Since the pandemic, millions of people have downloaded apps that promise to measure blood oxygen by pointing a phone camera at a finger. Does it work? Almost never the way people hope.
Some phones can produce reasonable estimates. Most "oximetry" apps on the app stores, though, aren't reliable for real medical use — and the gap between the two categories is nowhere near obvious from a store screenshot.
To calculate how much oxygen is in the air at your elevation, no sensor required, use the Oxymeter calculator.
Real oximetry vs. "camera oximetry"
How a real pulse oximeter works
A certified pulse oximeter uses two LEDs — one red (660 nm), one infrared (940 nm) — shining through the tissue of a finger. The ratio of how much each wavelength gets absorbed by oxygenated versus deoxygenated hemoglobin lets the device calculate SpO2 with an error of just ±2-3%.
That takes specific hardware: two calibrated LEDs and a photodetector positioned precisely on the other side of the finger. It's not something a smartphone's rear camera can replicate.
The problem with camera apps
Apps that use the camera and flash estimate saturation from skin color. Three problems, worst first:
- A phone's LED flash emits white light, not the specific wavelengths (660 nm / 940 nm) oximetry actually needs
- A camera sensor isn't built to measure differential light absorption in tissue
- The typical error runs ±10-15% or worse — a margin that makes the readings clinically useless
A "reading" of 95% from a camera app could correspond to a real value anywhere between 80% and 100%. Too imprecise for anything that actually matters.
Phones with a real SpO2 sensor: what exists
A handful of manufacturers have built genuine SpO2 sensors into phones or smartwatches.
| Device | SpO2 sensor | Approx. accuracy | Notes |
|---|---|---|---|
| Samsung Galaxy S8, S9, S10, S20 | Yes (rear) | ±3-5% | Dropped from S21 onward |
| Samsung Galaxy Watch 4/5/6/7 | Yes (wrist) | ±3-5% | Continuous overnight tracking |
| Apple Watch Series 6+ | Yes (wrist) | ±3-5% | Optimized for SpO2 ≥ 90% |
| Garmin Fenix 7/8, Forerunner 965 | Yes (wrist) | ±3-4% | Useful for trends at altitude |
| iPhone (any model) | No | N/A | No SpO2 sensor at all |
| Android without a dedicated sensor | No | N/A | Camera apps aren't trustworthy |
Wrist or finger: it matters
Wrist sensors (Apple Watch, Galaxy Watch, Garmin) are less accurate than fingertip devices for three reasons: perfusion is weaker at the wrist, the strap's pressure distorts the signal, and they're more exposed to motion artifacts.
For medical decisions, the finger is still the right call.
When the phone is enough
Tracking a trend in a healthy person
With a smartwatch carrying a real SpO2 sensor, you can watch how saturation shifts overnight (useful for flagging suspected sleep apnea), how it changes during exertion, or get a general reference at altitude — indicative only.
Personal curiosity, no medical stakes
Watching how your body responds during a steep hike, with a watch that has the right sensor, is genuinely interesting data. It's not a foundation for health decisions, though.
When you actually need a certified fingertip pulse oximeter
A certified (CE Class IIa) fingertip pulse oximeter is worth the money if:
- You have COPD, severe asthma, or heart failure — monitoring needs to be precise
- You're heading above 3,500 m — at that altitude, a watch's ±5% error can be the gap between two opposite calls
- You had COVID-19 with lung involvement — recovery needs to be tracked with numbers you can trust
- You're over 65 with a respiratory condition — your doctor may want precise reference values
- You're monitoring a family member at home — the ±2% precision of a fingertip device buys real peace of mind
Prices stay affordable:
- Contec CMS50D: $20-40 (CE Class IIa certified)
- Beurer PO80: $40-70 (CE Class IIa certified)
- Nonin GO2: $130-200 (FDA 510(k) certified — clinical standard)
We go deeper on this in choosing a reliable pulse oximeter.
Apps that actually help (the real ones)
"Camera oximetry" apps remain unreliable. Legitimate apps do exist, though — they pair with a fingertip pulse oximeter rather than replacing it.
| App | Function | Platform |
|---|---|---|
| Samsung Health | Reads SpO2 from a Galaxy Watch or phone sensor | Android |
| Health (Apple) | SpO2 history from Apple Watch | iOS |
| Garmin Connect | Overnight SpO2 history from Garmin devices | iOS / Android |
| Oxymeter.it | Calculates available O2 by elevation | Web / PWA |
None of these measure SpO2 on their own — they read it from the hardware sensor of whatever device they're paired with.
The practical answer: combine them
For anyone monitoring respiratory health or heading to altitude, the best strategy blends more than one tool.
- Certified fingertip pulse oximeter ($20-40) — for reference readings, morning and evening, especially with a respiratory condition or at altitude
- Smartwatch with SpO2 (if you already own one) — for overnight trends and desaturation alerts during sleep
- Oxymeter calculator — to know how much oxygen is in the air at your elevation, and give context to what you're reading
Left the pulse oximeter at home? Alternative ways to estimate SpO2 also cover the clinical signs worth recognizing without any device at all.
FAQ
Can you measure blood oxygen with a phone?
Only if the phone has a dedicated SpO2 sensor (some Samsung Galaxy models through the S20). Apps using the standard camera carry an error of ±10-15% — unacceptable for medical use.
Are iPhone oximetry apps reliable?
No. The iPhone has no SpO2 sensor. Apple Watch Series 6+ has a wrist sensor (±3-5%), more reliable than a camera but still behind a certified fingertip pulse oximeter.
What's the best app to measure oxygen on Android?
On Samsung Galaxy phones with an SpO2 sensor, Samsung Health gives acceptable readings for non-clinical use. On other Android phones without a sensor, no camera app is medically useful.
When do I need a real fingertip pulse oximeter?
If you have COPD, severe asthma, heart failure, are heading above 3,500 m, or need serious medical monitoring. A Contec CMS50D runs $20-40 and is far more precise than any phone app.
Living at altitude, or heading out on an expedition? Use the Oxymeter calculator to see how much oxygen is actually in the air at your elevation — and give context to whatever your pulse oximeter reads.
