Optical Heart Rate Accuracy on Garmin Watches: A Deep Dive

Whether you’re training by heart rate zones, fine-tuning your recovery days, or just curious about the data your Garmin watch spits out—you’ve probably asked: how accurate is this thing, really?

As someone who loves digging into the actual data and performance not just the marketing, I set out to critically explore the reliability of Garmin’s optical heart rate (OHR) monitors across different generations, and how they stack up to the gold standard: chest straps.

Let’s break it all down.

How Garmin’s Optical HR Works

Garmin watches use a technology called photoplethysmography (PPG). Those little green (and sometimes red) flashing lights on the back of your watch shine into your skin to detect blood volume changes, which they use to estimate your heart rate.

Cool? Absolutely. Perfect? Not even close.

Garmin Elevate Optical Sensor Generations

Garmin has gone through five major sensor generations. Here’s how they compare:

Generation	Found In Watches	Sensor Hardware	Accuracy (vs Chest Strap)	Key Improvements	Verdict
Gen 1	Forerunner 235, Fenix 3 HR, Vivoactive HR	Basic dual-LED	Poor (±10–20 bpm under motion)	First attempt, low sampling rate	Not reliable under load
Gen 2	Fenix 5 (non-Plus), Forerunner 735XT	Triple-LED, better firmware	Meh (±6–15 bpm)	Slightly better filtering	Still problematic in workouts
Gen 3	Fenix 5 Plus, Fenix 6 Series, FR 245/945	Green + red LEDs, PulseOx	Good (±2–6 bpm steady-state)	Faster response, more data	Great for steady runs, not intervals
Gen 4	FR 255/955, Venu 2, Instinct 2, Fenix 7	Smaller bump, refined contact	Very Good (±1–4 bpm)	Motion filtering improved	Close enough for most runners
Gen 5	Fenix 7 Pro, Epix Pro, Venu 3, FR 965	Elevate v5, multi-band LEDs	Excellent (±1–3 bpm)	Fast response, tighter sync	Best OHR Garmin’s made so far

Real-World Accuracy by Workout Type

Here’s how different generations perform in real training scenarios:

Scenario	Gen 1	Gen 3	Gen 5
Resting HR	±3 bpm	±1–2 bpm	<±1 bpm
Zone 2 steady run	±10 bpm	±2–4 bpm	±1–2 bpm
Tempo effort	±15 bpm	±3–6 bpm	±2–3 bpm
Sprints / Intervals	±20+ bpm	±5–10 bpm	±3–6 bpm
Response lag	5–10 sec	3–6 sec	~1–2 sec

Source: Based on testing by DC Rainmaker, DesFit, The5KRunner, and Garmin users on Reddit and Garmin Forums.

Cold, Sweat, and Sensor Fit Matter (A Lot)

Garmin’s optical sensors are incredibly sensitive to environmental conditions and how you wear the device. Here’s what can skew your readings the most:

Cold Weather

When you start a run in winter, blood flow to your extremities decreases, which reduces the PPG signal. Your HR could be 15–20 bpm off for several minutes—even if the watch warms up later.

Sweat

Heavy sweat or rain can disrupt the optical signal by scattering the light used to read blood flow. The sensor might drop out or lag during high-effort workouts when you’re sweating the most.

Improper Fit

This one’s huge. Loose watches = bad readings.

How to Wear Your Garmin for Best Accuracy:

• Tighten the strap so it doesn’t bounce, but don’t cut off circulation

• Wear it 1–2 inches above the wrist bone during workouts

• Wipe off sweat if readings drop or spike randomly

• Use a nylon strap if possible—it conforms better to your wrist for solid contact

Set Up Your Own HR Zones (Don’t Use Garmin’s Defaults)

Garmin’s built-in HR zones are generic and often wrong. Set yours manually using:

• A lab or lactate threshold test

• The 20-minute field test

• Real-world training data (e.g., long runs and tempo efforts)

You’ll get more accurate zone-based training if you stop relying on Garmin’s defaults.

Chest Strap: Still the Gold Standard

Despite how far wrist sensors have come, chest straps still outperform them in:

• Interval tracking

• Real-time responsiveness

• Wet/sweaty conditions

• Cold weather

They use ECG signals instead of light, making them more consistent and reactive.

What About Armbands?

If you don’t love wearing a chest strap, consider a forearm optical HR monitor like:

• COROS HR Armband

• Polar Verity Sense

These sit on fleshier parts of your arm and avoid wrist motion, so they deliver HR data that’s often within ±1–3 bpm of a chest strap.

Final Thoughts

If you have a new Garmin (Gen 4 or newer), you’re likely getting very good HR data for daily tracking, easy runs, and recovery if you are wearing the watch correctly and exercising in ideal conditions. But, if you’re serious about pacing intervals, threshold efforts, or races by heart rate, use a chest strap. Nothing else is as accurate, responsive, or stable across all conditions.

Note: Optical HR struggles during short intervals (e.g. 30–90 seconds). The watch can’t keep up with your heart rate spikes in real time. You might miss peaks completely or see delayed responses.

Personal note: I wouldn’t use HR for race pacing. Between nerves, heat, and cardiac drift, heart rate is often misleading during races. Run by effort, pace, and feel and review HR afterward, not during.

Limitations of This Review

While this guide draws on both firsthand testing and trusted expert reviews, there are some important limitations to keep in mind:

• Individual variability: Optical HR accuracy can vary significantly based on skin tone, wrist size, hair, tattoos, sweat rate, and how the watch is worn. Your experience might differ from test data.

• Sensor placement and fit are critical. A loose strap, poor contact, or cold conditions can drastically skew results, especially in wrist-based sensors.

• No clinical trials: This post is based on real-world testing and anecdotal reports, not lab-controlled studies. Some error margins are visual approximations from HR plots.

• Garmin doesn’t officially publish sensor generation names (e.g. “Elevate Gen 3”). These classifications are community-based and inferred from product teardowns and historical updates.

• Firmware and software updates can impact accuracy over time, meaning performance may improve (or regress) post-review.

For most runners and athletes, the insights here are directionally reliable—but always validate your own data against feel, effort, and common sense.