A pulse oximeter displays two main numbers: your blood oxygen level (SpO2) and your pulse rate (PR). A normal SpO2 reading is 95% or higher, and a normal resting pulse rate falls between 60 and 100 beats per minute. Some devices also show a waveform, a perfusion index, or both. Here’s how to get an accurate reading and understand what every number on the screen means.
How to Get an Accurate Reading
Clip the device onto your index, middle, or ring finger with the display facing you. Your finger should be clean, dry, and warm. Press the power button and keep your hand completely still while the device measures. Most oximeters stabilize within 5 to 10 seconds, but give it a full 15 to 30 seconds before you trust the number.
A few things can throw off the reading. Cold hands reduce blood flow to your fingertips, which makes it harder for the sensor to detect a pulse. If your fingers feel cold, warm them up for a minute or two before trying again. Nail polish, especially black, blue, brown, or dark purple shades, can interfere with the light sensor. A large systematic review found that black nail polish caused the most variation in readings, and in one study, the oximeter failed to produce a reading at all on 88% of fingers painted with black polish. Gel and acrylic nails can cause similar problems. For the most reliable result, use a bare fingernail.
Movement is the other common culprit. Even small hand tremors or fidgeting can distort the signal and produce wildly inaccurate numbers. Sit quietly with your hand resting on a surface at or below heart level.
Reading the SpO2 Number
The SpO2 percentage tells you how much of your blood’s hemoglobin is carrying oxygen. For a healthy adult at rest, 95% to 100% is normal. Anything below 95% is generally considered abnormal and worth paying attention to.
If your reading drops to 92% or lower, contact your healthcare provider. At 88% or lower, it’s a medical emergency. At oxygen levels below 80% to 85%, the brain begins to show measurable changes in function, and by around 67%, the skin can turn visibly blue, a sign called cyanosis.
One important exception: if you have a chronic lung condition like COPD, your normal baseline may be lower than 95%. Medical guidelines for COPD actually target oxygen saturations of 88% to 92%, because pushing oxygen levels higher in these patients can paradoxically increase risk. If you’ve been given a specific target range by your doctor, use that instead of the standard 95% threshold.
Reading the Pulse Rate
The number labeled PR or PRbpm is your pulse rate in beats per minute. A normal resting heart rate for adults is 60 to 100 bpm. Well-trained athletes often sit in the 40s or 50s, which is usually fine for them.
A resting pulse consistently below 60 or above 100 may be worth discussing with a provider, especially if it’s a change from your usual pattern or comes with symptoms like dizziness, chest tightness, or shortness of breath. Some oximeters also display a small heart icon that blinks with each detected beat. If that icon blinks erratically or the pulse number jumps around, the device may not be getting a clean signal. Reposition your finger and try again.
The Waveform (Pleth)
Many pulse oximeters, especially clinical-grade and some higher-end home models, display a moving wave at the bottom of the screen. This is called the plethysmograph, or “pleth” wave, and it represents the pulsing of blood through your finger with each heartbeat.
A good pleth wave looks like a series of smooth, evenly spaced peaks, similar to rolling hills. Each peak corresponds to one heartbeat. When this wave is consistent and well-defined, you can trust the SpO2 and pulse rate numbers on screen. If the wave is flat, jagged, or erratic, the device is struggling to detect blood flow reliably, and the SpO2 reading may be artificially high or low. Common causes include a loose fit on the finger, cold hands, or too much movement. Reposition the sensor and hold still until the waveform stabilizes before reading the numbers.
What the Perfusion Index Means
Some oximeters display a number labeled PI, which stands for perfusion index. This is essentially a signal-strength indicator. It measures how much blood is pulsing through the tissue at the sensor site compared to the baseline. A higher PI means stronger blood flow and a more reliable reading.
PI is displayed as a percentage, typically ranging from about 0.2% (very weak signal) to 20% (very strong). Research has found that a PI below 2% is associated with less accurate SpO2 readings. If your device shows PI and the number is very low, warming your hands or trying a different finger can improve blood flow and give you a more trustworthy result.
Skin Tone Can Affect Accuracy
Pulse oximeters work by shining light through your skin and measuring how much is absorbed by oxygenated versus deoxygenated blood. Darker skin pigmentation can affect how that light is absorbed, and the FDA has acknowledged that current devices show some accuracy differences across skin tones. In January 2025, the FDA proposed new draft guidance requiring manufacturers to test their devices across a wider range of skin pigmentations and to clearly label oximeters that have been validated for diverse populations.
What this means in practice: if you have darker skin, a pulse oximeter may slightly overestimate your actual oxygen level. A reading of 95% might correspond to a true oxygen level a few percentage points lower. This doesn’t make the device useless, but it does mean you should pay attention to trends and symptoms rather than treating any single number as absolute. If you feel short of breath or unwell and your reading looks borderline normal, take the symptoms seriously.
What Your Numbers Mean Together
The most useful approach is to look at all the numbers as a set rather than focusing on one in isolation. A normal SpO2 with a normal pulse rate and a strong, steady waveform is reassuring. An SpO2 of 94% with a weak, erratic waveform and a very low perfusion index is more likely a bad reading than a sign of low oxygen. Recheck before reacting.
If you’re monitoring a chronic condition, take readings at the same time each day, in the same position, using the same finger. This gives you a reliable baseline so you can spot meaningful changes. Write down the SpO2, pulse rate, and PI (if available) each time. A single low reading can be a fluke. A consistent downward trend over several readings is something to act on.