The idea: total first, then share

A recipe has one fixed amount of carbohydrate, set by what went in. Everything after that is a question of how you divide it. So work in two steps: get the total for the whole dish, then split it.

Step 1: add up the ingredients

Weigh each ingredient as it goes in, and find its carbohydrate per 100 g on the packaging. Multiply, then add.

The recipe below uses invented, rounded values so the arithmetic is easy to follow. They are not nutrition data for real pasta, passata or cheese. Use the numbers from your own packets.

Ingredient Weight Carbs per 100 g Carbs
Dry pasta 250 g 70 175.0 g
Passata 500 g 5 25.0 g
Cheese 100 g 1 1.0 g
Olive oil 20 g 0 0.0 g
Whole dish 201.0 g

Each line is weight × carbs per 100 ÷ 100. For the pasta: 250 × 70 ÷ 100 = 175.

Use the value that matches how you weigh. A per-100 figure for dry pasta goes with the dry weight, so weigh it before it goes in the pan.

Step 2, option A: equal portions

If you’ll serve the dish in equal portions, divide the total by the number of portions.

201 ÷ 4 = 50.25 g per portion

This is quick, but it only works if the portions really are equal. Spooning by eye rarely is.

Step 2, option B: weigh the finished dish

Weighing the dish lets you work out any portion, however you serve it.

  1. Weigh the empty pan or bowl.
  2. Weigh it again with the finished dish in it.
  3. Subtract the first from the second to get the weight of the dish.

Illustrative example. The empty pan weighs 850 g. With the finished dish it weighs 2,350 g, so the dish weighs 1,500 g. The pasta absorbed cooking water, which is why this is more than the raw ingredients.

201 × 100 ÷ 1500 = 13.4 g per 100 g of finished dish

Now weigh your plate and scale:

Plate Working Carbs
200 g 13.4 × 200 ÷ 100 26.8 g
320 g 13.4 × 320 ÷ 100 42.88 g
375 g (a quarter of the dish) 13.4 × 375 ÷ 100 50.25 g

The 375 g plate matches the equal-portion answer, as it should. Weighing is the better method when plates differ in size.

Where the estimate stops being exact

  • Cooking changes weight, not carbohydrate. Water absorbed or evaporated changes the weight of the dish and so its carbs per 100 g, but the total stays the same. Don’t reuse a per-100 figure from a different batch. Weigh each batch you make.
  • Only count what you eat. Peel, bones and any sauce or oil left in the pan don’t end up on the plate. Weigh the edible part of an ingredient, and if a lot is left behind, weigh that too.
  • Labels are rounded. Small rounding errors in each ingredient add up across a long list.
  • Ingredients without labels. For loose produce you can look up values in a public reference such as USDA FoodData Central or the UK’s CoFID dataset. Real foods vary, so treat the figure as an estimate.
  • It’s still an estimate. The method gives a reasoned figure, not a lab measurement. This guide covers the arithmetic only and is not medical advice.

Saving the work

For a one-off ingredient sum, the free carb calculator does the multiplication and shows the working. If you cook the same dishes often, the Android app lets you build a dish from ingredients, split it into equal portions or weigh the finished dish, and save it to reuse.

Sources

This guide explains arithmetic. It isn't medical advice and doesn't cover insulin or treatment decisions. Packaging is the best source for a specific product, because recipes and labels change.