A heart rate is considered abnormal when it falls outside the range of 60 to 100 beats per minute (bpm) for a resting adult. But that number alone doesn’t tell the whole story. A rate of 55 bpm can be perfectly healthy in a fit person, while a rate of 95 bpm might signal a problem in someone who is dehydrated or anemic. What matters is not just the number, but whether your heart is pumping enough blood to meet your body’s needs.
Normal Resting Heart Rate by Age
The 60 to 100 bpm standard applies to adults and adolescents age 13 and older. Children and infants run much faster. Newborns typically beat between 100 and 205 bpm, infants between 100 and 180, toddlers between 98 and 140, and school-age children between 75 and 118. These ranges apply when a child is awake and at rest. During sleep, rates naturally dip lower at any age.
To get an accurate resting reading, sit or lie down in a calm state for a few minutes first. Place your index and middle fingers on the inner wrist, feel for each pulse, and count the beats over a full 60 seconds. Checking after exercise, caffeine, or a stressful moment will give you a misleadingly high number.
When a Fast Heart Rate Is Abnormal
A resting heart rate above 100 bpm in an adult is called tachycardia. It’s one of the most common abnormal heart rate patterns, and it has a wide range of causes. Many of them are temporary and not dangerous: stress, dehydration, fever, caffeine, alcohol, or tobacco use can all push your rate above 100. Pregnancy also raises resting heart rate.
Other causes are more serious. An overactive thyroid gland speeds up your metabolism, which forces the heart to work harder. Anemia, where your blood carries less oxygen than normal, has the same effect. Heavy bleeding, damaged heart muscle, high blood pressure, and low blood sugar can all trigger a sustained fast rate. If your resting heart rate is frequently above 100 and you can’t point to an obvious cause like a cup of coffee or a stressful morning, it’s worth investigating.
When a Slow Heart Rate Is Abnormal
Bradycardia means your heart beats fewer than 60 times per minute at rest. For many people, this is completely normal. Well-trained athletes often have resting rates closer to 40 bpm because their hearts are efficient enough to pump the same amount of blood with fewer beats. A rate in the 40s or 50s is also common during deep sleep.
Bradycardia becomes a problem when the heart can’t pump enough oxygen-rich blood to the brain and body. The symptoms are distinct: dizziness or lightheadedness, unusual fatigue (especially during physical activity), confusion or memory problems, shortness of breath, and fainting or near-fainting. If a slow heart rate comes with any of these symptoms, the heart isn’t keeping up with demand.
Irregular Rhythm vs. Fast or Slow Rate
An abnormal heart rate isn’t always about speed. Sometimes the rhythm itself is the problem. Atrial fibrillation (AFib) is the most common example. In a healthy heart, electrical signals travel in an orderly path from the upper chambers to the lower chambers, producing a steady beat. In AFib, the signals in the upper chambers become chaotic, causing them to quiver rather than contract in a coordinated way. The lower chambers then beat fast and irregularly, often between 100 and 175 bpm.
What makes AFib different from simple tachycardia is the irregularity. If you check your pulse and the spacing between beats is uneven, skipping or stuttering rather than ticking along at a steady pace, that’s a different concern than a pulse that’s simply fast. AFib increases the risk of blood clots and stroke, so an irregular rhythm is worth taking seriously even if your average rate doesn’t seem extreme.
What Pushes Your Heart Rate Up or Down
Beyond medical conditions, several everyday factors influence where your resting heart rate lands. Caffeine and other stimulants speed it up. Dehydration reduces blood volume, forcing the heart to compensate by beating faster. Stress and anxiety trigger your body’s fight-or-flight response, releasing hormones that raise your rate. Even sitting up versus lying down makes a measurable difference.
On the slower side, certain medications deliberately lower heart rate. Beta-blockers, one of the most commonly prescribed heart medications, work by blocking the hormones that tell your heart to speed up. If you take a beta-blocker and your resting rate drops into the 50s, that’s usually the intended effect, not a sign of trouble. Other blood pressure and heart rhythm medications have similar effects.
Fitness level also plays a major role. Regular aerobic exercise strengthens the heart muscle so it can pump more blood per beat. Over months of consistent training, resting heart rate gradually decreases. This is why a rate of 48 bpm in a runner is healthy, while the same number in a sedentary person with no medication changes could signal a conduction problem in the heart.
Heart Rate During Exercise
Your heart rate is supposed to climb during physical activity, so a high number mid-workout isn’t abnormal. The question is how high is appropriate. You can estimate your maximum heart rate by multiplying your age by 0.7 and subtracting that number from 208. For a 40-year-old, that works out to about 180 bpm.
The American Heart Association recommends staying between 50% and 70% of your maximum for moderate exercise, and 70% to 85% for vigorous exercise. For that same 40-year-old, moderate intensity means roughly 90 to 126 bpm, and vigorous means 126 to 153 bpm. Going above 85% of your maximum is generally reserved for short bursts during interval training and isn’t sustainable or necessary for most people.
Symptoms That Signal a Problem
A heart rate outside the normal range doesn’t always produce symptoms. Many people discover an abnormal rate during a routine checkup or while wearing a fitness tracker. When the heart can compensate well enough, you may feel nothing at all.
The symptoms that do matter are the ones suggesting your organs aren’t getting enough blood. Dizziness, fainting, chest pain, shortness of breath, and unusual fatigue are the key warning signs regardless of whether your rate is too fast, too slow, or irregular. Chest pain lasting more than a few minutes, difficulty breathing, or fainting warrant emergency care. These symptoms can indicate that the heart’s electrical system or pumping ability has been compromised in a way that needs immediate attention.
A single high or low reading on a smartwatch, on the other hand, is rarely cause for alarm on its own. Context matters: what you were doing, how you were feeling, and whether the reading repeats consistently. Tracking your resting rate over days or weeks gives a much clearer picture than any single measurement.