A sigma level expresses how capable a process is of meeting customer requirements. The higher the sigma level, the fewer defects per million opportunities (DPMO).
Common reference values
Using the conventional 1.5-sigma long-term shift:
- 2 sigma: about 308,500 DPMO
- 3 sigma: about 66,800 DPMO
- 4 sigma: about 6,210 DPMO
- 5 sigma: about 230 DPMO
- 6 sigma: about 3.4 DPMO
How to calculate it
- Define what counts as a defect and an opportunity.
- DPMO = defects ÷ (units × opportunities per unit) × 1,000,000.
- Convert DPMO to a sigma level with a standard table or software.
Why it matters
Each step up the scale means far fewer defects, complaints and rework. Many organizations do not need six sigma everywhere; the aim is the right level for each critical-to-quality characteristic, improved where defects cost most.
