IIFYM Calculator (If It Fits Your Macros)

Written by Great Medico

Updated September 24, 2026 •2 min read

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Nutrition planning gets easier when a broad goal is translated into a concrete daily number. IIFYM Calculator (If It Fits Your Macros) does that using only the inputs and rules implemented in this tool.

What this calculator does

If It Fits Your Macros, or IIFYM, turns an estimated calorie target into daily protein, fat, and carbohydrate grams. This calculator adjusts calories for a maintain, lose, or gain goal and then allocates the macros. Nutrition equations are planning tools: real energy and nutrient needs still vary with physiology, goals, health conditions, and the quality of the input data.

How to use it

Enter or select Sex, Height, Weight, Age, Activity level, and Goal. Use the units offered by the calculator and keep measurements consistent when you plan to compare results over time.

How the calculation works

Maintenance calories come from Mifflin–St Jeor BMR multiplied by the selected activity factor. The engine subtracts 500 kcal for the lose setting or adds 300 kcal for gain. Protein is set at 1.8 g/kg, fat at 25% of calories, and carbohydrate receives the remaining calories.

Example

For the sample values prefilled in this calculator, the primary result is 2678 kcal/day (Macro calorie target), Protein is 135 g, and Fat is 74 g. This example is there to show how the inputs flow through the implemented equation; replace the sample values with your own measurements before using the result.

How to interpret the result

Use the result as a structured starting point for macro tracking. Protein and fat are set first in this model, so carbohydrates change the most when the calorie target changes.

Limitations and notes

Macro formulas are not individualized for pregnancy, kidney disease, diabetes therapy, elite sport, or eating-disorder care. Activity estimates can also be off, so body-weight trends, performance, hunger, and clinician guidance may justify adjustments.