Blood Pressure 100 Over 40–99: Which Are OK?
With an upper value of 100, the answer depends on your lower number. Here are all readings, sorted by what they mean.
On its own, a systolic (upper) value of 100 mmHg is Ideal (optimal) by the international categories (2023 ESH guideline) and Normal under the 2025 AHA/ACC guideline. The worse of your two numbers decides the category, so a raised diastolic (lower) value moves a 100 reading up, and a lower number below 60 makes it low: below 100/60 it counts as low, from 100/80 it counts as normal, from 100/85 it counts as high normal, from 100/90 it counts as stage 1.
Low blood pressure 100/40 to 100/59
Ideal (optimal) 100/60 to 100/79
Hypertension stage 1 100/90 to 100/99
What to do with a reading of 100 over …
A good upper value. Re-check at least once a year, and use the average of several readings rather than a single one. A lower number below 50 (from 100/49 down) is clearly low: measure again, and if it is confirmed or you feel dizzy, faint or confused, contact a doctor the same day. Keep your readings in a free blood pressure log and read how to measure correctly.
Sources: ESH 2023 guideline · AHA/ACC 2025 guideline.