Net Carbs Explained: How to Calculate + Our 100g Standard
The one-line definition
Net carbohydrates are the part of a food's carbohydrate content that your body actually digests and turns into blood sugar. The working formula, used by virtually every low-carb community and most labels in the United States, is:
Net carbs = total carbohydrates − dietary fiber
That is the entire calculation. On this site it is applied to USDA laboratory figures, per 100 grams of food, and the result drives every keto and low-carb verdict we publish. This guide walks through why the subtraction is legitimate, why sugar deliberately does not get subtracted, how our fixed 100-gram scale works, and where the convention breaks down — because knowing the edges of a rule is part of using it well.
What sits inside "total carbohydrates"
When a lab measures the carbohydrates in a food, the total line bundles three chemically different things:
Sugars. Glucose, fructose, lactose — small molecules that digest quickly and raise blood sugar within minutes. Honey, table sugar, the natural sweetness of an apple and the lactose in milk all land here.
Starches. Long chains of glucose found in grains, potatoes, beans and anything made from flour. They digest more slowly than sugar but almost completely, so gram for gram they contribute nearly the same carbohydrate load.
Fiber. Structural carbohydrates — cellulose, pectins, beta-glucans — that human enzymes cannot break down. Fiber travels to the large intestine largely intact, where gut bacteria may ferment part of it. Because it is not digested into glucose in the small intestine, it does not raise blood sugar the way the other two do.
All three are included in the total carbohydrate figure. That is why we display the relationship as a parent-child breakdown on every data card: total carbohydrates is the parent, and sugars and fiber are children listed underneath it. Sugars and fiber are parts of the total, not items to add on top. If you ever see a label or an app that lists "carbs 20 g, sugar 10 g, fiber 4 g" and then adds them together, that is an arithmetic error — the 14 g already live inside the 20 g.
Why fiber comes off — and why sugar stays
The subtraction exists because fiber is the one component of the total that largely escapes digestion. Subtracting it approximates the carbohydrate that will actually become glucose. A cup of raspberries and a tablespoon of sugar can carry a similar total-carbohydrate figure, but their net figures differ enormously, and the difference is almost entirely fiber.
Sugar, by contrast, stays in the net figure. There is a persistent myth that "natural sugar" or "fruit sugar" does not count. Chemically, fructose and glucose in a mango are the same molecules as in a soda; the mango brings fiber, water and micronutrients along, which slows absorption and changes the experience, but the sugar itself still digests. On our cards, sugars appear as a child line under the total — listed for context, never subtracted.
Starch also stays in, with no subtraction. Resistant starch — a fraction of starch that behaves like fiber — is real, but USDA SR Legacy tables do not report it separately for most foods, so we cannot subtract what has not been measured. We would rather leave a number slightly conservative than invent a correction.
Worked examples, straight from the data
These five examples use the exact USDA FoodData Central entries behind our verdict cards. Every figure is per 100 grams:
| Food | Total carbs | Fiber | Net carbs | Sugars (context) |
|---|---|---|---|---|
| Avocado, raw (FDC #171705) | 8.53 g | 6.7 g | 1.83 g | 0.66 g |
| Spinach, raw (FDC #169400) | 2.71 g | 2.1 g | 0.61 g | n/a |
| Almonds, raw (FDC #170158) | 21.01 g | 10.9 g | 10.11 g | 4.86 g |
| Sweet potato, raw (FDC #168484) | 17.72 g | 2.5 g | 15.22 g | 5.74 g |
| White rice, cooked (FDC #169711) | 21.09 g | 1.0 g | 20.09 g | 0.05 g |
Read the pattern: avocado and almonds carry deceptively high total carbohydrate, and only the fiber subtraction reveals why they belong on keto plates. White rice barely moves — 1 g of fiber against 21 g of total — which is exactly why it fails the keto line so decisively. The numbers agree with the lived experience of the diet, which is the minimum you should demand from a screening metric.
You can verify any of these yourself: every data card on this site prints its FDC ID, and the USDA FoodData Central database is public. If our figure and the government's ever disagree, one of us has a bug, and we want to know.
Why we score per 100 grams, not per serving
Most nutrition sites quote "per serving". We refuse, for three reasons.
A serving is whatever the manufacturer says it is. A serving of granola is 30 g on one box and 55 g on another. A serving of cooking spray is a fraction of a second. When the denominator is negotiable, the number attached to it is marketing before it is information.
A fixed scale makes foods comparable. On a per-100-gram basis, rice, quinoa and lentils sit in one honest ranking; potato, sweet potato and corn in another. You can compare anything to anything, which is precisely what a screening metric is for.
Scaling to your actual portion is one multiplication away. The math is linear:
| You eat | Arithmetic | Net carbs |
|---|---|---|
| 150 g avocado (about one whole fruit) | 1.83 × 1.5 | ≈ 2.7 g |
| 200 g cooked white rice (a generous bowl) | 20.09 × 2 | ≈ 40.2 g |
| 30 g almonds (a small handful) | 10.11 × 0.3 | ≈ 3.0 g |
| 250 g sweet potato (one large) | 15.22 × 2.5 | ≈ 38.1 g |
Scaling-table figures reuse the verified values from the five-food table above (FDC IDs listed there); the portions are ours, the arithmetic is linear.
That last row is the one that catches people: one large sweet potato is a very respectable-looking vegetable that consumes most of a strict keto day's entire carbohydrate budget. Per-100-gram thinking, applied to real portions, is what turns the verdict badges into useful decisions. Our daily food verdicts do this arithmetic for you in the "how to eat it" section.
Where the convention breaks down
Honesty about limits is part of the method, so here are the known edges of the net-carbs rule:
Sugar alcohols. Products sweetened with erythritol, xylitol and maltitol often advertise "net carbs" with sugar alcohols subtracted. Erythritol is indeed nearly calorie-free and minimally glycemic; maltitol is not, and is notorious for raising blood sugar. Our database does not carry sugar-alcohol figures for whole foods (they barely occur there), but for packaged products you should read the label and treat maltitol like sugar.
Added sugars. USDA SR Legacy reports total sugars but not the modern added-sugars line for most entries. A gram of sugar is a gram of sugar either way — it counts in full — but if you are tracking added sugar separately for other reasons, our figures cannot split it for you.
Labeling differences between countries. In the United States, fiber is already included in the total carbohydrate line, so subtraction is correct as written. In the European Union and the UK, fiber is typically listed outside the carbohydrate line. Subtracting again there would double-count. Our data is US-sourced and US-labeled throughout, so this trap does not arise on our pages — but it will if you mix our numbers with a British label.
It is a screening rule, not physiology. Two foods with identical net carbs can behave differently in your bloodstream — the speed of absorption, the food matrix, what you eat them with all matter. We do not have glycemic-index data of sufficient quality across the whole database, so we do not publish GI figures. The 5 g and 20 g verdict lines are decision aids, calibrated to be useful, not laboratory predictions of your glucose curve.
How verdicts use the number
Every food on this site gets two badges from one net-carb figure: Keto (YES at or under 5 g per 100 g, IN MODERATION up to 20 g, NO above) and Low-Carb (YES at or under 20 g, IN MODERATION above). The reasoning behind the two lines, and the foods that sit in the awkward middle band, is laid out in Keto vs Low-Carb: Which Is Right for You. The full methodological backdrop — sources, confidence grades and how to reproduce any figure — lives in About Our Data.
If you take one habit away from this guide, make it this: whenever you meet a new food, ask for its per-100-gram net carbs before anything else. Everything else on the label is negotiable presentation; that one number, scaled to your portion, is the decision.