There isn’t a “protein‑specific” equation inside either Mifflin‑St Jeor or Katch‑McArdle.
Those two equations only predict BMR (Basal Metabolic Rate)—i.e., the calories you burn at rest.

How practitioners link BMR formulas to daily‑protein targets

1. Step 1 – Estimate energy needs

   • Use Mifflin‑St Jeor or Katch‑McArdle to get BMR, then multiply by an activity factor to find TDEE.

2. Step 2 – Choose a protein‑percentage (or grams / kg) appropriate to the goal

   • Muscle gain / heavy training → ≈ 25–30 % of total calories

   • Maintenance / moderate training → ≈ 15–25 %

   • Fat‑loss phase → ≈ 25–35 % (to preserve lean mass)

3. Step 3 – Convert that calorie slice to grams of protein

   Protein (g)  =  (TDEE×Protein %)/4

   (Protein supplies 4 kcal · g⁻¹.)

Example using both formulas

1) Mifflin‑St Jeor

BMR ≈ 1 759 kcal  
TDEE ≈ 1 759 × 1.55 = 2 727 kcal
Protein ≈ (2 727 × 0.25) / 4 ≈ 170 g day⁻¹

2) Katch‑McArdle

Lean mass = 75 × (1 – 0.15) = 63.75 kg
BMR ≈ 370 + 21.6 × 63.75 = 1 744 kcal
TDEE ≈ 1 744 × 1.55 = 2 703 kcal
Protein ≈ (2 703 × 0.25) / 4 ≈ 169 g day⁻¹

Result: both routes land near 170 g protein per day.

Quick rule if you don’t want to bother with calories

Many coaches skip the calorie math and give protein directly per kilogram of body‑weight or lean mass:

So, while Mifflin‑St Jeor and Katch‑McArdle do not contain protein math, you can pair either BMR estimate with a protein percentage (or g / kg table) to build a “protein calculator.”