Converting between calories and kilojoules is essential for anyone tracking their energy intake in the UK. Whether you’re reading nutrition labels, planning meals, or monitoring your diet, this converter helps you switch between kcal (kilocalories) and kJ (kilojoules) instantly.
Conversion History
Kcal to kJ Conversion Table
This table shows common calorie to kilojoule conversions you’ll see on UK food packaging. Most nutrition labels display both kcal and kJ values side by side.
Kilocalories (kcal)
Kilojoules (kJ)
Common Food Reference
50 kcal
209 kJ
Small apple
100 kcal
418 kJ
Slice of wholemeal bread
150 kcal
628 kJ
Glass of semi-skimmed milk
200 kcal
837 kJ
Two Weetabix with milk
250 kcal
1,046 kJ
Small jacket potato
300 kcal
1,256 kJ
Tuna sandwich
500 kcal
2,092 kJ
Big Mac burger
1,000 kcal
4,184 kJ
Large takeaway pizza
2,000 kcal
8,368 kJ
Daily intake (average woman)
2,500 kcal
10,460 kJ
Daily intake (average man)
Conversion Formula
The maths behind converting calories to kilojoules is straightforward. Here’s what you need to know for both types of calories.
Kilocalories to Kilojoules
Energy (kJ) = Energy (kcal) × 4.184
This is the conversion you’ll use most often for nutrition labels and dietary tracking.
Small Calories to Kilojoules
Energy (kJ) = Energy (cal) × 0.004184
Used primarily in scientific contexts rather than everyday nutrition.
Step-by-Step Conversion
Identify whether you’re working with kcal (nutritional Calories) or cal (small calories). Most food labels show kcal.
Take your calorie value and multiply it by the appropriate conversion factor: 4.184 for kcal or 0.004184 for cal.
Round the result to a sensible number of decimal places. Most nutrition labels show kJ as whole numbers.
Example: A chocolate bar lists 230 kcal on the wrapper. To convert: 230 × 4.184 = 962 kJ (rounded to nearest whole number).
Why UK Labels Show Both
Since 2014, pre-packaged food in the UK must display energy content in both kJ and kcal. The kilojoule value always appears first, though most people find kilocalories more intuitive.
The reference intake for an average adult is 8,400 kJ or 2,000 kcal per day. You’ll see this figure on many food packages, helping you gauge what percentage of your daily energy a product provides.
Kilojoules are the metric (SI) unit of energy, whilst kilocalories remain popular in English-speaking countries. Both measure exactly the same thing: the energy your body gets from food and drink.
Daily Energy Requirements
Your energy needs depend on age, sex, weight, height, and activity level. The NHS provides these general guidelines:
Group
Kilocalories (kcal/day)
Kilojoules (kJ/day)
Average woman
2,000 kcal
8,400 kJ
Average man
2,500 kcal
10,500 kJ
Children (7-10 years)
1,800 kcal
7,500 kJ
Teenagers (11-14 years)
2,200 kcal
9,200 kJ
Very active adults
3,000+ kcal
12,500+ kJ
These figures are averages. Someone training for a marathon needs far more energy than someone with a desk job. Your individual requirements may differ significantly.
Energy Unit Conversions
Besides calories and kilojoules, energy can be expressed in other units. Here’s how they relate:
From
To
Multiply By
kcal
kJ
4.184
kJ
kcal
0.239
kcal
Joules (J)
4,184
cal
kJ
0.004184
kcal
Watt-hours (Wh)
1.162
kJ
Megajoules (MJ)
0.001
Calories in Popular UK Foods
Here’s what you’ll typically find in everyday British foods and drinks:
Food Item
Serving Size
kcal
kJ
Cup of tea with milk
250ml
20 kcal
84 kJ
Digestive biscuit
1 biscuit (15g)
71 kcal
297 kJ
Slice of toast with butter
1 slice
120 kcal
502 kJ
Chicken tikka masala
Per serving
465 kcal
1,946 kJ
Fish and chips
Average portion
840 kcal
3,515 kJ
Pint of lager
568ml
180 kcal
753 kJ
Mars bar
51g
229 kcal
959 kJ
Cheese and pickle sandwich
1 sandwich
380 kcal
1,590 kJ
FAQs
What’s the difference between calories and Calories?
The capital ‘C’ Calorie on food labels is actually a kilocalorie (kcal). One Calorie equals 1,000 small calories. When you see “200 Calories” on a packet of crisps, that’s really 200 kcal or 200,000 small calories. In everyday conversation and on UK labels, we use kcal and Calories interchangeably.
Why do UK food labels show both kJ and kcal?
EU regulations (which still apply to UK labelling) require both units. Kilojoules are the official SI (metric) unit, whilst kilocalories remain more familiar to consumers. The kJ value must appear first, but both give you the same information about energy content.
How many kilojoules should I eat to lose weight?
To lose roughly 0.5kg per week, you need a deficit of about 500 kcal (2,090 kJ) per day. For women, this might mean eating around 1,500 kcal (6,280 kJ) daily; for men, about 2,000 kcal (8,370 kJ). Always consult your GP or a registered dietitian before starting a weight loss programme, as individual needs vary considerably.
Are kJ and kcal measurements exact on food labels?
Not always. Research shows that declared values on UK packaging can vary from actual content by up to 20%. Manufacturing variations, rounding rules, and testing methods all affect accuracy. Labels provide a consistent baseline for comparison, but your body’s absorption and metabolism also vary based on factors like gut health and food preparation.
Which is better for tracking my diet: calories or kilojoules?
Either works perfectly well, as they measure the same thing. Most Brits find kcal more intuitive because the numbers are smaller and more widely discussed in diet advice. Kilojoules are standard in Australia and New Zealand. Pick whichever feels more natural to you and stick with it consistently.
Do I burn calories or kilojoules during exercise?
You burn both—they’re just different ways of measuring the same energy expenditure. A 30-minute jog might burn 300 kcal or 1,255 kJ. Fitness apps and gym equipment typically display kcal, but some show both. The conversion factor stays the same: multiply kcal by 4.184 to get kJ.
What’s a megajoule (MJ) and when is it used?
One megajoule equals 1,000 kJ or about 239 kcal. You’ll sometimes see MJ in scientific papers or when discussing total daily energy intake. For instance, the NHS recommendation of 8,400 kJ for women equals 8.4 MJ. It’s not commonly used on food labels, where kJ remains standard.
References
National Health Service. (2022). Food labels. NHS UK. Retrieved from https://www.nhs.uk/live-well/eat-well/food-guidelines-and-food-labels/how-to-read-food-labels/
National Health Service. (2025). Understanding calories. NHS UK. Retrieved from https://www.nhs.uk/live-well/healthy-weight/managing-your-weight/understanding-calories/
Food Standards Agency. (2018). Technical guidance on nutrition labelling. UK Government. Retrieved from https://www.food.gov.uk/business-guidance/nutrition-labelling
British Nutrition Foundation. (2021). Looking at nutrition labels. Nutrition.org.uk. Retrieved from https://www.nutrition.org.uk/creating-a-healthy-diet/food-labelling/
Public Health England. (2016). Government dietary recommendations. UK Government. Retrieved from https://assets.publishing.service.gov.uk/media/5a749fece5274a44083b82d8/government_dietary_recommendations.pdf
British Heart Foundation. (2021). A balanced diet: how many calories should I eat in a day? BHF. Retrieved from https://www.bhf.org.uk/informationsupport/heart-matters-magazine/nutrition/
