How mean arterial pressure is calculated
If you are here with a worksheet open and a quiz in the morning, you are in the right place. Most people who look up a MAP calculator are nursing, paramedic, or medical students checking whether the number they got by hand is the number the answer key wants, and that is exactly what this page is for. Type the reading, get the answer, and read the working, because on an exam the working is the part you have to reproduce without us.
Mean arterial pressure is the average pressure in the arteries across one full cardiac cycle. It is not the average of the two numbers in a blood pressure reading, and that is the single most common way students lose the mark. The cycle is not split evenly between the squeeze and the refill, so the average has to be weighted toward the part that lasts longer, which is diastole. That weighting is where the 2 in the formula comes from.
One thing to be clear about before the arithmetic: this page checks a calculation, not a person. It has no idea who the numbers belong to, what their baseline is, what they are being treated with, or what a monitor said thirty seconds later. A MAP is one number in a clinical picture, and the picture belongs to the clinician standing in the room.
The formula
SBP is the systolic pressure, the peak as the left ventricle ejects. DBP is the diastolic pressure, the trough as it refills. PP is the pulse pressure, the gap between them. Every value is in millimeters of mercury (mmHg).
The two MAP formulas look different and are the same equation. Expand the second one and watch it collapse into the first: DBP + (SBP − DBP) ÷ 3 becomes (3 × DBP + SBP − DBP) ÷ 3, which is (SBP + 2 × DBP) ÷ 3. Textbooks print one, lecturers often teach the other, and exam questions use both. If you know they are identical, neither one can surprise you. This calculator runs both and shows both, which is also a free self check: two routes to one answer means a slip in either shows up immediately.
Worked example
A reading of 120/80 mmHg:
Pulse pressure: PP = 120 − 80 = 40 mmHg.
Form one: MAP = (120 + 2 × 80) ÷ 3 = (120 + 160) ÷ 3 = 280 ÷ 3 = 93.3 mmHg.
Form two: MAP = 80 + 40 ÷ 3 = 80 + 13.33 = 93.3 mmHg.
Note that the answer is 93.3, not 100. Averaging 120 and 80 gives 100, and it is wrong, because diastole occupies more of the cycle than systole does. That gap of nearly seven points is what the weighting buys you.
An exam-style one, 148/92 mmHg: PP = 56. MAP = (148 + 184) ÷ 3 = 332 ÷ 3 = 110.7 mmHg, or 92 + 18.67 = 110.7 by the second form.
Why diastolic is counted twice
Take a resting heart rate of about 75 beats per minute. That is one cardiac cycle every 0.8 seconds, of which systole takes roughly 0.3 seconds and diastole roughly 0.5 seconds. Diastole is therefore about two thirds of the cycle and systole about one third, and the true mean pressure is the time-weighted average of the pressure over the whole cycle, not a plain average of the highest and lowest points.
Weighting diastolic at two thirds and systolic at one third is the arithmetic version of that. It is the reason MAP always sits closer to the bottom number than the top one: 120/80 gives 93.3, much nearer to 80 than to 120. If an answer ever comes out above the midpoint of the two numbers, you have flipped the weights.
Strictly, the real definition is not a formula at all. True MAP is the area under the arterial pressure waveform divided by the length of the cycle, which an arterial line does by integrating the actual trace. The one third and two thirds version is a rule of thumb that reproduces that integral closely enough at ordinary resting heart rates, which is why it survives in every textbook.
Why MAP and not just the systolic number
Organs are not perfused by the peak of the waveform. They are perfused by the pressure that is present on average, all cycle long, which is why MAP is the number that turns up in every perfusion calculation you will be asked about. Cerebral perfusion pressure is CPP = MAP − ICP. Renal, coronary, and splanchnic flow are all discussed against MAP rather than systolic. The classic autoregulation curve, the one where flow stays roughly flat across a range of pressures, is drawn against MAP, and in the traditional teaching that plateau runs somewhere around 50 to 150 mmHg before flow starts to follow pressure passively.
