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Free Health Tools
Tools to understand your well-being.
Research-backed calculators and factor explorers. No signup. Free forever.
26 tools
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6 categories
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No signup
Featured Tool
Our most-used comparison, backed by peer-reviewed data
Accuracy leader by metric
Oura
Apple
Garmin
WHOOP
Sleep
HRV
Steps
VO₂ max
Recovery
Most Popular
Most Accurate Wearable
Compare real accuracy data for Oura, Apple Watch, Garmin, WHOOP, Fitbit & Samsung across 9 health metrics backed by peer-reviewed research.
30+
peer-reviewed studies
6
wearables compared
9
health metrics
Open tool
From guessing to knowing
See how your food affects your sleep, energy & recovery.
Sleep
6 tools
What shapes how fast you fall asleep, how deep you go, whether you stay there, and which device measures it best.
Factors ranked by evidence
Staying Asleep Factors
Explore 27 research-backed factors that affect whether you stay asleep through the night: nutrition, supplements, exercise, environment & demographics ranked by evidence.
27 factors · 40+ peer-reviewed studies
Open
Time to fall asleep
Sleep Latency Factors
Explore 57 research-backed factors that affect how fast you fall asleep: environment, therapies, nutrition, supplements, exercise & demographics ranked by evidence.
57 factors · 65 peer-reviewed sources
Open
Sleep stages overnight
Deep Sleep Factor Explorer
What affects your deep sleep? 28 factors ranked by research, with evidence strength, doses, and mechanisms from 40+ studies.
28 factors · 5 categories
Open
Sleep stages overnight
REM Sleep Factor Explorer
What influences your REM (dream) sleep? Explore 37 research-backed factors across substances, supplements, exercise, environment & demographics, ranked by evidence strength, with mechanisms and sources.
37 factors · 5 categories
Open
Metrics tracked
Oura
Garmin
Apple
Fitbit
Sleep Metrics Comparison
Compare 38 sleep metrics tracked by Oura Ring, Fitbit, Apple Watch & Garmin across 10 categories. See exactly what each device measures while you sleep.
38 metrics compared · 10 sleep categories
Open
Sleep staging vs PSG
Oura
Apple
Fitbit
Garmin
Most Accurate Sleep Tracker
Which wearable is most accurate for sleep vs a lab PSG? Compare Oura, Apple Watch, Fitbit, WHOOP, Garmin & Withings on 4-stage staging (healthy vs clinical), deep & REM detection, sleep/wake agreement, and total-sleep-time bias.
Compare any 2–4 devices · Healthy vs clinical accuracy
Open
Heart Rate & HRV
3 tools
The inputs that move HRV and resting heart rate, and how accurately wearables read them.
HRV · beat-to-beat
HRV Factor Explorer
Explore 43 research-backed factors that affect Heart Rate Variability across supplements, lifestyle, exercise, micronutrients & demographics, ranked by evidence strength.
43 factors · 5 categories explored
Open
Resting heart rate
Resting Heart Rate Factors
37 research-backed factors that move your resting heart rate, sorted by exact bpm impact. See what actually lowers, raises, or doesn't change your RHR.
37 factors · Sortable by bpm impact
Open
HR accuracy vs strap
Fitbit
Garmin
Pixel
Oura
Most Accurate Heart Rate Wearable
Which wearable is most accurate for heart rate vs an ECG or chest strap? Compare Fitbit, Garmin, Apple Watch, Google Pixel, Polar, Oura & Xiaomi on error (MAPE), bias, limits of agreement and CCC, and see why arm motion, not intensity, is what breaks them.
Compare any 2–4 devices · 10 devices ranked head-to-head
Open
Stress & Recovery
2 tools
What recovery and stress scores are actually built from, and which ones are validated.
Readiness score
↑ Validated
Recovery Score Explorer
Compare recovery and readiness scores across 12 wearables (WHOOP, Oura, Garmin, Fitbit, Samsung, Polar, Ultrahuman, Coros, Amazfit, Suunto, Apple & RingConn), see the 35 factors that move yours, and find which scores are actually validated.
12 wearables compared · 35 factors
Open
Stress load
Stress Factor Explorer
What actually moves your wearable stress score? Explore every factor by device (Garmin, WHOOP, Oura, Fitbit, Samsung, Apple Watch), broken down by signal, direction & evidence.
10 wearables compared · 5 signal types mapped
Open
Activity & Fitness
3 tools
Steps, VO2 max and cardio fitness: what moves them and what your device gets right.
Daily step counts
Step Count Accuracy
See which wearable counts steps most accurately: Garmin, Apple Watch, Fitbit, Samsung, Oura and more, grouped into four evidence tiers.
