Diet Diary calculates weight change from the difference between the calories you eat in a day and the calories you burn (your calorie balance). This page explains the formulas used in the game and where they come from.
The game works in metric units internally (kilograms and centimeters) and shows pounds and feet/inches on screen. 1 lb = 0.4536 kg and 1 in = 2.54 cm.
1. Resting metabolism (BMR)
BMR (basal metabolic rate) is the energy your body uses just to stay alive, even when lying still. Diet Diary uses the Mifflin-St Jeor equation, which estimates it from sex, weight, height, and age.
Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161Example: a 35-year-old man, 5'9" (175.3 cm), 200 lb (90.7 kg)
10 × 90.7 + 6.25 × 175.3 − 5 × 35 + 5 = about 1,833 kcal
As your weight drops, so does your BMR. Even if you keep the same menu, weight loss slowly gets slower.
2. Calories burned per day
Daily calories burned are your BMR multiplied by your job's activity level (activity factor), plus the calories burned by exercise.
Calories burned = BMR × activity factor + exercise calories| Job | Activity factor |
|---|---|
| Desk job (mostly sitting) | 1.2 |
| On the go (some walking) | 1.375 |
| On your feet (retail, cooking) | 1.55 |
| Manual labor (construction, moving) | 1.725 |
These activity factors are widely used to estimate total daily calories from BMR.
3. Exercise calories (METs)
Exercise intensity is measured in METs (metabolic equivalents). Resting counts as 1 MET, and an activity's METs tell you how many times more energy it uses than resting.
Exercise calories = (METs − 1) × weight (kg) × time (hours) × 1.05One MET is subtracted because the resting portion is already counted in your BMR and activity factor. The 1.05 is the factor for converting METs to calories.
Example: a 200 lb (90.7 kg) person walks (3.5 METs) for 30 minutes
(3.5 − 1) × 90.7 × 0.5 × 1.05 = about 119 kcal
For the man in the example above with an on-the-go job, total calories burned for the day are 1,833 × 1.375 + 119 = about 2,639 kcal.
| Exercise | METs | Time |
|---|---|---|
| None | 0 | − |
| Walk | 3.5 | 30 min |
| Jog | 7 | 30 min |
| Weights (strength training) | 5 | 45 min |
| Swim | 6 | 40 min |
When you enter your own exercise under "Other," choose from four intensity levels: light (3 METs: yoga, strolling), moderate (5 METs: cycling, dancing), vigorous (8 METs: running, sports), or very vigorous (10 METs: HIIT, hill hiking).
On rainy days and days you skip your workout, exercise calories are 0.
4. Weight change
Each day, weight changes by the day's calorie balance divided by 7,200 kcal.
Weight change (kg) = (calories in − calories burned) ÷ 7,200Body fat tissue is about 80% fat, and 1 g of fat holds about 9 kcal of energy, so 1 kg of body fat is equivalent to about 7,200 kcal (1,000 g × 0.8 × 9 kcal). In pounds, that's about 3,300 kcal per lb.
You may have heard the common "3,500 calories per pound" rule. That works out to about 7,700 kcal per kg, a little higher than the roughly 7,200 kcal per kg (about 3,300 kcal per lb) used in this game.
Example: a daily balance of −700 kcal
Per day: 700 ÷ 7,200 = about 0.097 kg (about 0.21 lb). Per week: about 0.68 kg (about 1.5 lb).
What the scale shows
Real bathroom scales go up and down from day to day because of water and bloating. In the game, the weight shown also varies within ±0.25 kg (about ±0.55 lb) of the calculated weight. On days you eat salty foods (such as a cheesesteak, burger & fries, BBQ ribs, pizza, or chips), water retention adds 0.3 kg (about 0.66 lb) to the reading. Reaching your goal and the result you share are based on the calculated weight, without this variation.
5. Metabolic adaptation and muscle
- Metabolic adaptation: if your daily balance keeps coming in below −500 kcal, your body goes into energy-saving mode and your BMR drops by 0.15% per day, down to a maximum of 6%. When you eat more again, it recovers by 0.3% per day.
- Muscle: you gain muscle on days you do strength training and slowly lose it on days you don't. More muscle raises your BMR by up to 5%.
Both are simplified versions of how real bodies respond, made easy to follow as a game. The numbers are estimates.
6. BMI and weight categories
BMI (body mass index) is a measure of body size calculated from weight and height.
BMI = weight (kg) ÷ (height (m) × height (m))BMI = 703 × weight (lb) ÷ (height (in) × height (in))Example: 200 lb, 5'9" (69 in)
703 × 200 ÷ (69 × 69) = about 29.5 (Overweight)
Weight categories follow the CDC/WHO adult BMI categories.
| BMI | Category |
|---|---|
| Below 18.5 | Underweight |
| 18.5 to below 25 | Healthy weight |
| 25 to below 30 | Overweight |
| 30 to below 35 | Obesity (Class 1) |
| 35 to below 40 | Obesity (Class 2) |
| 40 or higher | Obesity (Class 3) |
BMI categories are for adults. BMI is a screening measure, not a diagnosis, and does not directly measure body fat.
You can't set a goal weight that would put your BMI below 16. If the goal is below BMI 18.5, you'll see a warning.
Sources
- Mifflin MD, St Jeor ST, et al. "A new predictive equation for resting energy expenditure in healthy individuals." The American Journal of Clinical Nutrition, 1990; 51(2): 241–247.
- Herrmann SD, et al. "2024 Adult Compendium of Physical Activities." 2024.
- Centers for Disease Control and Prevention (CDC). "Adult BMI Categories."
- U.S. Department of Health and Human Services. Physical Activity Guidelines for Americans.
The calculations in the game are simplified estimates based on the sources above, designed to be easy to play. Actual results vary with each person's body and lifestyle. Talk to a doctor, registered dietitian, or other health professional about real weight loss and health decisions.