Mean arterial pressure formula and normal range

A blood pressure reading gives two numbers, but organs do not experience two numbers. They experience an average pressure pushing blood through them across the whole heartbeat. That average is the mean arterial pressure, or MAP, and it is a common calculation on nursing exams and a routine number in acute care.

You can work any example by hand with the formula below, or check your answers with the mean arterial pressure calculator. This is a study aid, not clinical guidance; your program, facility policy and state board decide practice.

The formula

The MAP estimate the NCLEX-RN course teaches is:

MAP = diastolic + one-third of the pulse pressure

You will also see it written as MAP = DP + (SP - DP) / 3, where SP is systolic and DP is diastolic. Both say the same thing. The pulse pressure is the part you have to find first.

Pulse pressure comes first

Pulse pressure is the systolic pressure minus the diastolic pressure.

Pulse pressure = systolic - diastolic

It is normally about 40 mm Hg. A blood pressure of 120/80 gives 120 - 80 = 40. A blood pressure of 128/78 gives 50. The course source notes add that pulse pressure narrows when blood volume falls, and define a narrowed pulse pressure as one under 25 percent of the systolic value.

Step by step

1. Write the reading as systolic over diastolic.
2. Subtract diastolic from systolic to get the pulse pressure.
3. Divide the pulse pressure by 3.
4. Add that result to the diastolic.

Worked example 1: a typical reading

Blood pressure 120/80 mm Hg.

• Pulse pressure: 120 - 80 = 40
• One-third of 40: 13.3
• MAP: 80 + 13.3 = 93.3 mm Hg

Worked example 2: a lower reading

Blood pressure 90/60 mm Hg.

• Pulse pressure: 90 - 60 = 30
• One-third of 30: 10
• MAP: 60 + 10 = 70 mm Hg

That sits at the low end of the usual range, which is covered next.

Worked example 3: a high reading

Blood pressure 150/90 mm Hg.

• Pulse pressure: 150 - 90 = 60
• One-third of 60: 20
• MAP: 90 + 20 = 110 mm Hg

The MAP is above the usual range, as you would expect from a reading with both numbers raised.

The benchmarks: 60 and 70 to 100

Two numbers come up with MAP.

• A MAP of at least 60 mm Hg is needed to perfuse vital organs. This is how the course notes teach it. Below that, the pressure pushing blood through the brain, kidneys and heart may be too low.
• The usual range is about 70 to 100 mm Hg. The course notes teach this as a normal MAP. The same notes list a MAP at or above 65 among the hypovolemic shock monitoring points, so you may see 65 in shock contexts. Targets for a given client come from the provider's orders and unit policy.

Reference ranges differ by lab and source, and targets vary by client, and the MAP is a calculated estimate rather than a measured value. So treat the benchmarks as anchors for recognizing a problem, not as a replacement for the whole picture.

Narrowed pulse pressure: the number that hides

Here is why pulse pressure matters even though MAP is the "average". A MAP can look acceptable while the pulse pressure tells you something is wrong.

Take a reading of 92/76 mm Hg, as in a course-notes example.

• Pulse pressure: 92 - 76 = 16 mm Hg
• One-third of 16: 5.3
• MAP: 76 + 5.3 = 81.3 mm Hg

The MAP is inside the usual range, but a pulse pressure of 16 is far below the normal of about 40, and it is well under 25 percent of 92 (which is 23). A narrowed pulse pressure suggests hypovolemic shock. In that setting, the pulse pressure narrows as blood volume falls, so it is worth checking alongside the heart rate and the trend.

A second example: 100/80 mm Hg gives a pulse pressure of 20 and a MAP of 86.7. The MAP looks fine, but 20 is under 25 percent of the systolic value of 100, so the pulse pressure is narrowed. It is the trend from the client's earlier readings that tells you which way things are going.

The opposite pattern also shows up on the exam. In the late, dangerous pattern called the Cushing triad (bradycardia, irregular respirations and high blood pressure with a wide pulse pressure), a reading such as 188/64 gives a pulse pressure of 124. Here the MAP is 105.3, which is high. The wide pulse pressure with a slow heart rate and irregular breathing is what points to rising intracranial pressure. Shock generally gives the opposite picture: low pressure and a fast heart rate (neurogenic shock is the exception, with a slow heart rate and low pressure).

Common mistakes

• Averaging the two numbers. The mean of 120 and 80 is 100, but the MAP is 93.3. The formula weights diastolic more heavily because of the one-third.
• Adding one-third of the systolic. The third applies to the pulse pressure, not to the systolic pressure.
• Forgetting to subtract first. If your answer is larger than the systolic, you skipped the pulse pressure step.
• Treating the number alone as the answer. An exam stem usually wants the next nursing action. A MAP below 60, or a new low reading in a client who was stable, is reassessed and reported; see NCLEX priority and delegation questions for how to rank that against the other clients on the unit.

Where this appears on the exam

MAP sits in the physical assessment and monitoring unit of the free NCLEX-RN flashcard course, under vital-sign trends and hemodynamic monitoring, with a typed card where you calculate the answer. How the NCLEX-RN CAT exam works explains the computerized adaptive format that delivers these items. The weighting of this content is in the client needs categories explained. Number interpretation also ties into which lab values the NCLEX expects you to know, since the course notes teach that a high lactate means tissues are not getting enough oxygen, as in shock.

If you are studying for the NREMT instead, the same vital-sign trend logic runs through EMT patient assessment, step by step, and the arithmetic habits transfer to IV flow rate and drip rate formulas. The Glasgow Coma Scale calculator is another quick scoring drill.

How to make the formula stick

Formulas fail under exam pressure when they were only ever read. Two habits help:

• Calculate, don't recognize. Do five readings from memory on a blank page, then check. The testing effect explains why producing the answer builds recall that rereading does not.
• Review the formula a little each day. A one-line formula is perfect for spaced repetition, which brings it back just before you would forget it.

When you are ready for the whole exam, how to study for the NCLEX-RN lays out a plan, and the NCLEX-RN exam prep course has the full set of cards. Check nclex.com and ncsbn.org for the current exam rules.