acalculator

How long will my weight gain take?

Enter your body details, goal weight and the calories you plan to eat each day. See how many weeks it takes to reach your goal, the date, and how many calories you burn now.

Your numbers

Units
Sex
Multiplies your BMR by 1.2, 1.375, 1.55, 1.725 or 1.9.
More options
BMR formula
You reach your goal on
December 21, 2026

Eating 2,200 calories a day, you reach your goal weight in about 11.7 weeks, on December 21, 2026.

Weeks to your goal
11.7
Days to your goal
82
Calories you burn now (TDEE)
1,740
Daily surplus now
460
Calories you burn at your goal weight
1,802
Calories for about 1 lb a week now
2,240
Gain in the first week
0.9 lb
Weeks by the 3,500-calorie rule
10.9

Answer for the example date Wednesday, September 30, 2026. It changes to today's date when the page loads.

You reach your goal on: December 21, 2026. Eating 2,200 calories a day, you reach your goal weight in about 11.7 weeks, on December 21, 2026.

Weeks to your goal by calories you eat a day

This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use

How to calculate

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.

Example with the default inputs (Sex Female, Age 30, Height 5 ft 5 in, Weight 120 lb, Activity Light: exercise 1 to 3 days, Goal weight 130 lb, Calories you eat a day 2,200, Start date September 30, 2026, BMR formula Mifflin-St Jeor) on the example date Wednesday, September 30, 2026: Eating 2,200 calories a day, you reach your goal weight in about 11.7 weeks, on December 21, 2026.

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).

  • 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.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Sex Female, Age 30, Height 5 ft 5 in, Weight 120 lb, Activity Light: exercise 1 to 3 days, BMR formula Mifflin-St Jeor, Goal weight 130 lb, Calories you eat a day 2,200, Start date 2026-10-01 gives Calories you burn now (TDEE) 1,739.630536, Daily surplus now 460.369464, Calories you burn at your goal weight 1,801.999486, Calories for about 1 lb a week now 2,239.630536, Weeks to your goal 11.670555, Days to your goal 82, You reach your goal on 2026-12-22, Gain in the first week 0.9 lb, Weeks by the 3,500-calorie rule 10.860842.Source: Mifflin et al. 1990; 3,500 kcal per lb (Wishnofsky 1958)
  2. Sex Male, Age 25, Height 5 ft 10.1 in, Weight 143.3 lb, Activity Moderate: exercise 3 to 5 days, BMR formula Mifflin-St Jeor, Goal weight 154.3 lb, Calories you eat a day 3,000, Start date 2027-01-01 gives Calories you burn now (TDEE) 2,545.875, Daily surplus now 454.125, Weeks to your goal 13.307564, You reach your goal on 2027-04-05, Weeks by the 3,500-calorie rule 12.136651.Source: Mifflin et al. 1990
  3. Sex Female, Age 40, Height 5 ft 3 in, Weight 110.2 lb, Activity Sedentary: little or no exercise, BMR formula Harris-Benedict, Goal weight 121.3 lb, Calories you eat a day 1,900, Start date 2026-11-15 gives Calories you burn now (TDEE) 1,478.9076, Weeks to your goal 14.035088, Days to your goal 99, You reach your goal on 2027-02-22.Source: Roza and Shizgal 1984 (revised Harris-Benedict)

How it works

Daily needs. Your total daily energy expenditure at weight w (kg) is TDEE(w) = f × BMR(w), where f is the activity factor (1.2 sedentary, 1.375 light, 1.55 moderate, 1.725 very active, 1.9 extra active). BMR uses height in centimetres and age in years:

  • Mifflin-St Jeor (default): men 10 × w + 6.25 × height − 5 × age + 5; women 10 × w + 6.25 × height − 5 × age − 161.
  • Revised Harris-Benedict (under "More options"): men 88.362 + 13.397 × w + 4.799 × height − 5.677 × age; women 447.593 + 9.247 × w + 3.098 × height − 4.330 × age.

Call a the weight coefficient of the chosen formula: 10 for Mifflin-St Jeor, 13.397 (men) or 9.247 (women) for Harris-Benedict. Each kilogram you gain raises your TDEE by s = f × a calories a day.

Energy balance. A kilogram of body weight is taken as K = 3,500 ÷ 0.45359237 = 7,716.18 calories (3,500 per pound; Wishnofsky, 1958). You eat I calories a day. The daily surplus is S = I − TDEE(w), and weight rises at S ÷ K kilograms a day. Because S shrinks by s for each kilogram gained, the surplus falls smoothly:

S(t) = S₀ × e^(−t ÷ τ), with τ = K ÷ s days,

where S₀ = I − TDEE(current weight) is the surplus on day 0 and t is in days.

Time to the goal. At the goal weight g the surplus is S_g = S₀ − s × (g − current weight). Then:

days to goal = τ × ln(S₀ ÷ S_g)

  • Weeks to goal = days ÷ 7.
  • The goal date is the start date plus the days, rounded up to a whole day.
  • TDEE at the goal weight = I − S_g.
  • Gain in the first week = (S₀ ÷ s) × (1 − e^(−7 ÷ τ)).
  • Calories for about 1 lb a week now = TDEE(current weight) + 500 (3,500 calories a week).
  • For comparison, weeks by the 3,500-calorie rule = (g − current weight) × K ÷ S₀ ÷ 7, as if the surplus stayed at S₀.

