Katch-McArdle vs Mifflin-St Jeor: Which Formula Should You Trust?

When two calorie formulas produce different numbers, it is easy to assume one must be correct and the other must be wrong. That is usually the wrong frame.

Both Katch-McArdle and Mifflin-St Jeor are estimation tools. The better formula depends less on internet arguments and more on the quality of the information you can provide.

What Mifflin-St Jeor uses

Mifflin-St Jeor estimates resting energy needs from:

• body weight
• height
• age
• sex

That makes it practical for most people because those inputs are easy to provide. If you do not have a reliable body-fat estimate, this formula is usually the cleaner choice.

What Katch-McArdle uses

Katch-McArdle works from lean body mass, not just total scale weight. In theory, that can make it more tailored when body composition differs meaningfully between people at the same body weight.

The catch is obvious: the formula is only as useful as your body-fat estimate.

When Katch-McArdle can help

It can be helpful when:

• you already have a reasonably trustworthy body-fat estimate
• body composition is an important reason your scale weight may be misleading
• you want a lean-mass-aware comparison against a standard weight-based formula

For example, someone with higher lean mass may prefer to see how a lean-mass-aware formula changes the estimate.

When Mifflin-St Jeor is the safer default

Mifflin-St Jeor is usually the safer default when:

• you do not know your body-fat percentage
• your body-fat estimate comes from a rough visual guess
• you want fewer moving parts in the input flow
• speed and consistency matter more than theoretical precision

This is why many calculators make Mifflin-St Jeor the standard path and only branch when body-fat input is available.

The hidden problem: bad body-fat data

People often trust body-fat numbers more than they should. Consumer devices, mirror guesses, and casual estimates can be noisy. If the body-fat input is weak, a formula that depends on lean mass can look more advanced while actually becoming less reliable.

In that case, the more sophisticated formula is not the better one.

Formula choice does not remove the need for calibration

This is the part people skip. Even if you choose the better formula for your situation, you still do not get an exact maintenance number. Activity reporting, adherence, and individual variation still matter.

Formula selection can improve the starting point, but it cannot replace feedback from:

• body-weight trend
• training response
• recovery
• appetite

Which one should you trust?

Use Mifflin-St Jeor when you want a reliable default with common inputs. Use Katch-McArdle when you have a body-fat estimate you actually trust and you want a lean-mass-aware result.

If both formulas land near the same range, that is often a useful signal that your estimate is directionally sound. If they are far apart, it is a sign to be cautious, not a reason to blindly pick the higher or lower number.

Bottom line

The best formula is the one supported by the best inputs. Mifflin-St Jeor is usually the stronger default for general use. Katch-McArdle becomes more valuable when body-fat data is good enough to justify the extra complexity.