Four steps: warm your hand and remove nail polish from one finger, clip the pulse oximeter onto that finger, keep the hand still and relaxed below heart level, and wait until the display stops changing. Then read the two labelled numbers: SpO2, oxygen saturation as a percentage, and PR, pulse rate in beats per minute.
The steps follow the FDA's 2025 Pulse Oximeter Basics consumer update and NHS England's 2022 COVID-19 home-monitoring advice; if your device's instructions differ, follow them. Technique matters because MedlinePlus notes that a reading may be 2% to 4% higher or lower than your actual blood oxygen level.
How do I use a fingertip pulse oximeter, step by step?
Sit down and rest first, warm your hand, and remove any nail polish or false nail from the finger you will use. Switch the pulse oximeter on, clip it onto your index or middle finger, and keep that hand still, relaxed and below the level of your heart. Wait until the display settles on one steady number, then write down the SpO2 and pulse rate with the date and time.
- Warm up and clear the finger. NHS England's 2022 COVID-19 easy-read leaflet asks for a hand warm to the touch, rested on your chest for five minutes. Remove nail polish, as the FDA advises, and any false nail, as NHS England adds.
- Clip it on. Switch it on and use your index or middle finger, as the NHS England leaflet suggests.
- Keep the hand still and relaxed. The FDA says to keep it "warm, relaxed, and held below the level of the heart", and not to move it. NHS England's COVID-19 self-monitoring diary instead rests the hand on the chest at heart level. If your clinician or the device instructions name a position, use theirs.
- Wait for one steady number, then record. Write both numbers down, with the date and time, once they stop changing.
How does a pulse oximeter work?
A pulse oximeter shines red and infrared light through the fingertip to a sensor on the other side of the clip. Oxygen-rich and oxygen-poor haemoglobin absorb those wavelengths differently, and the device tracks the small pulsing changes that arterial blood makes with each heartbeat to estimate the percentage of haemoglobin carrying oxygen. That estimate, made from light rather than a blood test, is the SpO2 reading.
The StatPearls chapter on pulse oximetry (2023) puts the wavelengths at about 660 nm for red light and about 940 nm for infrared.
How do you read the numbers on a pulse oximeter?
Read the label beside each number, not its position, because screen layouts differ between models. The number marked SpO2, usually with a % sign, is oxygen saturation. The number marked PR, often with bpm, is the pulse rate, which the FDA describes as similar to heart rate, so PR is the number to read for heart rate. Some models add a third number or indicator showing the strength of the signal.
The FDA's Pulse Oximeter Basics update says "Oxygen saturation values are between 95% and 100% for most healthy individuals but sometimes can be lower in people with lung and heart problems." That is not a personal target: your clinician sets the number that matters for you. For the pulse figure, see what the pulse number on a monitor means.
How long should you keep a pulse oximeter on your finger?
Keep the clip on until the display settles on one steady number. The FDA's 2025 Pulse Oximeter Basics consumer update says to wait a few seconds until the reading stops changing, while NHS England's 2022 COVID-19 self-monitoring diary asked people to keep the pulse oximeter in place for at least a minute, or longer if the reading keeps changing, after resting for at least five minutes.
The same NHS England diary asked people to record the highest result once the reading had not changed for five seconds. If the numbers never settle, the device may not be picking up a clear pulse, which StatPearls links to severe low blood pressure, cold extremities or excessive motion. Warm the hand, keep it still, check the clip and try again. If your clinician has given you a routine, follow that.
What can make a pulse oximeter reading wrong?
Cold fingers, movement, nail polish or false nails, poor circulation and bright light shining into the sensor can all make a fingertip reading less accurate, and StatPearls notes that readings in people with darker skin may be overestimated by about 2%. Skin thickness and current tobacco use are also on the FDA's list.
| Factor | What it does to the reading | What helps | Source |
|---|---|---|---|
| Cold hands | Weakens the pulse signal; StatPearls cites about 33°C (91.4°F) for reliable readings | Warm the hand before clipping on | FDA; StatPearls |
| Movement | Excessive motion can interfere with the reading | Keep the measured hand still | FDA; StatPearls |
| Nail polish or false nails | May block the light; black or blue polish and artificial nails are most often blamed, though studies are mixed | Remove polish; avoid a false nail | FDA; NHS England; StatPearls |
| Bright ambient light | Light leaking into the sensor may produce an inaccurate reading | Cover or shade the finger and clip | StatPearls |
| Poor circulation | A weak fingertip pulse weakens the signal | Rest and warm the hand first | FDA; StatPearls |
| Skin pigmentation | Can read falsely high in darker skin, which may hide low oxygen | Do not adjust the number; ask your clinician | FDA; StatPearls |
| Tobacco use, skin thickness | Listed by the FDA as affecting accuracy | Mention it to your clinician | FDA |
The FDA's advice holds whatever the cause: "Do not rely only on a pulse oximeter." Contact a health care provider if you are concerned about a reading, or if your symptoms are serious or get worse.
