Protein provides about 4 kilocalories per gram, but food labels rarely show how many of a product's calories come from protein. This protein calories per 100 g calculator converts the protein figure on any nutrition label into calories using that standard 4 kcal per gram rule. A food with 25 g of protein per 100 g, for example, delivers roughly 100 kcal from protein alone — before fat and carbohydrates are counted.
Add the fat and carbohydrate values from the same label and the calculator also works out the total calories per 100 g, the percentage of calories supplied by protein, and how those numbers scale to the serving size you actually eat. It is a fast, reliable way to compare packaged foods, evaluate high-protein claims, and plan meals around a protein or calorie target.
The tool applies the same Atwater conversion factors (4-4-9 kcal per gram) used on nutrition labels worldwide.
🧮 Protein Calories per 100 g Calculator
Enter the values from your food label — all amounts are grams per 100 g of food.
Step-by-Step Guide
Start with the nutrition facts panel on the food package. On most European-style labels the values are given per 100 g, which is exactly what this calculator expects.
- Enter the protein value in grams per 100 g — this is the only required field.
- Optionally add the carbohydrate and fat values from the same label.
- Tick Add alcohol if you are checking a drink label that lists alcohol per 100 ml.
- Tick Calculate for my serving size and enter the portion you actually eat, in grams or ounces.
- Press Calculate to see protein calories per 100 g, total calories, the protein share, and serving-scale values.
The 4-4-9 Rule: How It Works
Most nutrition labels use the Atwater system, which assigns approximate metabolizable energy values of 4 kcal per gram for protein and carbohydrate, 9 kcal per gram for fat, and 7 kcal per gram for alcohol. The math is simple:
- Protein calories per 100 g = protein (g) × 4
- Total calories per 100 g = (protein × 4) + (carbs × 4) + (fat × 9) + (alcohol × 7)
- Percentage of calories from protein = protein calories ÷ total calories × 100
The FAO's technical report on food energy conversion factors explains why these rounded values are used on labels worldwide.
Reference: Protein in Common Foods
| Food (cooked/ready to eat) | Protein g/100 g | Protein kcal/100 g |
|---|---|---|
| Chicken breast, grilled | 31 | 124 |
| Whey protein powder | 80 | 320 |
| Greek yogurt, plain | 10 | 40 |
| Whole egg, boiled | 13 | 52 |
| Tofu, firm | 8 | 32 |
| Lentils, boiled | 9 | 36 |
| Peanut butter | 25 | 100 |
Tips & Common Mistakes
Frequently Asked Questions
Why is protein counted as exactly 4 kcal/g?
It is a rounded average from the Atwater system. Real metabolizable energy varies between about 3 and 5 kcal/g depending on the protein source, but 4 kcal/g gives consistent, practical label values.
Does cooking change protein calories per 100 g?
Cooking changes weight through water loss or gain, so the protein per 100 g shifts even though the total protein in the food stays similar. Always use cooked weights and label values that match the food state you are measuring.
Where can I find accurate nutrition data?
The USDA's FoodData Central is a free, authoritative database with detailed entries for thousands of foods, including protein, fat, and carbohydrate values per 100 g.