MAP is also where the physiology plugs in. MAP is approximately cardiac output multiplied by systemic vascular resistance (plus central venous pressure, which is small enough that most teaching drops it). That equation is why a patient can be vasodilated with a fine cardiac output and still have a low MAP, and it is why the two numbers you were given at the start are only ever the surface of the thing.
For reference ranges, textbooks generally cite roughly 70 to 100 mmHg as a normal MAP, and critical care commonly treats 60 to 65 mmHg as the floor to hold for organ perfusion, with 65 mmHg the initial target named in the Surviving Sepsis Campaign guidance for adults in septic shock. Those are the numbers to know for an exam. They are ranges taught as reference points, and no range substitutes for the judgment of whoever is actually caring for the patient.
When the one third rule drifts
Being able to say where a formula stops working is usually worth more marks than the formula itself. The one third and two thirds split is an assumption about the shape of the cardiac cycle, so it drifts whenever that shape changes.
- Tachycardia. As heart rate climbs, diastole shortens far more than systole does. At 140 beats per minute diastole is no longer two thirds of the cycle, so a formula that still weights it two thirds tends to underestimate the true mean.
- Very stiff arteries or aortic regurgitation. A widened pulse pressure changes the waveform shape, and the wider the gap between systolic and diastolic, the more approximate the estimate becomes.
- Irregular rhythms. In atrial fibrillation, beat-to-beat pressure varies, so a single pair of numbers has less to say about the average of anything.
- The monitor already knows. An automated cuff computes its own MAP from the oscillometric envelope, and an arterial line integrates the waveform directly. Both will often differ a little from the formula, and neither is being sloppy. They are measuring a different way, and the invasive one is the closest thing to the true value.
None of that makes the formula wrong. It makes it what it is: an estimate that is quick, closely enough right at rest, and completely reproducible by hand, which is why it remains the version you have to be able to do on paper.
What pulse pressure tells you
Pulse pressure is the smaller number this calculator hands back, and it earns its own exam questions. It is the gap the heart opens with each stroke, driven mainly by stroke volume and by the stiffness of the large arteries.
Textbooks describe a pulse pressure over about 60 mmHg as wide, and it is classically associated with stiff arteries in older adults, aortic regurgitation, and high output states such as anemia, fever, and thyrotoxicosis. A pulse pressure under about 25 mmHg, or under about a quarter of the systolic value, is described as narrow, and is classically associated with low stroke volume: hypovolemia, cardiogenic shock, tamponade, and aortic stenosis. Learn the two lists as textbook associations, because that is what they are. A number in isolation names no diagnosis.
Common readings and their MAP
Useful for spot checking a worksheet. Every row here was produced by this page's own code, not typed in by hand.
| Reading (SBP/DBP) | Pulse pressure | MAP |
|---|---|---|
| 80/50 | 30 | 60.0 |
| 90/50 | 40 | 63.3 |
| 90/60 | 30 | 70.0 |
| 100/70 | 30 | 80.0 |
| 110/70 | 40 | 83.3 |
| 120/80 | 40 | 93.3 |
| 130/85 | 45 | 100.0 |
| 140/90 | 50 | 106.7 |
| 160/100 | 60 | 120.0 |
Two patterns worth carrying into the exam room. First, the answer always lands nearer the diastolic number, every time, without exception. Second, a reading can look reassuring at a glance and still produce a MAP under the floor: 90/50 reads like a mild low pressure and calculates to 63.3, below the 65 mmHg line critical care texts talk about. That is the whole argument for calculating MAP rather than eyeballing a pair of numbers.
The mistakes that cost the marks
Four of them, in the order we see them. Averaging the two numbers (120 and 80 gives 100, and the answer is 93.3). Weighting systolic twice instead of diastolic, which produces 106.7 for the same reading and is the one that hides best, because the number looks perfectly plausible. Forgetting to divide by 3 after adding. And entering the reading backwards, which this page catches for you rather than quietly returning a number: 80/120 is not a low MAP, it is a typo.
If you are working through cardiovascular material more broadly, the BMI calculator and BMR calculator get the same step-by-step treatment, and the BAC calculator walks the Widmark formula the same way.