NourmioHow to convert kilojoules to calories on food labels
Convert food energy from kJ to kcal, scale it to your portion and see which number to use when an imported food label already lists both.
Divide kilojoules by 4.184 to estimate kilocalories. If the label lists both kJ and kcal for the same portion, use the printed kcal number. When the label gives only kJ per 100 g, first scale to the amount you ate, then convert.
Convert the amount you actually ate
For this food-energy conversion, 1 kcal equals 4.184 kJ. A food Calorie on a US Nutrition Facts panel is a kilocalorie.
Suppose a label lists 800 kJ per 100 g and you ate 75 g. Use the weight to find the energy in your portion, then divide by 4.184.
| Step | Result |
|---|---|
| 800 kJ × 75 ÷ 100 | 600 kJ in the portion |
| 600 ÷ 4.184 | About 143.4 kcal |
| Rounded to the nearest whole kcal | 143 kcal |
If the panel gives a number per serving and you ate exactly one serving, divide that number by 4.184 directly. For half a serving, start from half the stated kJ.
Use the printed kcal when both units are present
The UK Weetabix Original table lists 574 kJ and 136 kcal for two biscuits (37.5 g). Converting 574 kJ directly gives about 137.2 kcal. For that portion, use the printed 136 kcal. The table already gives the energy in the unit the diary asks for.
A printed pair need not match a direct conversion exactly. For example, European labeling rules give separate factors in kJ and kcal for nutrients. The Weetabix table does not state the calculation behind this specific pair.
Check the column before entering the number
Look for “per 100 g,” “per serving” or a named portion, especially if the label has more than one column. Keep that basis with the number you use. If the label shows kcal and you ate a different portion, scale the printed kcal instead of doing an extra conversion. The same serving-size check applies to the other amounts you enter.