Every blood pressure reading has two numbers — something like 128/82, said as "128 over 82". Here's what they actually measure, and how to think about them without worrying over every single reading.
The two numbers
| Number | Name | What it measures |
|---|---|---|
| The top one | Systolic | The pressure in your arteries when your heart squeezes |
| The bottom one | Diastolic | The pressure when your heart relaxes between beats |
Both are measured in "millimetres of mercury" (mmHg) — a leftover from the days when blood pressure machines used a column of mercury.
Why the numbers keep changing
Blood pressure is not a fixed quantity like your height. It rises and falls all day with activity, stress, caffeine, temperature, pain, sleep and dozens of other things. Two readings taken minutes apart can differ noticeably — that's normal, not a faulty machine.
A single reading is a snapshot of one moment. What your doctor works from is the pattern across many readings — which is exactly what a diary shows.
Home readings and clinic readings are different
Many people measure higher at the clinic than at home ("white coat" effect), and some measure higher at home than at the clinic. Because of this, the reference points doctors use for home readings are lower than the clinic ones you may have heard — which is why comparing your home numbers against clinic thresholds can be misleading.
Your own doctor may give you a personal target to aim below. If they do, enter it in your diary and the chart will show it as a line — until then, the diary uses the common home guide as a general reference.
What to focus on instead of single readings
- Measure the same way each time — the measuring guide shows how.
- Record every reading, including the good ones.
- Look at your weekly averages, not today's number.
- Bring the whole record to your appointment and let your doctor interpret it.
Track the pattern, not a single number
A dated record you can share with your doctor.
Sources
- National Heart Foundation of Australia, Blood pressure and heart health — https://www.heartfoundation.org.au/your-heart/blood-pressure-and-your-heart
This is general information and does not replace advice from your doctor or healthcare professional. Health Diary does not monitor your readings. If you feel unwell or a reading worries you, contact your doctor's rooms, or call 000 in an emergency.