How It Works
biological age, calculated from your health data
Step 1
Data collection
- Apple Watch
- Whoop
- Oura Ring
- Garmin
- Fitbit
- Any HealthKit Device
Gist reads from Apple HealthKit. Works with Apple Watch, Oura, Whoop, Garmin, Fitbit, and any device that syncs to HealthKit.
Step 2
Metrics and optimal ranges
Each metric scores 0 to 100 against its optimal range. The ranges are deliberately tight, tighter than the public-health guidance they start from. Within a category, the weight decides how much each metric moves the score. The overall score is the plain average of whichever categories have data.
Sleep
Three metrics from the last 7 nights. Total duration counts more than the stage split.
| Metric | Optimal range | Weight |
|---|---|---|
| Sleep Duration | 7.5-8.5 hours | 3x |
| Deep Sleep | 18-25% | 2x |
| REM Sleep | 20-25% | 2x |
Activity
Six metrics, all 7-day averages. Steps carry the most weight, stand hours the least.
| Metric | Optimal range | Weight |
|---|---|---|
| Daily Steps | 8,000-12,000 | 3x |
| Active Energy | 400-700 kcal | 2.5x |
| Exercise Time | 45-90 min | 2.5x |
| Distance | 6-10 km | 2x |
| Flights Climbed | 8-20 floors | 1.5x |
| Stand Hours | 10-14 hours | 1x |
Heart
Resting heart rate and HRV carry the most weight. Lower is better for one, higher for the other.
| Metric | Optimal range | Weight |
|---|---|---|
| Resting Heart Rate(Lower is better) | 45-58 bpm | 3x |
| HRV (SDNN)(Higher is better) | 50-100 ms | 3x |
| Walking Heart Rate | 80-105 bpm | 2x |
| Respiratory Rate | 12-15 breaths/min | 1.5x |
| Average Heart Rate(All-day average) | 60-80 bpm | 1x |
Body
VO2 Max counts most. If your watch does not measure it, Gist estimates it from five other metrics.
| Metric | Optimal range | Weight |
|---|---|---|
| VO2 Max(Higher is better) | 42-55 mL/kg/min | 3x |
| Estimated Cardio Fitness(Only when VO2 Max is missing or stale) | Derived score, 0-100 | 2.5x |
| Walking Speed(Higher is better) | 2.8-4.0 mph | 2x |
| BMI | 18.5-24.0 | 2x |
| Body Fat | 10-22% | 2x |
| Walking Steadiness(Higher is better) | 80-100% | 1.5x |
Step 3
Scoring
Standard scoring
Anywhere inside the optimal range scores 100. Outside it, you lose 50 points for every full range-width you miss by, down to 0. With a steps range of 8,000 to 12,000, 6,000 steps scores 75 and 4,000 scores 50.
Lower is better
Resting heart rate and walking heart rate. At or below the lower bound scores 100. Across the range the score slides from 100 down to 70. Above the upper bound it keeps falling, 50 points per range-width.
Higher is better
HRV, VO2 Max, walking speed, and walking steadiness. At or above the upper bound scores 100. Across the range the score climbs from 70 to 100. Below the lower bound it keeps falling, 50 points per range-width.
Time windows
Daily metrics (sleep, steps, heart rate, walking speed) are 7-day averages. VO2 Max looks back 180 days but must be under 90 days old. BMI and body fat look back a year but must be under 180 days old. Walking steadiness uses the latest reading from the last 30 days. Anything older than its cutoff is dropped, not scored.
Estimated cardio fitness
If there is no VO2 Max reading, or the newest one is over 90 days old, Gist builds a stand-in from the last 7 days of walking speed and resting heart rate (3x each) plus active energy, exercise minutes, and walking heart rate (2x each). It needs at least two of those. The stand-in gets a 2.5x weight instead of VO2 Max's 3x, because it is inferred rather than measured.
Step 4
The formula
Gist Age = Chronological Age + Age Offset
Age Offset = (50 - Overall Score) / 7.5 · Range: ±6.7 years
| Score | Age offset | Meaning |
|---|---|---|
| 100 | -6.7 years | Exceptional health |
| 80 | -4 years | Excellent health |
| 50 | 0 years | Average health |
| 20 | +4 years | Below average |
| 0 | +6.7 years | Poor health markers |
Bonus
Pace of aging
The same overall score also becomes a pace. 1x is normal aging. Below 1x you are aging slower than the calendar. Negative numbers mean the score is high enough that the model reads it as getting younger.
| Score | Pace | What it means |
|---|---|---|
| 100 | -1.0x | Aging in reverse |
| 90 | -0.5x | Significant reversal |
| 80 | 0x | Stable / not aging |
| 65 | 0.5x | Aging slower than normal |
| 50 | 1.0x | Normal aging rate |
| 25 | 2.0x | Aging twice as fast |
| 0 | 3.0x | Accelerated aging |
Research
References
The optimal ranges start from the guidance below and are then tightened. Treat these as the sources, not the exact bands the app uses.