Your smartwatch promises to count every step, measure every heartbeat, and calculate every calorie burned. But how accurate are these numbers really? Understanding the limitations and strengths of fitness tracking technology helps you interpret your data meaningfully and get genuine value from your wearable.

The truth is nuanced: smartwatches are remarkably accurate for some metrics while remaining approximate for others. Let's explore what you can trust and what requires a healthy dose of scepticism.

Step Counting: Surprisingly Good, With Caveats

Step counting was one of the first features in wearables, and modern smartwatches have refined it substantially. Studies consistently show that quality smartwatches count steps within 5-10% of actual steps during normal walking.

However, accuracy varies based on activity:

  • Normal walking: Highly accurate (within 5%)
  • Running: Generally accurate, though stride length changes can cause slight overcounting
  • Pushing a shopping cart or pram: Often undercounts because arm swing is restricted
  • Cycling: May incorrectly count pedal rotations as steps on some devices
  • Desk work: May occasionally count typing or gestures as steps

Practical Advice

Focus on trends rather than absolute numbers. Whether your watch says 9,847 or 10,153 steps matters less than whether you're consistently hitting your targets week over week. The relative accuracy is excellent even if absolute accuracy has small variations.

Heart Rate Monitoring: Good Baseline, Variable During Exercise

Smartwatches use photoplethysmography (PPG)—shining green LED light into your skin and measuring blood flow changes—to estimate heart rate. This technology has improved dramatically, but it faces inherent challenges.

When Heart Rate Accuracy Is High

  • At rest or during low-intensity activity
  • During steady-state cardio like jogging or cycling
  • When the watch fits snugly and correctly positioned
  • On lighter skin tones (more on this below)

When Accuracy May Suffer

  • During high-intensity interval training with rapid heart rate changes
  • During activities with significant wrist movement (rowing, boxing)
  • In cold weather when blood flow to extremities is reduced
  • On darker skin tones, where some sensors struggle with light absorption
  • With tattoos on the wrist area
  • When the watch band is too loose or too tight

The Skin Tone Factor

Research has shown that optical heart rate sensors can be less accurate on darker skin tones due to how melanin affects light absorption. Premium watches with advanced sensor arrays have largely addressed this issue, but it remains a consideration with budget devices. If you notice inconsistent readings, trying a different watch position or brand may help.

How to Improve Heart Rate Accuracy

Several simple adjustments can significantly improve your readings:

  • Wear position: Position the watch about one finger-width above your wrist bone, not directly on it
  • Band tightness: Snug enough that light doesn't leak underneath, but not so tight it restricts blood flow
  • Warm up: Let the sensor establish a baseline for a few minutes before checking accuracy during workouts
  • Keep sensors clean: Sweat, sunscreen, and debris can interfere with optical sensors

For maximum accuracy during serious training, consider a chest strap heart rate monitor. These use electrical signals (like an ECG) rather than optical measurement and are significantly more accurate during intense exercise. Most smartwatches can pair with chest straps and use that data instead.

Calorie Burn Estimates: Use With Caution

This is where accuracy becomes genuinely challenging. Calorie estimates combine multiple data points—heart rate, movement, user profile (age, weight, height)—into algorithmic calculations. The problem? Individual metabolism varies enormously.

Studies suggest smartwatch calorie estimates can be off by 20-40% or more, and errors aren't consistent. Your watch might overestimate during one activity and underestimate during another.

Important Consideration

If you're using calorie data for weight management, treat the numbers as rough estimates. They're useful for comparing relative effort between workouts but shouldn't be trusted for precise calorie counting or "eating back" exercise calories.

GPS Distance and Pace: Generally Reliable

GPS accuracy in smartwatches has improved substantially. Under good conditions (clear sky, open areas), expect distance accuracy within 2-3% on quality devices. This is sufficient for most training purposes.

Accuracy decreases in:

  • Urban canyons with tall buildings
  • Dense forest cover
  • Immediately after starting (before GPS lock is established)
  • Indoor environments (where GPS is unavailable)

For the most accurate GPS tracking, wait for your watch to establish a GPS lock before starting your activity. Many watches display a signal strength indicator—wait until it shows a strong connection.

Sleep Tracking: Useful Trends, Imprecise Details

Sleep tracking uses movement and heart rate patterns to estimate when you're asleep and what sleep stage you're in. While the technology has improved, it cannot match the accuracy of clinical polysomnography (sleep studies that measure brain waves).

What sleep tracking does well:

  • Tracking total time in bed
  • Identifying general sleep/wake patterns
  • Showing consistency trends over time

What it struggles with:

  • Accurately differentiating sleep stages (especially REM vs. light sleep)
  • Detecting when you're lying still but awake
  • Tracking naps or non-traditional sleep schedules

For a deeper dive into getting value from sleep data, see our sleep tracking guide.

Blood Oxygen (SpO2): Screening Tool, Not Medical Device

Blood oxygen monitoring has become common in smartwatches, especially since the COVID-19 pandemic raised awareness of oxygen saturation. However, these sensors are designed for wellness insights, not medical diagnosis.

Smartwatch SpO2 readings can vary by 2-4% from medical-grade pulse oximeters. For healthy individuals, this variance is usually inconsequential. But if you're monitoring SpO2 for health concerns, rely on a medical device rather than your smartwatch.

Key Takeaway: Accuracy by Metric

  • Steps: Highly reliable (within 5-10%)
  • Resting heart rate: Generally accurate (within 5 bpm)
  • Exercise heart rate: Variable (use chest strap for precision)
  • GPS distance: Good under open sky (within 2-3%)
  • Calories: Rough estimate only (20-40% variance)
  • Sleep stages: Directionally useful, not precise
  • SpO2: Wellness screening only

The Bottom Line

Smartwatch fitness tracking is a remarkable technology that provides genuine value—when you understand its limitations. Use the data for motivation, trend tracking, and relative comparisons rather than absolute measurements. Your watch is a wellness tool, not a clinical instrument.

The most accurate metric is still the simplest: are you moving more, sleeping better, and feeling healthier? If your smartwatch helps drive those outcomes, the precise numbers matter less than the positive direction.