Lean Mass Gain Calculator

Calculator inputs

Use the form below to estimate calorie needs, macro targets, and expected lean mass progress from a controlled bulk.

Unit system

Sex

Age (years)

Weight (kg)

Height (cm)

Body fat (%)

Activity factor

Training level

Training days per week

Sleep hours per night

Weekly gain target (% body weight)

Protein target (g/kg)

Fat target (g/kg)

Custom surplus (optional kcal/day)

Reset

Example data table

Sample scenario for a moderately active intermediate lifter using a controlled surplus.

Input or output	Example value
Sex	Male
Age	28 years
Weight	78 kg
Height	178 cm
Body fat	14%
Activity factor	1.55
Training level	Intermediate
Sleep	7.5 hours
Weekly gain target	0.30%
Estimated target calories	3,020 kcal/day
Protein target	156 g/day
Projected lean gain	0.52 kg/month
Projected fat gain	0.31 kg/month

Formula used

1) Lean body mass

Lean Body Mass = Body Weight × (1 − Body Fat % / 100)

2) Basal metabolic rate

BMR = 370 + (21.6 × Lean Body Mass in kg)

3) Total daily energy expenditure

TDEE = BMR × Activity Factor

4) Weekly scale gain target

Weekly Gain in kg = Body Weight × Weekly Gain % / 100

5) Surplus from target gain

Daily Surplus ≈ (Weekly Gain in kg × 7700) / 7

6) Macro targets

Protein = Body Weight × Protein Factor

Fat = Body Weight × Fat Factor

Carbs = (Target Calories − Protein Calories − Fat Calories) / 4

7) Lean gain projection

Projected lean gain is limited by training status, sleep, age, body fat range, and the chosen surplus. The calculator estimates a realistic monthly ceiling, then partitions total monthly gain into lean and fat mass.

How to use this calculator

Choose metric or imperial units.
Enter body weight, height, age, and body fat percentage.
Select your activity level and training experience.
Add training days, sleep hours, and your weekly gain target.
Set protein and fat targets in grams per kilogram.
Leave custom surplus blank for an automatic recommendation.
Press the calculate button to show the result above the form.
Review calories, macros, lean gain pace, and the Plotly projection chart.
Download the result as CSV or PDF when needed.

FAQs

1) What does lean mass gain mean here?

It refers to projected increases in lean tissue during a calorie surplus. The estimate tries to separate likely lean gain from likely fat gain using body composition, recovery, and training status.

2) Why does the calculator ask for body fat percentage?

Body fat helps estimate lean body mass, which improves BMR and FFMI calculations. It also affects how efficiently extra calories may support muscle gain rather than fat storage.

3) Is the calorie surplus always the recommended one?

No. You can leave the custom surplus blank to use the recommended value, or enter your own daily surplus. Higher surpluses usually increase total gain faster, but often reduce lean efficiency.

4) Why is my projected lean gain lower than expected?

Advanced trainees gain muscle more slowly than beginners. Low sleep, very low or high body fat, older age, and aggressive gain targets can also reduce how much of the monthly gain is likely to be lean.

5) Should I use actual protein intake or a target?

Use the protein target you plan to follow. The calculator turns that intake into daily macro goals. Many lifters bulk effectively in the 1.6 to 2.2 g/kg range.

6) What is FFMI and why is it included?

FFMI stands for Fat-Free Mass Index. It relates lean body mass to height, giving a quick way to compare muscularity across different body sizes more fairly than body weight alone.

7) Can I use this during a recomp phase?

You can, but it is designed for lean bulking. Recomposition often uses maintenance calories or a smaller surplus, so the gain forecasts may look more conservative than a dedicated recomp model.

8) How often should I update my inputs?

Update every two to four weeks, or sooner if body weight changes quickly. Recalculate after major shifts in training volume, activity, sleep quality, or body fat estimates.