Every number in HealthOS that isn't directly measured by a device is an estimate — and we label it as such. Here's the exact math behind every calculation.
BMI is a ratio of weight to height. It's quick to compute and useful as a rough baseline, but it says nothing about body composition — muscle, fat, or distribution. We show it with a trend line and always display your goal-weight BMI alongside your current BMI.
Categories: Under 18.5 = Underweight · 18.5–24.9 = Healthy · 25–29.9 = Overweight · 30+ = Obese
BMR is the number of calories your body burns at complete rest — just to keep your organs running. We use the Mifflin-St Jeor equation (1990), which outperforms the older Harris-Benedict formula for most adults.
If your connected scale or device reports a BMR directly (e.g., via bioimpedance), we use that measured value instead of the formula, and label it "(device)".
TDEE = BMR × activity multiplier, adjusted for actual measured activity when available. We use a tiered approach: if your device or MFP reports exercise calories, we add those directly to your sedentary base (BMR × 1.2) rather than guessing a fixed multiplier.
Activity multipliers for the calculator: Sedentary 1.2 · Lightly Active 1.375 · Moderately Active 1.55 · Very Active 1.725 · Extremely Active 1.9
One pound of fat ≈ 3,500 kcal. A sustained deficit of 500 kcal/day predicts ~1 lb/week of fat loss. We accumulate your deficit daily for 7-day and 30-day periods, then convert to predicted pounds.
The "Theory vs Reality" ratio compares your calorie-math prediction to what the scale actually shows. Below 80% usually indicates water retention, adaptive metabolism, or logging gaps. Above 100% can happen when you underestimate activity or overestimate portion sizes.
We calculate three scenarios based on your current 30-day regression slope: Best (1.5× current pace), Expected (current pace), and Slow (0.5× current pace). A range is more honest than a single date because real-world loss pace varies week to week.
The slope comes from linear regression on the last 30 days of weight data. We cap the projection at 365 days to avoid showing implausibly distant dates for very slow trends.
VO2 Max is your maximum oxygen uptake — the gold-standard measure of cardiovascular fitness and a strong predictor of longevity. Lab measurement requires a treadmill test. The Uth-Sørensen formula gives a reasonable estimate from resting and max heart rate.
We pull your resting HR from overnight heart rate samples stored from your Withings device. The norms shown are age-adjusted for males and females (40-49 reference range for our current user base).
The 30-day trend line uses ordinary least squares (OLS) linear regression on your last 30 daily weight entries. This filters out day-to-day noise from water retention and shows the real underlying direction.
We also display a 7-day moving average (orange line) alongside raw daily weights (blue) to make the noise vs. signal distinction visual and immediate.
Heart Rate Variability (HRV) is measured directly by your Withings device during sleep and synced via the Withings API. A higher HRV generally indicates better recovery capacity and autonomic nervous system balance.
The recovery grade compares today's HRV to your value from 7 days ago:
If no 7-day baseline exists, we grade on absolute HRV value (A=≥50, B=≥35, C=≥25, D=≥15, F=<15 ms).
The report card grades 5 dimensions on a 7-day rolling window. Grades are letter grades (A–F) aggregated to an overall GPA-style score.
| Metric | A | B | F |
|---|---|---|---|
| Sleep | ≥8h | ≥7h | <5h |
| Exercise | 5+ days | 4 days | 0–1 days |
| Steps | 6+ days at goal | 5 days | 0–1 days |
| Nutrition | 7-day food streak | 5-day streak | No log |
| Recovery (HRV) | HRV +5% vs 7d | Within ±5% | −25% or worse |
HealthOS connects your devices and calculates all of these in real time — no spreadsheets, no guessing.
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