How do you calculate mean arterial pressure?
Add the systolic to twice the diastolic, then divide by 3. MAP is the average pressure in the arteries over one cardiac cycle. It is the pressure that drives blood into the organs, which is why it is watched so closely in sick patients.
MAP = (systolic + 2 × diastolic) ÷ 3
The same result can be written as diastolic + (systolic − diastolic) ÷ 3. The calculator also shows the pulse pressure: systolic − diastolic.
How to use the calculator
- Enter the Systolic pressure in mmHg.
- Enter the Diastolic pressure in mmHg. It must be lower than the systolic, or the calculator stops with an error.
- Read the Mean arterial pressure, rounded to a whole number, and the Pulse pressure below it.
- The working line shows the unrounded value to one decimal place, so you can check the arithmetic.
Worked examples
Example 1 – a textbook reading. Blood pressure 120/80 mmHg. (120 + 2 × 80) ÷ 3 = 280 ÷ 3 = 93.3 → MAP 93 mmHg. Pulse pressure 120 − 80 = 40 mmHg.
Example 2 – a low reading. Blood pressure 90/50 mmHg. (90 + 2 × 50) ÷ 3 = 190 ÷ 3 = 63.3 → MAP 63 mmHg, pulse pressure 40 mmHg. The result is below 65 mmHg, so the low-perfusion warning appears. A systolic of 90 can look acceptable on its own while the MAP is already under the usual threshold.
Example 3 – a wide pulse pressure. Blood pressure 160/70 mmHg. (160 + 2 × 70) ÷ 3 = 300 ÷ 3 = MAP 100 mmHg, with a pulse pressure of 90 mmHg. Very different readings can share the same MAP: 130/85 also gives 100.
Quick reference
MAP for some common readings, as the calculator displays them.
| Blood pressure | MAP | Pulse pressure |
|---|---|---|
| 80/50 mmHg | 60 mmHg | 30 mmHg |
| 90/60 mmHg | 70 mmHg | 30 mmHg |
| 100/60 mmHg | 73 mmHg | 40 mmHg |
| 110/70 mmHg | 83 mmHg | 40 mmHg |
| 120/80 mmHg | 93 mmHg | 40 mmHg |
| 140/90 mmHg | 107 mmHg | 50 mmHg |
| 160/100 mmHg | 120 mmHg | 60 mmHg |
Common mistakes
- Taking the simple average. (120 + 80) ÷ 2 = 100 overestimates the true mean, because diastole lasts longer than systole.
- Swapping the two values. The diastolic must be the lower number; the calculator refuses inputs where it is not.
- Treating formula and monitor as the same measurement. A cuff or arterial line that displays MAP has measured it; the formula estimates it from two points. Small differences are expected.
- Reading 65 mmHg as a universal target. It is an adult reference point. Targets can be set higher or lower for individual patients, and children have their own ranges.
Limits
The one-third weighting assumes a normal resting heart rate. In tachycardia, diastole shortens and the true mean moves towards the systolic value, so the formula tends to underestimate it. Arrhythmias, aortic regurgitation and very stiff arteries add further error. The result is only as good as the cuff reading: a cuff of the wrong size or a moving arm distorts both numbers.
A falling MAP rarely travels alone. The shock index can flag trouble while the pressure still looks fine, and in suspected infection the qSOFA score adds the bedside picture. For fluid management, a running fluid balance helps.
Frequently asked questions
What is a normal mean arterial pressure?
In healthy adults at rest, MAP usually lies between about 70 and 110 mmHg. In critical care, 65 mmHg is the most widely quoted lower threshold: the Surviving Sepsis Campaign, for example, uses it as the initial target in septic shock. The calculator flags any value below 65 mmHg.
Why is the diastolic pressure counted twice?
Because the heart spends about two thirds of each beat in diastole. At a normal resting heart rate, roughly one third of the cycle is systole and two thirds diastole. The arterial pressure therefore sits closer to the diastolic value for most of the time, and the formula weights it accordingly.
Is the calculated MAP the same as the one on the monitor?
Not exactly. Automatic cuffs measure MAP directly, at the point of maximum oscillation, and then estimate systolic and diastolic from it. Arterial lines average the whole waveform. Both can differ from the formula by a few mmHg, and a measured MAP is usually the more accurate figure.
What is pulse pressure?
Pulse pressure is systolic minus diastolic. At rest it is typically around 40 mmHg in young adults. A wide pulse pressure is common with stiff arteries in older people. A narrow one can go with a low stroke volume, for example in hypovolaemia, so it is worth a second look.
Is there another way to write the formula?
Yes: diastolic + (systolic − diastolic) ÷ 3, which is the diastolic plus one third of the pulse pressure. It gives exactly the same number, because it is the same formula rearranged. Some people find it quicker for mental arithmetic: for 120/80, 80 + 40 ÷ 3 = 93.3.
Can I use this calculator for children?
Yes for the arithmetic, but not for the warning. The formula is the same at any age, while normal values depend on age and size. The 65 mmHg warning is an adult threshold and does not apply to children, so it’s best to compare a child’s MAP with paediatric reference ranges instead.