Heart rate 300 on an adult - what does it mean?
A heart rate of 300 bpm is above any expected maximum heart rate, even during the hardest exercise. Check the reading first (devices can miscount); if it is real, it needs urgent medical care.
Where 300 bpm sits on the adult scale
- Very low
- Low (bradycardia)
- Slightly low
- Normal
- Elevated (tachycardia)
- High
- Very high
What a resting pulse of 300 means
At rest, a sudden racing heartbeat of 300 bpm is usually not ordinary sinus rhythm. Typical causes are supraventricular tachycardia (SVT), atrial flutter or fibrillation, and — less often but more dangerous — ventricular tachycardia.
Also double-check the reading: wrist devices can lock onto your movement cadence during exercise and report it as heart rate.
Ranges apply to adults and children aged 10 and older, measured at rest (American Heart Association). By the numbers: at 300 bpm the heart beats about every 0.20 seconds — roughly 432,000 beats a day.
300 bpm during exercise
This table compares 300 bpm with the common estimate of maximum heart rate (220 minus age). Individual maximums can differ by 10–20 bpm.
300 bpm is above the estimated maximum heart rate at every age in the table. It is above any expected maximum, even during the hardest exercise.
| Age | Est. max (220 − age) | 300 bpm is | Training zone |
|---|---|---|---|
| 20 | 200 bpm | 150% of max | above estimated max |
| 30 | 190 bpm | 158% of max | above estimated max |
| 40 | 180 bpm | 167% of max | above estimated max |
| 50 | 170 bpm | 176% of max | above estimated max |
| 60 | 160 bpm | 188% of max | above estimated max |
| 70 | 150 bpm | 200% of max | above estimated max |
How the zones work: Heart Rate Zones: The 5 Training Zones by Age.
How to get an accurate resting reading
- Sit or lie quietly for 5 minutes. No caffeine, nicotine or exercise in the 30 minutes before.
- Place two fingertips (not your thumb) on the inside of your wrist below the thumb, or gently beside your windpipe on one side of the neck.
- Count the beats for 30 seconds and double it — or count a full 60 seconds if the beat feels irregular.
- Repeat on a few mornings before getting up and use the average.
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Nearby values
Measured on a child instead? See heart rate 300 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.