Heart rate is measured by detecting individual heartbeats and counting how many occur in a set time period, expressed as beats per minute (bpm). You can do this with nothing more than two fingers pressed against an artery, or you can rely on sensors that track blood flow optically or electrical signals from the heart itself. Each method has trade-offs in convenience, accuracy, and detail.
Checking Your Pulse by Hand
The simplest way to measure heart rate is to feel your pulse at a point where an artery runs close to the skin. Two spots work best. The radial artery, on the thumb side of your inner wrist, sits in the groove between the wrist bone and the nearest tendon. The carotid artery runs along each side of your neck, in the soft groove just beside your windpipe. Place the tips of your index and middle fingers (never your thumb, which has its own pulse) over either spot and press lightly until you feel a rhythmic throb.
Once you find the pulse, count the beats. The most accurate approach is to count for a full 60 seconds. A common shortcut is to count for 15 seconds and multiply by four, or count for 30 seconds and multiply by two. The shorter the counting window, the more a single miscount throws off your result. If you miss one beat in a 15-second count, your final number is off by four. Over 60 seconds, that same error only shifts the total by one.
How Fitness Trackers and Smartwatches Work
Most wrist-worn devices use a technique called photoplethysmography. A small LED on the back of the watch shines light into your skin. With each heartbeat, blood surges through tiny vessels near the surface, briefly changing how much light gets absorbed versus reflected. A photodetector next to the LED picks up those fluctuations, and the device’s software translates the pattern into a beat-by-beat heart rate reading.
Green light is the most common wavelength used because it’s absorbed well by blood, making the signal easier to detect. Some newer devices add red or infrared LEDs, which penetrate deeper into tissue and can improve accuracy in certain conditions. The reflected light signal is extremely faint, measured in microamperes, so the software applies heavy filtering to extract the pulse wave from background noise.
What Affects Wearable Accuracy
Several factors can throw off optical readings. Skin pigmentation plays a role: melanin absorbs green light, which reduces the amount of light bouncing back to the sensor and can weaken the signal. Motion is another major factor. Arm movement during exercise creates artifacts in the signal that look similar to pulse waves, making it harder for the device to distinguish real beats. Sweat pooling under the sensor, changes in blood flow to the skin during intense effort, and a loose-fitting band all add further noise. For the cleanest reading from a wearable, keep the band snug, hold your arm still for a few seconds, and wipe moisture from under the sensor if you’ve been sweating heavily.
Electrocardiogram (ECG) Measurement
An electrocardiogram measures heart rate by recording the electrical signals your heart generates with each beat. Electrodes placed on the skin detect these tiny voltage changes, producing the familiar squiggly line on a monitor or paper strip. Each heartbeat shows up as a distinct spike called the R wave, and the time between consecutive R waves (the R-R interval) determines heart rate.
On a standard ECG recording that scrolls at 25 millimeters per second, each large grid square represents 0.2 seconds. A quick way to estimate heart rate is to count the number of large squares between two R waves and divide 300 by that number. Three large squares between beats means a rate of 100 bpm. Four large squares gives 75. Five gives 60. For more precision, you can count the small squares (each worth 0.04 seconds) and divide 1,500 by that number.
These shortcuts only work when the rhythm is regular. If beats are spaced unevenly, clinicians count the number of R waves over a longer stretch, typically 10 seconds, then multiply to get the per-minute rate. ECG is the gold standard for heart rate measurement because it captures the heart’s actual electrical activity rather than inferring beats from blood flow. It also reveals information no other method can, like whether the upper and lower chambers are beating in sync, by separately measuring the intervals between P waves (which represent the upper chambers firing) and R waves.
Chest Strap Monitors
Chest straps sit just below the breastbone and detect the same type of electrical signal an ECG uses, though with far fewer electrodes and much less detail. Because they sit close to the heart and stay firmly in place against the torso, they’re less susceptible to motion artifacts than wrist-worn optical sensors. That’s why they remain the preferred tool for athletes and researchers who need reliable beat-by-beat data during vigorous movement. The trade-off is comfort: many people find a strap around the chest less convenient than a watch.
What Counts as a Normal Heart Rate
A healthy resting heart rate for adults 15 and older falls between 60 and 100 bpm. Well-trained endurance athletes often sit below that range, sometimes in the 40s or low 50s, because a stronger heart pumps more blood per beat and doesn’t need to beat as frequently at rest.
Children and infants have significantly faster resting rates. Newborns range from 100 to 160 bpm. By ages one to three, the typical range narrows to 80 to 130. Between six and ten years old, it drops further to 70 to 110. By the early teenage years, rates settle into the 60 to 105 range and gradually approach adult norms. These higher childhood rates reflect smaller hearts that hold less blood per beat and need to cycle faster to meet the body’s oxygen demands.
Getting the Most Accurate Reading
Regardless of the method you use, a few simple practices improve accuracy. Measure at rest: sit or lie quietly for at least five minutes before checking. Caffeine, stress, dehydration, and recent physical activity all elevate heart rate temporarily and can give you a misleadingly high number if you’re trying to establish your baseline. Take readings at the same time of day, since heart rate naturally fluctuates from morning to evening.
If you’re comparing a wearable to a manual count and the numbers don’t match, the manual count at the wrist or neck is generally more trustworthy for a single resting measurement. Wearables shine in continuous tracking over hours or days, catching trends and patterns that a spot check would miss. For clinical accuracy during a medical evaluation, nothing replaces an ECG.