Science & formulas behind your data

Trend = average of your last 7 weigh-ins

Your weight fluctuates every single day — sometimes by 2 to 5 pounds — because of water retention, digestion, sodium, hormones, and dozens of other factors. A single number on the scale is noisy. The trend line smooths all of that out so you can see what your weight is actually doing over time. Focus on the trend, not the daily reading.

BMI = weight (kg) ÷ height (m)²

BMI is a rough screening tool that divides your weight by your height squared. It was designed for populations, not individuals. It does not account for muscle mass, bone density, age, or ethnicity — so a muscular athlete might show as "overweight" even though they are perfectly healthy. DayByDay shows BMI as a general reference point, not a verdict on your health.

¹ World Health Organization BMI classification: Underweight <18.5, Normal 18.5–24.9, Overweight 25–29.9, Obese ≥30.

Men: (10 × kg) + (6.25 × cm) − (5 × age) + 5 Women: (10 × kg) + (6.25 × cm) − (5 × age) − 161

Your BMR is the number of calories your body burns at complete rest — just to keep you alive. Think of it as your body's idle speed. You burn more calories the moment you add any activity, even just walking to the kitchen. As you lose weight and lean mass, your BMR shifts too, which is why DayByDay recalculates it as your numbers change.

¹ Mifflin MD, St Jeor ST et al. (1990). A new predictive equation for resting energy expenditure. Am J Clin Nutr.

Men: 64 − (20 × height ÷ waist) Women: 76 − (20 × height ÷ waist)

RFM estimates your body fat percentage using only two measurements: your height and your waist circumference. No special equipment needed — just a tape measure. Research suggests RFM is more accurate than BMI for most people because it accounts for where fat is actually stored. You'll find this in the Measurements section once you've logged at least one waist measurement.

¹ Woolcott OO, Bergman RN. (2018). Relative Fat Mass (RFM) as a new estimator of whole-body fat percentage. Scientific Reports.

WHtR = waist ÷ height (both in the same unit)

WHtR is one of the simplest ways to check whether your waist size puts you at higher health risk. The rule of thumb: keep your waist to less than half your height. So if you are 70 inches tall, aim for a waist under 35 inches. It does not matter whether you use inches or centimeters as long as both measurements use the same unit.

¹ Ashwell M, Gunn P, Gibson S. (2012). Waist-to-height ratio as a predictor of cardiometabolic risk in adults. Nutr Rev.

FFMI = lean body mass (kg) ÷ height (m)²

FFMI works like BMI but focuses on muscle and lean tissue instead of total body weight. It tells you how much of your mass is functional — not fat. When you are losing weight, one of the goals is to hold onto as much lean mass as possible. Tracking FFMI over time helps you see whether you are losing fat or losing muscle. You need a body fat percentage to calculate this one.

¹ Kouri EM et al. (1995). Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med.

EWL% = (weight lost ÷ excess weight) × 100 Excess weight = starting weight − ideal weight

EWL% measures how much of your total "excess" weight you have lost. It is particularly useful when tracking progress on GLP-1 medications, because researchers and clinicians often report outcomes in terms of excess weight lost rather than total pounds lost. For example, losing 30 lbs when you had 60 lbs to lose is 50% EWL — a meaningful milestone.

¹ Ideal weight is often estimated using a target BMI of 25 or the Devine formula. DayByDay uses your personal goal weight as the target.

Level(t) = Dose × e^(−0.693 × t ÷ half-life)

This formula uses exponential decay to estimate how much medication is likely in your system at any point in time. It is the same math used to describe how drugs are eliminated by the body. The curve starts high right after your dose and gradually falls until your next injection. This is an estimate based on the drug's typical half-life — not a blood test.

¹ Half-lives used: semaglutide (Ozempic/Wegovy) ≈7 days; tirzepatide (Mounjaro/Zepbound) ≈5 days; liraglutide (Saxenda) ≈13 hours. Individual metabolism varies.