Heart rate 21 on an adult - what does it mean?
A resting heart rate of 21 bpm is very low for an adult. Outside of elite endurance athletes and deep sleep, a pulse under 40 is unusual: see a doctor promptly, and call 911 (US) or your local emergency number if you feel faint, have chest pain or are short of breath.
Where 21 bpm sits on the adult scale
- Very low
- Low (bradycardia)
- Slightly low
- Normal
- Elevated (tachycardia)
- High
- Very high
What a resting pulse of 21 means
Very slow resting rates in people who are not highly trained can mean the heart's electrical system is not keeping pace — for example sinus node dysfunction or a heart block — or can be a medication effect (beta blockers, some calcium channel blockers, digoxin). An underactive thyroid, low body temperature and electrolyte problems are other causes.
Rule out a measuring error first: a weak or irregular pulse is easy to undercount by hand, and some wrist wearables miss beats. Count for a full 60 seconds at the wrist or use a chest-strap or blood-pressure monitor.
Ranges apply to adults and children aged 10 and older, measured at rest (American Heart Association). By the numbers: at 21 bpm the heart beats about every 2.86 seconds — roughly 30,240 beats a day.
21 bpm during exercise
21 bpm is below half of the estimated maximum heart rate at every adult age, so it is a resting-range value, not an exercise intensity. During light activity most adults are already above 90 bpm.
For a reliable resting value, sit quietly for five minutes first and average a few mornings: Resting Heart Rate: What It Says About Your Health. You can also measure by tapping along on the start page.
Check another reading
Nearby values
Measured on a child instead? See heart rate 21 in a child.
Sources: American Heart Association: All About Heart Rate · MedlinePlus: Pulse · ACSM position stand on exercise intensity (2011)
This page explains a single reading in general terms. It is not a diagnosis. Heart rate alone cannot tell whether the rhythm is regular or whether the heart is healthy — if you are worried, talk to a doctor.