A blood pressure reading has two numbers written as one over the other, like 120/80. The top number (systolic) measures the pressure in your arteries when your heart beats. The bottom number (diastolic) measures the pressure when your heart rests between beats. Both numbers are recorded in millimeters of mercury, abbreviated as mm Hg.
What Each Number Means
The top number, systolic pressure, reflects the maximum force your blood pushes against artery walls during a heartbeat. It’s influenced by how forcefully your heart pumps, how elastic your arteries are, and how much resistance your smaller blood vessels create. For people over 50, this number becomes especially important because arteries stiffen with age and plaque builds up over time, making systolic pressure a stronger predictor of heart disease risk.
The bottom number, diastolic pressure, captures the lowest pressure in your arteries while your heart muscle relaxes between beats. A high diastolic number suggests your blood vessels are under constant strain, even when your heart isn’t actively pumping. In younger adults, an elevated diastolic reading can be an early signal worth paying attention to.
Blood Pressure Categories
Either number being too high is enough to place you in a higher category. Here’s how the ranges break down:
- Normal: Below 120 systolic and below 80 diastolic
- Elevated: 120 to 129 systolic and below 80 diastolic
- Stage 1 hypertension: 130 to 139 systolic or 80 to 89 diastolic
- Stage 2 hypertension: 140 or higher systolic or 90 or higher diastolic
- Hypertensive crisis: Higher than 180 systolic or higher than 120 diastolic
A reading of 180/120 or above is a medical emergency, especially if accompanied by chest pain, shortness of breath, blurred vision, severe headache, confusion, or nausea. That combination requires a 911 call.
How to Get an Accurate Reading
The way you sit, where your arm rests, and what you did five minutes beforehand all change the number on the screen. Small positioning mistakes can add 20 or more mm Hg to your reading, turning a normal result into one that looks like hypertension.
Before you measure, sit in a chair with your back fully supported for at least five minutes. Put both feet flat on the floor with your legs uncrossed. Rest the arm wearing the cuff on a table so it’s level with your chest. A full bladder alone can inflate your systolic reading by up to 33 mm Hg, so use the bathroom first. Having your arm below heart level can add anywhere from 4 to 23 mm Hg. Don’t talk during the reading, and stay still.
The cuff itself matters too. Standard adult cuffs fit arm circumferences between about 27 and 34 centimeters. If your mid-upper arm measures 34 to 44 centimeters, you need a large adult cuff. Arms over 44 centimeters require an extra-large. A cuff that’s too small squeezes unevenly and produces falsely high readings.
When and How Often to Measure
If you’re tracking your blood pressure at home, take it at least twice a day: once in the morning before eating or taking any medication, and once in the evening. Try to measure at the same times each day so you can spot real trends rather than normal daily fluctuations.
Each time you sit down to check, take two or three readings a minute or two apart rather than relying on a single measurement. Individual readings bounce around naturally. Averaging a few back-to-back results gives you a more reliable picture. Write down or log every reading with the date and time so you can share the full record with your doctor rather than trying to remember a number from three weeks ago.
Why Readings Vary Between Arms
It’s common to get slightly different numbers from your left and right arm. A gap of less than 10 mm Hg is normal and nothing to worry about. But if you consistently see a difference of 10 to 15 mm Hg or more in systolic pressure between arms, that can signal blocked or narrowed blood vessels, a condition called peripheral artery disease. People with that pattern also face a higher long-term risk of heart disease.
If you notice a persistent gap, measure on both arms a few times to confirm it’s not a one-off. The arm with the consistently higher reading is the one you should use for all future measurements, since it gives the more accurate picture of the pressure your body is actually dealing with.
White-Coat Effect
Some people consistently read higher in a medical office than at home. This “white-coat effect” can add up to 26 mm Hg to a reading, purely from the stress of being in a clinical setting. Home monitoring helps separate genuine hypertension from anxiety-driven spikes. If your doctor’s office readings run high but your home log shows normal numbers, bring that log to your next appointment. The reverse also happens: some people read normal at the doctor’s office but high at home, which is equally important to catch.