How to calculate calories and kilojoules in your portion of food

by Ali on June 12, 2008

If you’re confused on what exactly a calorie or a kilojoule is, read the previous part in this series – Understanding calories, kilocalories, and kilojoules.

Some people are put off the idea of counting calories because they suspect it will be a big hassle – especially if maths was their worst subject at school! But you don’t need to be an Excel whiz, or have a friendly geek in the office, to work out the calories in your lunch.

How do I work out the calories in a portion of food?

All you need to know is:

  1. The number of calories per 100g of the food
  2. The size of the portion you’re eating, in grams

Then divide “number of calories per 100g” by 100, then multiply the result by “portion size in grams” and divide the total by 100.

Food packaging in the UK always gives calories per 100g, and often gives the calories (and the size, eg. “1 biscuit”) of what’s considered a “portion” – make sure that you adjust correctly if your portion is larger or smaller than the one given on the packet.
For example, my bran flakes box is labelled with this:
333 kcal per 100g
160 kcal per 30g serving with 125ml of semi-skimmed milk

However, if I eat a 40g serving with 175ml of skimmed milk, the 160 kcal figure won’t be accurate for my serving!
So to calculate the calories in my breakfast, I would:

  • Multiply 333 (“calories per 100g”) divided by 100 = 3.33 calories per gram
  • Multiply this by 40 (“portion size in grams”) = 133.2 calories for the bran flakes.

Then I work out the calories in my 175mls of skimmed milk (the label tells me there’s 35 calories per 100ml):
35 divided by 100 = 0.35 calories per ml
Multiply by 175 = 61.25 calories

So my bran flakes and milk have a total of 133.2 + 61.25 = 194.45 calories.

Sometimes, a label will only give figures per 100g even when the “portion” is obvious (eg. “one cereal bar”). For example, I have a box of cereal bars which tells me they contain 383 kcals per 100g … but doesn’t give me the figure per cereal bar, or even the weight of each bar.

However, the box does give the total weight (most food packaging should do this): 210g. There’s 10 cereal bars so each weighs 21g:
383 / 100 * 21 = 80.43 calories per cereal bar.

Help! Numbers make my head spin… isn’t there some easy way to count calories?

You don’t have to get out a paper and pen and scribble down numbers every time you eat something, and you don’t need to keep the labels from everything you eat. There are many online sites which help you track calories by looking up lots of common foods in a database.

I’ve reviewed Weight Loss Resources in the past and strongly recommend it as a great calorie-counting and food-diary resource.

If you’re in the US, I’ve heard a lot of good things about Calorie King, which has similar features.
And if you want to work out the calories in a recipe (whether it’s your grandma’s special flapjacks, or just a “recipe” such as milk and cereal), try my calorie calculator spreadsheet.

All you need to enter is the calories per 100g, the amount of each ingredient you’re using, and the number of servings it makes. The spreadsheet does all the fiddling multiplying and dividing for you!

What about recipes from a book? Do these have calorie counts?

Many modern recipe books do give calorie counts (and often grams of fat too) per recipe. I’m fond of the Woman’s Weekly series, which are Australian – some only give kilojoules but it’s easy to convert kilojoules to calories.

I also like the recipe finder from Sainsbury’s (a large UK supermarket chain) — this gives calories for each recipe and you can even search for “under 200 calories” etc.

And the recipes which I put on The Office Diet always have calorie counts, and other nutritional information, to help you make an informed choice!

Read the next part in the dieting basics series: “How many calories should I be eating?” to find out your recommended daily calorie intake – or get the whole series (plus lots of extra exclusive content) in my 88 page ebook, Dieting Basics – currently just $4!

Dieting Basics - Click to Read More

(Main post image by kodama (home))

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