046171932,
      "simpleWeeks": 10.8608419662
    },
    "text": "Eating 2,200 calories a day, you reach your goal weight in about 11.7 weeks, on December 21, 2026."
  },
  "examples": [
    {
      "given": {
        "sex": "female",
        "age": 30,
        "height": 1.651,
        "weight": 54.4310844,
        "activity": "light",
        "formula": "mifflin",
        "goal": 58.9670081,
        "intake": 2200,
        "start": "2026-10-01"
      },
      "expect": {
        "tdee": 1739.6305355000002,
        "surplus": 460.3694644999998,
        "tdeeGoal": 1801.9994863750003,
        "poundWeek": 2239.6305355000004,
        "weeks": 11.670555056510436,
        "days": 82,
        "date": "2026-12-22",
        "firstWeek": 0.4150461719318276,
        "simpleWeeks": 10.860841966202996
      },
      "source": "Mifflin et al. 1990; 3,500 kcal per lb (Wishnofsky 1958); closed-form solution in content.mdx"
    },
    {
      "given": {
        "sex": "male",
        "age": 25,
        "height": 1.78,
        "weight": 65,
        "activity": "moderate",
        "formula": "mifflin",
        "goal": 70,
        "intake": 3000,
        "start": "2027-01-01"
      },
      "expect": {
        "tdee": 2545.875,
        "surplus": 454.125,
        "weeks": 13.30756391370777,
        "date": "2027-04-05",
        "simpleWeeks": 12.136650822178781
      },
      "source": "Mifflin et al. 1990; 3,500 kcal per lb; hand calculation in content.mdx"
    },
    {
      "given": {
        "sex": "female",
        "age": 40,
        "height": 1.6,
        "weight": 50,
        "activity": "sedentary",
        "formula": "harris",
        "goal": 55,
        "intake": 1900,
        "start": "2026-11-15"
      },
      "expect": {
        "tdee": 1478.9076,
        "weeks": 14.035088215421426,
        "days": 99,
        "date": "2027-02-22"
      },
      "source": "Roza and Shizgal 1984 (revised Harris-Benedict); 3,500 kcal per lb; hand calculation in content.mdx"
    }
  ],
  "sources": [
    "Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247. https://doi.org/10.1093/ajcn/51.2.241",
    "Roza AM, Shizgal HM. The Harris Benedict equation reevaluated: resting energy requirements and the body cell mass. Am J Clin Nutr. 1984;40(1):168-182. https://doi.org/10.1093/ajcn/40.1.168",
    "FAO/WHO/UNU. Human energy requirements (2004), section 5.3.1, Classification of physical activity levels (PAL = TEE ÷ BMR). https://www.fao.org/4/y5686e/y5686e07.htm",
    "Wishnofsky M. Caloric equivalents of gained or lost weight. Am J Clin Nutr. 1958;6(5):542-546. https://doi.org/10.1093/ajcn/6.5.542",
    "Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837. https://doi.org/10.1016/S0140-6736(11)60812-X"
  ],
  "related": [
    "weight-loss",
    "calorie",
    "tdee"
  ],
  "changelog": []
}