Weight Gain Calculator — Fixed Scenarios

Current mass, hypothetical target mass and duration define a constant-rate scenario. The result does not forecast an individual change in weight, muscle or fat, establish a safe rate, or provide a treatment plan.

The resting-energy reference uses the simplified Mifflin–St Jeor equation published in 1990, derived in healthy adults aged 19–78. The worksheet limits that reference to this age range; age alone does not establish individual suitability.

Reference resting energy (kcal/day) = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (years) + 5 for the male coefficient, or −161 for the female coefficient. This predicts resting energy, not recommended intake.

The selected local factors 1.2, 1.375, 1.55, 1.725 and 1.9 are scenario assumptions. The Mifflin paper does not validate these activity factors or the complete weight-gain model. The reference × factor is rounded before adding the rounded surplus.

Modeled mass difference (kg) = surplus (kcal/day) × duration (days) ÷ 7,700 (kcal/kg).

Each month represents 4.33 weeks. For 60 → 70 kg over 5 months: 10 ÷ (5 × 4.33) = 0.4619 kg/week; 0.4619 × 7,700 ÷ 7 rounds to 508 kcal/day. For age 30, height 175 cm, male coefficient and factor 1.55: resting reference 1,548.75 × 1.55 rounds to 2,401 kcal/day. Adding 508 produces the illustrative budget of 2,909 kcal/day.

With the local 7,700 kcal/kg assumption, 0.25, 0.5 and 0.75 kg/week correspond to 275, 550 and 825 kcal/day. These are arithmetic comparisons, not safety grades or predictions of tissue composition.

NHS adult guidance separately suggests trying 300–500 additional kcal/day. That guidance is not an exact weekly mass conversion. Real responses change with metabolism and other factors.

The six meal labels demonstrate dividing the scenario budget. The calculation first rounds budget ÷ 6, then multiplies by 1.2, 0.6, 1.3, 0.7, 1.4 and 0.8 and rounds each amount. The multipliers sum to 6; sequential rounding can make the final sum slightly different from the budget.

Breakfast 20%, morning snack 10%, lunch 21⅔%, afternoon snack 11⅔%, dinner 23⅓% and evening snack 13⅓% describe the unrounded allocation. They are local illustrations, not required meal frequency, food quantities or a personal nutrition plan. Oatmeal with fruit, rice with vegetables and a smoothie are examples of food names, not calorie-verified portions.

The selected factor changes arithmetic; the worksheet does not measure training, determine an exercise frequency or prescribe sets and repetitions. It cannot establish how exercise will divide a mass change between muscle and fat.

• Do not treat the scenario energy budget as measured maintenance energy or prescribed intake.
• Do not interpret an entered target or duration as a safe or achievable plan.
• Do not infer a food portion or protein target from the illustrative meal labels.
• The fixed coefficient does not account for metabolic adaptation, pregnancy, childhood, illness recovery or eating disorders.

• Mifflin–St Jeor, February 1990: resting-energy equation
• Hall et al., 2011: dynamic energy balance and limits of static predictions
• NHS: Healthy ways to gain weight, reviewed 2023-03-28

Sources accessed and assumptions checked: 2026-09-30. These sources do not validate the site's local activity or meal factors.