Mean Arterial Pressure (MAP) Calculator

Mean Arterial Pressure Calculator

Mean arterial pressure (MAP) is the average pressure inside a person's arteries over one complete heartbeat. This calculator estimates it from an ordinary blood pressure reading using the standard one-third approximation $M \approx D + (S - D)/3$, and reports the pulse pressure alongside it.

What the calculator does

It takes the two numbers of a cuff blood pressure reading, both in millimetres of mercury (mmHg):

• Systolic blood pressure (SBP), the higher number. It is the pressure while the heart is squeezing blood out.
• Diastolic blood pressure (DBP), the lower number. It is the pressure while the heart is relaxed between beats.

It returns two numbers, also in mmHg: the mean arterial pressure and the pulse pressure. Both are rounded to one decimal place, so the figure shown is the figure computed. The page works in mmHg only and has no kPa option.

How to calculate mean arterial pressure

Pulse pressure is the gap between the two readings:

$P = S - D$

Mean arterial pressure is the diastolic value plus one third of that gap:

$M \approx D + \frac{S - D}{3}$

where M is the mean arterial pressure (MAP), S the systolic pressure (SBP), D the diastolic pressure (DBP) and P the pulse pressure, all in mmHg. The sign is approximately equal to on purpose: this relation estimates MAP rather than defining it, as the next section sets out.

The same rule is often written $M \approx (S + 2D)/3$. The two forms are the same expression rearranged and always give the same answer. The calculator uses the first form because it shows the structure: the result starts at the diastolic pressure and rises only a third of the way toward the systolic pressure. That is why MAP always sits nearer the lower number than the higher one (Guyton and Hall, Textbook of Medical Physiology, 14th edition).

Why the formula uses one third

MAP has an exact definition. It is the pressure curve averaged over time across one cardiac cycle, that is, one whole heartbeat:

$M = \frac{1}{T} \int_{a}^{b} p(t) \, dt$

where p is the arterial pressure at time t, the beat runs from a to b, and T is its length. Computing that average needs a continuous pressure trace, which a cuff does not produce (Klabunde, Cardiovascular Physiology Concepts, 3rd edition).

The one-third rule is a shortcut around that integral. At a normal resting heart rate, the relaxed part of the beat takes roughly two thirds of the cycle and the squeezing part roughly one third. Weighting diastole twice as heavily as systole approximates the true average without a pressure trace. This is the formula given by DeMers and Wachs in "Physiology, Mean Arterial Pressure", StatPearls (Bookshelf ID NBK538,226), which also limits it to normal resting heart rates.

The result is therefore an approximation of MAP, not a measurement of it.

Worked example

For a reading of 120/80 mmHg, the pulse pressure is 120 − 80 = 40 mmHg, and the mean arterial pressure is 80 + 40 / 3 = 93.333… mmHg, shown as 93.3 mmHg.

The rearranged form gives the same answer: (120 + 2 × 80) / 3 = 280 / 3 = 93.333…

More readings, worked the same way:

The last row divides evenly, so no rounding happens: 100 + 20 = 120 exactly.

What pulse pressure is

Pulse pressure is the size of the swing between the peak and the trough of each beat. A 120/80 reading has a pulse pressure of 40 mmHg. The calculator shows it because it is the quantity the one-third weighting is applied to, so it makes the arithmetic easy to follow.

Where this estimate is unreliable

The formula is an approximation, and it is a fixed one. It has no way to know anything about the heartbeat it is describing.

• Fast heart rates. As heart rate rises, diastole shortens, so it no longer fills about two thirds of the cycle. The one-third formula then reads lower than a directly measured MAP. Razminia and colleagues reported this in Catheter Cardiovascular Interventions (2004; 63(4):419–425) and proposed a heart-rate-corrected formula in its place. This calculator does not implement that correction and has no heart rate input.
• Irregular rhythms. Any single-beat formula assumes the beats are alike. When beat-to-beat pressure varies, one reading does not represent the average.
• Cuff readings are themselves estimates. SBP and DBP from an arm cuff are not the same measurements as an arterial line records, so a MAP derived from them is not the same quantity as an intra-arterial MAP or the MAP an oscillometric monitor computes internally.

The calculator does arithmetic only. It does not diagnose, dose, or offer any clinical guidance, and it presents no reference ranges.

Values the calculator rejects

The input boxes carry a minimum and a maximum, but those are only HTML attributes: a pasted or typed value can pass straight through them. The same limits are re-checked in the calculation code, which is where they take effect.

400 mmHg is an implausibility limit, not a medical one. Real cuff and arterial-line readings never come near it. It is there so that a value which is not a blood pressure at all produces an error instead of a number. A reading of exactly 400/397 is accepted and gives a MAP of 398.0.

Conventions this calculator takes

Two choices here are open to argument, so they are worth stating.

It rejects a reading where diastolic is not below systolic. Some tools return the diastolic value itself when the two are equal, since the pulse pressure is zero. This one refuses instead. With no gap to weight, the formula is not describing anything, and an inverted reading (such as 80/120) is far more likely to be the two numbers entered in the wrong boxes than a real measurement.

It keeps the plain one-third weighting. Corrected formulas exist, including the heart-rate-dependent one from Razminia and colleagues. Using one would need a heart rate input and would move the page off the widely cited standard formula. The calculator stays on the standard formula and names its weakness rather than quietly adjusting for it.

One detail follows from rounding to one decimal place: clinical charts usually record MAP as a whole number. A MAP shown here as 93.3 would commonly be written as 93.

Frequently asked questions

How do you calculate MAP from blood pressure? Subtract the diastolic reading from the systolic reading, divide that difference by 3, and add the result to the diastolic reading. For 120/80 mmHg: (120 − 80) / 3 = 13.3, and 80 + 13.3 = 93.3 mmHg.

Is MAP just the average of the two blood pressure numbers? No. The plain average of 120 and 80 is 100 mmHg, but the MAP is 93.3 mmHg. The heart spends more of each beat relaxed than contracting, so the diastolic value counts twice as much as the systolic one in the formula.

Why is MAP closer to the diastolic number? Because the formula adds only one third of the pulse pressure to the diastolic value. The remaining two thirds of the gap are never added, so the answer can never reach halfway to the systolic reading.

What is pulse pressure, and why is it shown? Pulse pressure is the systolic reading minus the diastolic reading. It is displayed because the MAP formula works by taking one third of it.

How accurate is the one-third formula? It is an approximation built on the assumption that diastole lasts about twice as long as systole, which holds at a normal resting heart rate. At faster heart rates diastole is shorter, and the formula returns a value below the measured MAP.

Does the calculator accept kPa? No. It reads and reports mmHg only.