Blood pressure should be measured in both arms during your first check, then taken on whichever arm gives the higher reading from that point forward. There is no universal rule that one arm is always correct. The American Medical Association recommends measuring both arms initially and using the higher-reading arm for all future monitoring.
Why Both Arms Matter at First
Most people assume blood pressure is the same in both arms, but it often isn’t. A small difference of a few points is normal, yet some people have a consistent gap that, if missed, could lead to underestimating their true blood pressure. If your doctor only ever checks your left arm and your right arm runs 8 points higher, your readings have been too low all along. That’s why clinical guidelines call for checking both arms during an initial assessment.
In practice, this step is frequently skipped. Primary care providers have cited time constraints as the main barrier, and studies confirm the guidance is not routinely followed. If your doctor has never checked both arms, it’s worth asking for it at your next visit.
Use the Higher Arm Going Forward
Once you know which arm reads higher, that becomes your measurement arm for all future readings, whether at the clinic or at home. Consistency matters: switching arms between readings introduces variability that makes it harder to track trends over time. If you’ve been diagnosed with high blood pressure or you’re monitoring at home, always use the same arm.
The American Heart Association notes that most people won’t see a big difference between arms. If your initial two-arm check shows readings within a couple of points of each other, either arm is fine. Just pick one and stick with it.
When the Difference Between Arms Is a Warning Sign
A gap between your two arms can be more than a measurement nuisance. A large-scale meta-analysis published in Hypertension found that cardiovascular risk begins to rise when the systolic (top number) difference between arms reaches 5 mmHg or more. The researchers proposed that a 10 mmHg difference should now be considered the upper limit of normal.
The numbers are striking. Among people with high blood pressure, those with a gap of 10 mmHg or more between arms had roughly three times the risk of dying from cardiovascular causes compared to those with no meaningful difference. European and UK guidelines flag a difference above 15 mmHg as carrying additional cardiovascular risk, but the evidence suggests concern should start lower than that.
A persistent gap between arms can signal narrowing in the arteries that supply blood to the upper body. In younger people, this sometimes happens when a muscle compresses an artery or a structural issue restricts flow. In older adults, the usual culprit is atherosclerosis, the same plaque buildup that causes heart attacks and strokes. Less common causes include aortic aneurysm, inflammation of blood vessels, and thoracic outlet compression.
If your doctor finds a significant difference, they may investigate the blood vessels in your arms and chest to look for blockages or structural problems.
Getting an Accurate Reading on Either Arm
Which arm you choose matters less than how you position it. A 2023 trial published in JAMA Internal Medicine found that arm position dramatically affects readings. The reference standard is sitting with your arm resting on a desk or table so the middle of the cuff sits at heart level. When participants let their hand rest on their lap instead, or let their arm hang at their side, readings were artificially elevated. The arm needs to be supported and at the right height.
Cuff placement also affects accuracy. The center of the inflatable bladder inside the cuff should sit directly over the brachial artery, which runs along the inner side of your upper arm. Most cuffs have an arrow or marker indicating where the artery line should be. Misaligning the cuff by even 90 degrees can distort results because the pressure isn’t transmitting evenly to the artery underneath.
A few other basics that affect accuracy regardless of arm:
- Sit with your back supported against a chair, feet flat on the floor, for at least five minutes before measuring.
- Use the right cuff size. A cuff that’s too small inflates readings; too large deflates them.
- Take multiple readings. Clinical protocols typically involve three consecutive measurements. At home, two readings one minute apart, averaged together, give a more reliable number than a single check.
- Avoid caffeine, exercise, and smoking for at least 30 minutes before measuring.
Home Monitoring: Practical Tips
If you’re tracking blood pressure at home, do your initial two-arm comparison using the same monitor. Take three readings on each arm, sitting quietly between them, and compare the averages. Whichever side is consistently higher becomes your monitoring arm. Write it down so you don’t forget weeks later.
Upper-arm monitors are more reliable than wrist models because the cuff sits closer to heart level naturally and compresses the brachial artery directly. If you do use a wrist monitor, you’ll need to be even more careful about keeping the cuff at heart height, since small changes in wrist position create bigger errors.
Recheck both arms once a year or if your readings change unexpectedly. A new difference between arms that wasn’t there before could reflect a developing vascular issue worth mentioning to your doctor.