Carb counting for beginners: a guide for insulin users

If you take mealtime insulin, you’ve probably been told to “count your carbs”. It sounds simple, and then you’re standing in a kitchen trying to work out how many grams of carbohydrate are in a bowl of pasta. This guide covers the basics: what carbs are, where to find the numbers, and where people most often go wrong.

What counts as a carb?

Carbohydrates, or carbs, are the part of food that raises blood sugar the most directly. According to the CDC, there are three kinds:

• Sugars: the natural sugar in fruit and milk, and added sugar in things like soda and sweets.
• Starches: grains such as wheat and oats, starchy vegetables such as potatoes and corn, and dried beans, lentils and peas.
• Fiber: found in plant foods. The CDC notes that sugars and starches raise blood sugar, but fiber doesn’t.

So carbs aren’t just “sugar”. Bread, rice, pasta, cereal, potatoes, fruit, milk, yogurt and beans all contain them.

How do you count carbs for insulin?

Carb counting means adding up the grams of carbohydrate in what you eat and drink. The CDC puts it plainly: if you take mealtime insulin, you count carbs to match your insulin dose to the carbs in your food.

How much insulin you need for a given amount of carbs is personal, which is where your insulin-to-carb ratio comes in.

What is an insulin-to-carb ratio?

An insulin-to-carb ratio describes how many grams of carbohydrate one unit of your mealtime insulin covers. Diabetes UK explains that everyone has their own ratio, and it can even differ from meal to meal. It’s something to work out with your diabetes team, not something to copy from someone else or an online calculator.

How to read a food label

Carbs are measured in grams, and packaged food lists them on the label. Labels look different depending on where you live.

In the US (Nutrition Facts label):

1. Check the serving size first. Everything on the label refers to that amount, not necessarily the whole package.
2. Look at Total Carbohydrate. The American Diabetes Association notes that this number includes all carbs: sugar, starch and fiber.
3. If you eat two servings, double the number.

In the UK and Europe:

Labels usually show carbs per 100 g and sometimes per portion. Diabetes UK advises working out the carbs for the amount you’re actually going to eat. For example, if a cereal has 60 g of carbs per 100 g and you eat 40 g of it, that’s 24 g of carbs.

What is a carb serving (or “carb choice”)?

You’ll often hear carbs described in servings or “choices”. Both the CDC and the ADA describe one carb serving as about 15 grams of carbohydrate.

The catch: a carb serving isn’t always what you’d think of as a serving of food. A typical plate of pasta can easily be several carb servings. When in doubt, count grams rather than portions.

Weighing vs. estimating carbs

When you’re starting out, weighing food is the most reliable way to learn. Diabetes UK suggests a flat, digital kitchen scale that can be zeroed. With practice, many people find they can estimate carbs by eye.

Estimates are rarely perfect, and that’s normal. A 2018 review in Nutrients found that people often estimate within about 10–15 g of the real amount. It also found that in some studies only about half could estimate accurately. That’s why it helps to re-check your estimates against a scale or label from time to time.

Do you count total carbs or net carbs?

Count total carbohydrate. Diabetes UK stresses counting the total carbs, not just the “of which sugars” line, and on US labels the ADA notes that total carbohydrate already includes fiber. You may see “net carbs” on packaging or diet websites, which usually means some carbs, such as fiber, have been subtracted. If you want to count that way, ask your dietitian first.

Common carb counting mistakes

• Counting only sugar. Sugar is just one part of total carbs.
• Ignoring the serving size. The label’s numbers are for one serving. The package might hold three.
• Treating a “serving” of food as one carb serving. As above, one plate is often more than 15 g.
• Forgetting drinks. Juice, milk, smoothies and sweetened coffee all contain carbs.
• Forgetting fat and protein. The same Nutrients review notes that meals rich in fat and protein can cause a longer blood sugar rise, lasting 3–4 hours after eating. Carbs are the main thing to count, but they aren’t the whole story. A late, rich dinner is also one of the causes of high blood sugar in the morning.

Where to learn more

Carb counting is a skill, and it’s much easier with support. The ADA recommends seeing a registered dietitian or diabetes educator, who can build an eating plan around you. In the UK, Diabetes UK points to structured education courses such as DAFNE. Your diabetes team can refer you.

The short version

• Carbs include sugars, starches and fiber, and they’re in far more foods than just sweets.
• Count the total carbs on the label, for the amount you actually eat.
• One carb serving is about 15 g, which is often less than a typical portion.
• Weigh food while you learn, and expect estimates to be a little off.
• Your insulin-to-carb ratio is personal. Work it out with your diabetes team.
• Mealtime insulin keeps working for hours after you eat. See what insulin on board means.