Methods and sources
How our free tools work
Every number our tools show comes from a published public guideline, not from AI and not from our own opinion. This page lists each formula and cut-off, and where it comes from.
Method version 2026.10-a · sources reviewed October 2026
BMI and healthy weight band
BMI is weight in kilograms divided by height in metres squared. We use two sets of cut-offs:
- Standard: underweight below 18.5, healthy 18.5–24.9, above healthy 25–29.9, higher from 30. Source: World Health Organization.
- Lower cut-offs: healthy 18.5–22.9, increased risk 23–27.4, higher risk from 27.5. Used when you choose a South Asian, East or Southeast Asian, Middle Eastern or North African, Black African or African-Caribbean family background. Sources: NICE (UK) and WHO / The Lancet.
The healthy weight band is the weight range that gives a BMI from 18.5 up to the start of the raised range for your height.
Waist-to-height ratio
Waist divided by height. Under 0.5 is the healthy range, 0.5–0.59 is increased risk and 0.6 or more is high risk. It is most useful when BMI is under 35. Source: NICE (UK).
Daily calories
Resting energy uses the Mifflin–St Jeor equation (American Journal of Clinical Nutrition), multiplied by an activity factor:
- Mostly seated: × 1.2
- Lightly active: × 1.375
- Moderately active: × 1.55
- Very active: × 1.725
A gentle loss target is shown only if your BMI is above the healthy range for your background. It is your maintenance minus 500 kcal or 20%, whichever is smaller, and never below 1,200 kcal for women or 1,500 kcal for men. Below that, we suggest professional support instead. Source: NIH National Heart, Lung, and Blood Institute (US).
A gentle gain target (maintenance + 300 kcal) is shown only when your weight is in or below the healthy range.
Protein
Grams per kilogram of body weight a day, by activity:
- Mostly seated: 0.8–1 g/kg
- Lightly active: 0.8–1.2 g/kg
- Moderately active: 1–1.4 g/kg
- Very active: 1.2–1.6 g/kg
The lower end starts at the adult recommended intake of 0.8 g/kg. From age 65 the minimum is 1 g/kg. Above a BMI of 25, we use the weight at BMI 25 for your height rather than your current weight, so the target is not overstated. Sources: US National Academies (also used by Health Canada), ACSM, Academy of Nutrition and Dietetics, Dietitians of Canada, Journal of the American Medical Directors Association.
Fibre and fluids by country
Guidance differs between countries, so these follow where you live.
| Where you live | Fibre a day (women / men) | Drinks a day (women / men) | Source |
|---|---|---|---|
| United States | 25 / 38 g (51+: 21 / 30 g) | about 2.2 / 3.0 L | US National Academies (also used by Health Canada) |
| Canada | 25 / 38 g (51+: 21 / 30 g) | about 2.2 / 3.0 L | US National Academies (also used by Health Canada) |
| United Kingdom | 30 / 30 g | 1.5–2 L | UK Scientific Advisory Committee on Nutrition, NHS (UK) |
| Ireland or elsewhere in Europe | 25 / 25 g | about 1.6 / 2.0 L | European Food Safety Authority |
| Australia | 25 / 30 g | about 2.1 / 2.6 L | NHMRC (Australia) / Ministry of Health (NZ) |
| New Zealand | 25 / 30 g | about 2.1 / 2.6 L | NHMRC (Australia) / Ministry of Health (NZ) |
| India | 25 / 25 g | about 1.6 / 2.0 L | World Health Organization, European Food Safety Authority |
| Somewhere else | 25 / 25 g | about 1.6 / 2.0 L | World Health Organization, European Food Safety Authority |
Health self-check
Five questions about your goal and routine. Each answer adds points to a support area, and the highest total is suggested. It does not ask about symptoms and does not screen for any condition.
What these tools cannot do
- They are for adults aged 18 and over. They are not suitable during pregnancy or breastfeeding, for children, or for anyone with an eating disorder.
- They give estimates for groups of people. Your own needs depend on muscle, medication, health conditions, sleep and more.
- They do not diagnose anything and are not a substitute for advice from your doctor.
Spotted an error, or a guideline that has changed? Email us and we will review it.