How to Calculate a Purchasing Managers' Index
A purchasing managers' index equals the percent of firms reporting better conditions plus half the percent reporting no change, so a reading above 50 means expansion.
The PMI formula
Calculator
Enter the shares reporting better, no change and worse to get the diffusion index and where it sits against 50.
Share of survey responses saying activity improved on last month.
Share saying conditions held steady. These enter at half weight.
Share saying activity fell. These carry no weight at all.
Better responses plus half the flat responses give a reading of 52.5.
- Direction signal
- Expansion
- Points above or below 50
- 2.5
- Net share reporting better
- 5%
- Response shares add to
- 100%
The 50 line is the balance point, so the reading itself says which way the sector moved over the month.
The reading sits 2.5 points from the dividing line, which is a measure of how broad the move was.
Better minus worse is 5%, and 50 plus half of that reproduces the reading exactly.
The three shares should sum to 100 percent, since every response falls into one of the three buckets.
How to calculate PMI, step by step
- 1Collect the three response shares. Each survey answer is better, the same, or worse than last month, and the three shares add to 100 percent.
- 2Weight the responses. Better counts in full, no change counts half, and worse counts zero. That weighting is what makes this a diffusion index rather than an average of amounts.
- 3Add the weighted shares. PMI = percent better + 0.5 × percent no change. The result always lands between 0 and 100.
- 4Read it against 50. Above 50 more firms improved than declined, below 50 more declined than improved, and exactly 50 means the two sides balance.
- 5Check with the net form. The same reading equals 50 plus half the net share reporting better, so the two routes should agree on any set of responses.
Worked example: PMI
Suppose 30% of purchasing managers report better conditions, 45% report no change, and 25% report worse. PMI = 30 + (0.5 × 45) = 30 + 22.5 = 52.5. That sits 2.5 points above the 50 line, so the sector expanded over the month. The net check agrees: better minus worse is 30 − 25 = 5 points, and 50 + (0.5 × 5) = 52.5.
PMI questions
Why does 50 separate growth from decline?
At 50 the share reporting better exactly offsets the share reporting worse, so activity is flat. The index counts the direction each firm reports rather than the size of the move, which is why the dividing line sits at the midpoint.
Does a falling index above 50 mean the sector is shrinking?
No. Any reading above 50 still means more firms improved than declined, so a drop from 58 to 53 is slower growth rather than decline. Only a reading under 50 signals contraction.
Why is no change weighted at half?
The half weight splits flat responses evenly between the two directions, so firms reporting no movement pull the index toward 50 instead of toward either extreme.
Get AP Econ exam tips in your inbox
Occasional emails with study tips, new interactive graphs, and exam-season reminders. Free, no spam.
No spam. Unsubscribe anytime. Read our privacy policy.
Keep track of what you have studied
A free EconLearn account adds progress tracking, your quiz history, and achievements. Studying here is free either way, and there is nothing to pay for as a student.
Create a free accountAlready have one? Sign in
Last updated