Defect Rate, PPM & Sigma Level Converter

Enter one value you know, such as defect rate, PPM, DPMO, yield, sigma level, Z value or Cpk, and the tool converts it to all the others. Use it to express a quality target in another measure or to check a Six Sigma level.

How to use

1. Under "Value to enter", choose the kind of value you have and type the number. The other measures appear at once.
2. To work from counts, choose "DPMO from defects, units and opportunities" and enter "Defects", "Units inspected" and "Opportunities per unit".
3. In the "Sigma level conversion table", the row of the current result is highlighted.
4. For several process steps, enter one yield per line in "Yield of each process step (%)" to get the "Rolled throughput yield".

How it works

• Z = Φ⁻¹(1 − defect probability), and sigma level = Z + 1.5. The 1.5 is the convention that the process mean drifts by 1.5σ over the long term.
• Cpk ≈ Z ÷ 3 assumes all defects come from the nearer spec limit. When defects come from both sides, the real Cpk is higher.
• With zero defects, Z and the sigma level are infinite and cannot be calculated.

Questions and answers

How many PPM is Six Sigma?

3.4 DPMO under the 1.5σ shift convention. Without the shift, the one-sided tail beyond 6σ is about 0.001 PPM. The table shows both values.

How do I convert PPM to a sigma level?

Turn the PPM into a probability, find the standard normal Z value for it and add 1.5. For example, 233 PPM gives Z of about 3.5 and a sigma level of 5.0.

What is the difference between PPM and DPMO?

PPM counts defective parts per million parts. DPMO counts defects per million defect opportunities. With 25 defects, 500 units and 10 opportunities per unit, DPMO is 5,000.

What defect rate does a Cpk of 1.33 mean?

Z = 3 × 1.33 = 3.99, which is about 33 PPM beyond one spec limit. For a centred process with two spec limits, it is twice that, about 66 PPM.