What the FAT Index measures

The Fat Index scores any food on a single question: how much fat does it deliver relative to its calorie cost?

Fat is the most energy-dense of the four macronutrients, delivering roughly nine calories per gram — more than twice the caloric value of protein or carbohydrates. This makes it a powerful satiety tool in small quantities and a significant calorie contributor in large ones. For decades, fat carried an undeserved reputation as the primary driver of weight gain and poor health. That reputation has been substantially revised by the research. Fat is necessary, satiating, and largely fine — the question is less how much and more what kind.

The index scores on a 0 to 100 scale. A score of 100 represents the theoretical ceiling — a food where every calorie comes from pure fat, delivering approximately 11 g per 100 kcal. Pure oils sit at or near this ceiling. In practice, foods like almonds, avocado, and full-fat cheese score in the high range, while lean proteins and most grain products score low.

important distinction: this index is neutral

Unlike the Protein Index and the Fibre Index — where a higher score is an unambiguous positive signal — the Fat Index does not carry a directional verdict. A high score means this food is fat-dense. A low score means it is not. Neither is inherently better.

What matters alongside the score is what the fat is doing nutritionally — and that depends on its source. This is a distinction the index score alone cannot make. Two foods can score identically on the Fat Index while being nutritionally very different: one whose fat comes from olive oil and whole almonds, and one whose fat comes from partially hydrogenated palm oil in a packaged biscuit. The score is the same. The nutritional reality is not.

This is why the index includes a secondary quality note when saturated fat data is available, and why the footnote points explicitly to the ingredient list as the place to look for fat source information.

How to read your score

The scale uses the same logarithmic curve as the Protein Index. Differences in fat density matter more at the low end of the spectrum — the gap between a very low-fat food and a moderately fat food is more practically significant than the gap between two already fat-dense foods. The curve reflects this.

A note triggers above a score of approximately 83, which corresponds to around 7.5 g of fat per 100 kcal. This is not a warning — it is a prompt to look further. At this level of fat density, the source of that fat becomes the most relevant piece of information the score cannot give you.

The saturated fat field

When your nutrition label includes a saturated fat breakdown — not all do — entering it activates a secondary note below the score. This note tells you what proportion of the total fat is saturated, and whether that proportion is above or below half.

Saturated fat is not uniformly harmful. Full-fat dairy, eggs, and red meat are naturally high in saturated fat and carry nutritional value alongside it. The note is not a verdict. It is context — information that helps you place the score within a fuller picture of the food’s fat profile.

If the label does not list saturated fat separately, leave the field blank. The score calculates from total fat and calories alone and is fully valid without it.

Reference points

These are approximate scores for common foods:

Pure oil — 100
Almonds — 89
Avocado — 87
Cheddar cheese — 85
Whole eggs — 81
Salmon — 76
Chicken thigh — 70
Full fat Greek yoghurt — 67
Chicken breast — 44
Wholegrain bread — 39
White rice, cooked — 10
Pure sugar — 0

A note on zero-fat foods

Foods with no fat — pure sugar, most refined grain products, or even veggies like lettuce — score zero on this index. As with the Protein Index, a zero score is not a verdict on the food overall. It means fat is not present in any meaningful quantity. Whether that is a problem depends entirely on what the food is and what role it plays in your diet.

How to use this in practice

In label mode, enter total fat in grams and calories in kcal as they appear on your label. If the label also lists saturated fat, enter that in the optional field. Weight is there for your reference if you are weighing ingredients and does not affect the score.

In quick mode, enter fat per 100 kcal directly. If your label shows values per 100 g, use the label mode instead — it handles the conversion automatically.

The Fat Index is most useful when read alongside the Protein Index. A food that is high on both is likely to be satisfying but calorie-dense — nuts, cheese, fatty fish. A food that is high on fat and low on protein may be calorie-dense without the satiety benefits that protein provides. Seeing both scores together tells you more than either does alone.

What comes next

The Fibre Index — the third of the five base indexes — measures fibre density per 100 kcal. Fibre is, in our view, the most underrated number on any nutrition label and the one that most reliably distinguishes a food that sustains you from one that merely fills you briefly. It is also the variable that does the most work in the Sugar Buffer Index, which will eventually show how fibre modifies the metabolic impact of a food’s sugar content.