No answer when the goal is not above your current weight; when S₀ is 0 or less (you would not gain weight at this intake); when S_g is 0 or less (your weight levels off at current weight + S₀ ÷ s, below the goal); or when it would take more than 3,652 days (10 years).

The chart shows the weeks to your goal for daily intakes from 1,500 to 4,000 calories, with everything else as you entered it.

Rounding. Results are shown rounded half up from the decimal value, so a TDEE of 1,774.5 shows as 1,775. A computer’s arithmetic can leave a result such as 31.5 stored as 31.499999999999996, so each result is first rounded to 12 significant digits, which gives back the decimal value, and is then shown rounded half up. The goal date rounds the days up, as above.

Assumptions

  • The model is a simple energy balance. Real bodies adapt to overfeeding too (Hall and others, 2011), so real gain can differ.
  • Age, height and activity stay as entered for the whole time.
  • The formulas are for adults. They do not apply to children, or during pregnancy or breastfeeding.
  • Nothing is rounded until the result is shown, except the goal date, which rounds the days up.
  • This calculator gives an estimate for planning. It is not medical advice.

Worked examples by hand

A 30-year-old woman, 5 ft 5 in (165.1 cm), lightly active, from 120 lb (54.431 kg) to 130 lb (58.967 kg) on 2,200 calories a day, starting October 1, 2026. BMR = 544.31 + 1,031.88 − 150 − 161 = 1,265.19. TDEE = 1,265.19 × 1.375 = 1,739.63. S₀ = 2,200 − 1,739.63 = 460.37. s = 1.375 × 10 = 13.75, so τ = 7,716.18 ÷ 13.75 = 561.18 days. She gains 10 lb = 4.536 kg, so S_g = 460.37 − 13.75 × 4.536 = 398.00 (her TDEE at 130 lb is 1,802.00). Days = 561.18 × ln(460.37 ÷ 398.00) = 81.69, which is 11.67 weeks. Rounded up to 82 days, she reaches 130 lb on December 22, 2026. First week: (460.37 ÷ 13.75) × (1 − e^(−7 ÷ 561.18)) = 0.415 kg (0.92 lb). By the 3,500-calorie rule it would be 10 × 3,500 ÷ 460.37 ÷ 7 = 10.86 weeks. For about a pound a week at the start she would eat 1,739.63 + 500 = 2,240 calories.

A 25-year-old man, 178 cm, moderately active, from 65 kg to 70 kg on 3,000 calories a day, starting January 1, 2027. BMR = 650 + 1,112.5 − 125 + 5 = 1,642.5. TDEE = 1,642.5 × 1.55 = 2,545.875. S₀ = 454.125. s = 15.5, τ = 497.82 days. S_g = 454.125 − 15.5 × 5 = 376.625. Days = 497.82 × ln(454.125 ÷ 376.625) = 93.15, 13.31 weeks; 94 days after January 1, 2027 is April 5, 2027. The 3,500-calorie rule gives 12.14 weeks.

Harris-Benedict: a 40-year-old woman, 160 cm, sedentary, from 50 kg to 55 kg on 1,900 calories a day, starting November 15, 2026. BMR = 447.593 + 462.35 + 495.68 − 173.2 = 1,232.42. TDEE = 1,232.42 × 1.2 = 1,478.91. S₀ = 421.09. s = 1.2 × 9.247 = 11.096, τ = 695.38 days. S_g = 421.09 − 11.096 × 5 = 365.61. Days = 695.38 × ln(421.09 ÷ 365.61) = 98.25, 14.04 weeks, rounded up to 99 days: February 22, 2027.

Other questions people ask

How many calories do I need to gain weight?

More than you burn. Your total daily energy expenditure (TDEE) is your basal metabolic rate times an activity factor; eating above it gives a surplus. By the 3,500-calorie rule, 500 calories a day above your TDEE is about a pound a week at the start. The calculator shows that number as "Calories for about 1 lb a week now".

How long will it take to reach my goal weight?

It depends on the surplus. For example, a 30-year-old woman who is 5 ft 5 in, 120 lb and lightly active burns about 1,740 calories a day. Eating 2,200 a day, she reaches 130 lb in about 11.7 weeks.

Why is this slower than the 3,500-calorie rule?

A heavier body burns more calories, so as you gain, the same intake gives a smaller surplus each week. This calculator lets your needs rise as your weight rises, so the time to your goal is longer than the simple rule says.

Why does it say my weight levels off before my goal?

At a fixed intake, your weight moves toward the weight at which you burn exactly what you eat. If that weight is below your goal, you would never reach the goal at that intake. Eat more, or check your activity level.

Is this for muscle gain or fat gain?

The maths counts body weight only. How much of the gain is muscle depends on training, protein and many other things this model does not include.

Who should get advice before trying to gain weight?

Anyone who is underweight without knowing why, has lost weight without trying, or has a medical condition. A doctor or registered dietitian can check the cause and suggest a safe plan.