How accurate are home pulse oximeters?
A pulse oximeter reading may be 2% to 4% higher or lower than your actual blood oxygen level, according to MedlinePlus. Over-the-counter oximeters sold in shops and online are most commonly "intended for general wellness or sporting/aviation uses and not intended for medical purposes" and "do not generally undergo FDA premarket review", according to the FDA's executive summary for its November 2022 advisory committee meeting.
Prescription oximeters must pass clinical testing, yet the FDA's 2013 premarket guidance accepts an error score (Arms) of up to 3% for fingertip clips, and the 2022 summary explains that at an Arms of 3%, a reading falls within 3 points of an arterial blood test roughly 68% of the time.
Does skin tone affect pulse oximeter readings?
It can. In January 2025 the FDA said that "current scientific evidence suggests there are some accuracy differences in pulse oximeter performance between individuals with lighter and darker skin pigmentation". A falsely high number can hide low oxygen: in a 2020 study of hospital patients by Sjoding and colleagues in the New England Journal of Medicine, summarised by AHRQ PSNet, low oxygen that the oximeter missed was about three times as common in Black patients as in White patients. The FDA's January 2025 recommendations to manufacturers were issued as draft guidance and exclude wellness, sport and aviation oximeters.
Can a pulse oximeter detect carbon monoxide poisoning?
No, a standard pulse oximeter cannot rule it out. Blood disorders such as carboxyhaemoglobinaemia, caused by carbon monoxide, and methaemoglobinaemia change how blood absorbs light and "lead to false readings", according to StatPearls, which advises confirmation with a co-oximeter. If you suspect carbon monoxide exposure, the NHS advises stopping any appliance that may be the cause, if you can, going outside and getting medical advice as soon as possible. The NHS says to call 999 or go to A&E if someone is finding it hard to breathe, has suddenly become confused, has lost consciousness, is feeling weak or has chest or muscle pain; outside the UK, call your local emergency number, such as 911 in the US.
Are smartwatch oxygen readings as reliable as a fingertip oximeter?
Not for medical decisions. Apple, for example, states that measurements from its Blood Oxygen app "are not intended for medical use and are only designed for general fitness and wellness purposes." Many shop-bought fingertip oximeters are also sold as wellness products, so neither type should replace a device your clinician has asked you to use.
For prescription oximeters, the FDA's 2013 premarket guidance accepts a slightly larger error for reflectance sensors, which read light bounced back from the skin, than for fingertip clips that shine light through the finger: an Arms of 3.5% against 3.0%. Wellness watches are not held to those limits. Apple's own instructions name skin blood flow, some tattoos and motion as possible interference. For blood pressure on the wrist, see can an Apple Watch measure blood pressure?
How do I log pulse oximeter readings?
Write each pulse oximeter reading down with the date and time, which the FDA advises so you can track any changes. Record SpO2 and pulse rate as two separate numbers, and add a short note on anything that could have affected the reading, such as cold hands, movement or nail polish. NHS England's COVID-19 home-monitoring diary also recorded how the person felt at the time of each reading.
In the Monitor2Log diary, oxygen saturation with pulse is typed in by hand; the photo scan reads blood pressure monitors and handwritten blood pressure notebook pages only, not oximeter screens. To keep several measurements together, see keeping a blood sugar and blood pressure log. The printable blood pressure log has a notes column that can hold an SpO2 figure, though it has no column made for one.
Sources
- U.S. Food and Drug Administration, 2025, Pulse Oximeter Basics consumer update.
- U.S. Food and Drug Administration, 2022, executive summary for the Anesthesiology Devices Advisory Committee meeting on pulse oximeter accuracy.
- MedlinePlus, U.S. National Library of Medicine, 2024, pulse oximetry medical test page.
- StatPearls, NCBI Bookshelf, 2023, Pulse Oximetry chapter (Torp, Modi, Pollard, Simon).
- NHS England, 2022, self-monitoring COVID-19 diary with pulse oximeter instructions.
- NHS, 2025, carbon monoxide poisoning symptoms and when to get emergency help.
General information only, not medical advice. When and how often to check your oxygen level, and what your readings mean for you, are decisions for your own clinician.