Parts per million, or PPM, is a way of stating a defect rate when the rate is so small that percentages become awkward. Saying a plant ships 0.0024 percent defective is hard to read; saying it ships 24 PPM is not. PPM = (Defective Units ÷ Units Inspected or Shipped) × 1,000,000. The base can be units inspected, units produced or units shipped, and you must always say which one you used.
Converting between percent and PPM is a matter of moving the decimal point four places. One percent is 10,000 PPM. So 0.5 percent is 5,000 PPM, 0.05 percent is 500 PPM and 0.001 percent is 10 PPM. Going the other way, divide PPM by 10,000 to get percent: 150 PPM is 0.015 percent.
In a fastener plant making cold-headed bolts the volumes run into millions a month, which is exactly where PPM earns its keep. A defect rate of 120 PPM sounds small, but across 2.5 million bolts it is 300 bad parts in customers' hands.
Standard Procedure
- 1Count the defective units found in the period.
- 2Count the total units in the same base (inspected, produced or shipped) and write down which base you used.
- 3Divide defective by total to get a decimal fraction.
- 4Multiply the fraction by 1,000,000 and round to a whole number.
- 5To convert a percentage to PPM, multiply the percentage by 10,000.
Lesson Summary
- ✓PPM = (defective ÷ total) × 1,000,000; always state the base.
- ✓Move the decimal four places to switch between percent and PPM.