How the numbers are made

The idea

We estimate how much energy you use on a rest day and on a training day, then take a share off (to lose weight) or add a share (to gain). Everything runs in your browser. Every number below is an estimate: bodies differ, and the result is a starting point to adjust from your own progress.

1. Energy you use at rest

First we estimate the fat-free mass in your body, then the energy that mass uses at rest.

BMR = 370 + 21.6 × fat-free mass (kg)

• Source: Katch and McArdle's equation, which is based on fat-free mass and is widely published.
• How sure: It depends on your body-fat estimate, which is itself usually off by a few points.

2. Daily activity

Your everyday movement, outside training, multiplies your resting energy.

Activity factor: 1.40, 1.55, 1.75 or 1.90 for the four levels

• Source: Bands from the FAO/WHO/UNU 2001 report on human energy requirements. Which value inside each band we use is our own, cautious choice.
• How sure: People tend to overestimate how active they are, so the questions ask you to pick the lower option when unsure.

3. Digesting food

Digesting and absorbing food uses energy too, more on whole foods and protein than on processed food.

Thermic factor = 1 / (1 − share), with a share of 7, 10 or 14 %

• Source: Westerterp (2004) puts it at 5 to 15 % of daily expenditure. The whole versus processed difference is from Barr and Wright (2010).
• How sure: The three shares are our choice inside the published range. Real values vary from person to person.

4. Lifting sessions

A training day adds the extra energy of the session, from your active minutes or your number of sets.

Extra energy = (4.5 − 1) × body weight (kg) × hours, with 2.5 minutes per set

• Source: The Compendium of Physical Activities lists resistance training between 3.5 and 6.0 METs. 4.5 sits between the moderate and heavy entries.
• How sure: Intensity varies a lot between sessions, so this is a middle estimate.

5. Deficit or surplus

To lose weight we take a share off your daily energy, to gain we add one. The suggested share depends on your body-fat category, or your experience when gaining.

Target = expenditure × (1 − deficit) or × (1 + surplus)

• Source: Weekly loss rates follow Helms and colleagues (2014) and Garthe and colleagues (2011). Gain rates follow Iraki and colleagues (2019). Slower rates for the very lean are our own.
• How sure: The pace shown is an upper estimate: real change is usually slower, because the 7,700 kcal per kg rule overstates long-term change (Hall and colleagues, 2011).

6. Safety floors

We never show a target below 1,200 kcal (women) or 1,500 kcal (men), or below 80 % of your resting energy, without a warning.

Floors: 1,200 / 1,500 kcal and 80 % of BMR

• Source: Conventional guidance for unsupervised calorie targets.
• How sure: These are warnings, not a diagnosis. Anyone with a medical condition should ask a professional.

A worked example

A 34-year-old woman, 62 kg, 27 % body fat, lightly active, training 3 days a week, who wants to lose weight. This is run through the same calculation as the calculator.

Fat-free mass

62 kg × (1 − 27 / 100) = 45.3 kg

Basal metabolic rate (BMR)

370 + 21.6 × 45.3 = 1,347.6 kcal

Daily activity factor

Lightly active: 1.55 × 0.9 (digestion is counted separately) = 1.4

Thermic effect of food

1 / (1 − 0.1) for your diet = 1.11

Training cost per session

(4.5 − 1) × 62 kg × 45 min / 60 = 162.8 kcal

Rest-day expenditure

1,347.6 × 1.4 × 1.11 = 2,088.8 kcal

Training-day expenditure

(1,347.6 × 1.4 + 162.8) × 1.11 = 2,269.6 kcal

Weekly average

(3/7) × 2,270 + (4/7) × 2,089 = 2,166.3 kcal

Energy balance factor

1 − 0.21 = 0.8

Target intake (average)

2,166.3 × 0.8 = 1,722 kcal

Result: about 1804 kcal on a training day and 1661 kcal on a rest day.

Changes to the method

• 14 October 2026: Every value now comes from a published reference or is a stated assumption of this project, and each is listed on this page.

What this is not

This is information, not medical or nutrition advice. If you have a medical condition, are pregnant or under 18, or have a history of disordered eating, please talk to a doctor or registered dietitian.