20+ peer-reviewed studies · Head-to-head comparison
Open
VO2 max estimate
VO2 Max Factor Explorer
Explore 39 research-backed factors that raise or lower VO2 max (training, nutrition, environment, lifestyle & clinical), ranked by evidence strength, with doses and sources.
39 factors · 6 categories
Open
Accuracy vs lab CPET
Garmin
Apple
Polar
Fitbit
Most Accurate VO2 Max Wearable
How accurately do Garmin, Apple, Polar, Fitbit, WHOOP, Oura, Samsung & Coros estimate VO2 max vs a lab CPET? Compare 9 devices by method and see which brands are actually independently validated.
9 devices compared · Independent vs vendor validation
Open
Calories & Nutrition
4 tools
How food affects your sleep, calorie burn accuracy, what is actually in your food, and which supplements the research supports.
REM minutes vs control
How Food Affects Sleep
Foods and drinks for each part of sleep: what caffeine, alcohol, carbs, protein, late dinners and more did to deep sleep, REM, falling asleep and night waking, every finding graded by evidence with industry funding flagged.
32 findings, 4 sleep stages · Graded by evidence
Open
Reported vs actual
Calorie Burn Accuracy Calculator
Enter your wearable's reported calorie burn and see the likely actual range, with peer-reviewed per-activity accuracy for Apple Watch, Fitbit, Garmin, WHOOP, Oura & Samsung.
6 devices × 7 activities · Peer-reviewed per-activity data
Open
Macros & calories
Protein
Carbs
Fat
Food Scanner
Snap a photo of any meal and get instant calories, macros, health score, and nutrition insights powered by AI.
Instant calorie count · Macro & vitamin breakdown
Open
Graded by evidence
Supplements by Metric
Pick a wearable metric (sleep latency, deep sleep, staying asleep, HRV, resting heart rate, or recovery) and see which of 27 supplements the research actually supports, graded by evidence with industry-funding flags and the popular stuff that doesn't work.
6 metrics × 27 supplements · Evidence-graded with funding flags
Open
Devices & Buying Guides
7 tools
Device accuracy, head-to-head comparisons, and what to buy.
Published features
Oura
Ultra
RingConn
CUDIS
Smart Ring Comparison & Picker (2026)
Compare all 9 current smart rings from Oura, RingConn, Ultrahuman and CUDIS: a priority-based ring finder, every spec model by model, a 50-feature software matrix, the validation record for each brand, and the real multi-year cost.
9 models, 4 brands · Ring finder by priority
Open
Features vs value
Oura Ring vs RingConn (2026)
Compare Oura (Ring 5, Ring 4) and RingConn (Gen 3, Gen 2, Gen 2 Air) side by side: every spec, a feature-by-feature software matrix, validation posture, and the real multi-year cost with Oura's membership included.
5 models compared · Feature-by-feature matrix
Open
Relative thickness
Oura Ring 5 vs 4 vs 3 Comparison (2026)
Compare Oura Ring 5, Ring 4, and Gen 3 side-by-side. Size & dimensions, peer-reviewed accuracy data (Dial 2025, Khan 2025), and the real 3-year cost with subscription math included.
3 generations compared · Peer-reviewed accuracy data
Open
Ranked by evidence
Oura Ring 5 vs 4: What's Different (2026)
Oura Ring 5 vs Ring 4, decided on the numbers: every spec difference, a direct upgrade verdict, peer-reviewed accuracy (Dial 2025, Robbins 2024), and the real 3-year cost with membership.
Two generations, head-to-head · Peer-reviewed accuracy
Open
Head-to-head accuracy
Fitbit Air vs WHOOP: Accuracy & Cost (2026)
Compare Fitbit Air, WHOOP 5.0, and WHOOP MG side by side: heart-rate, sleep, and calorie accuracy with numbers, sensors, battery, price, and 3-year cost of ownership.
35 specs across 5 categories · HR/sleep/calorie accuracy
Open
Sensor & software focus
Hardware & Software Differences
See what actually makes Garmin, Whoop, Oura, Apple Watch, and Fitbit different: hardware sensors, health metrics, and proprietary software compared.
Hardware vs software breakdown · 25 proprietary algorithms
Open
Ranked by evidence
Wearable Accuracy: What Actually Affects It
Does hair, a tattoo, cold hands or which wrist change your readings? 12 questions people ask, 8 things that help and 6 things that quietly hurt, with what the studies found, what each brand says, and the free fixes that cut heart rate error from 20% to 7%.
12 questions answered · 8 free fixes ranked
Open
iOS, Android & web
These tools are a snapshot.Kygo is the full picture.
Connect your wearable. Log your meals. See how food affects your sleep, HRV, energy, and recovery.
Works with
Oura
Apple
Fitbit
Garmin
WHOOP
Polar
Health