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Defect rate: formula, example and how to reduce it

The scrap/defect rate shows the share of unusable or defective products in output. Together with FPY/RTY, PPM/DPMO, and COPQ, it reflects the real impact of quality on margin, capacity, and OEE.

What the metric measures

The share of products/operations that do not meet specifications: final waste (scrap) and rework (rework). It makes it possible to find "hot spots" by line, shift, SKU, and cause.

Calculation

  • Defect Rate, % = Defective units / All units × 100
  • Scrap Rate, % = Unusable units written off / All units × 100
  • FPY, % = Good units on the first pass / All units × 100
  • RTY, % = Product of FPY across operations
  • PPM = Defective units × 10⁶ / All units; DPMO — for multiple defect opportunities
  • COPQ = Scrap + rework + returns + warranty costs + downtime

Agree on policies: what counts as a defect, how rework is accounted for, when it is recorded, and a single denominator (units/batches/hours).

Calculation example

12,000 parts were produced in a week and 300 rejected at inspection. Defect rate = 300 / 12,000 × 100 = 2.5%. At a unit cost of 150 the loss is 45,000 a week, not counting rework and delayed orders.

Data sources

QMS/ERP/1C (nonconformances, write-offs), MES/SCADA/PLC (defect events), quality systems (FPY/testing), CMMS (impact of downtime), WMS (batch quarantine), CRM/complaints.

Interpretation

  • Scrap↑ — unstable process settings/tooling/materials, operator errors, startup defects after setup
  • Rework↑ — hidden loss of capacity and rising cost of goods
  • Assess process capability (Cp/Cpk) and the effect of TPM/SMED/DOE and standardization

Useful breakdowns

  • Line / unit / shift / crew
  • SKU / recipe / tooling; supplier / material batch
  • Defect type (code), routing stage; internal / external / field
  • "After setup" vs steady-state operation

Dashboard visualizations

  • Trends in Scrap/Defect Rate, FPY/RTY
  • Pareto chart of defect types and causes
  • SPC p-/u-charts for stability
  • Heatmap by line/shift/SKU; PPM/DPMO by supplier
  • Correlation "after setup → scrap"

How to set targets

Set targets for FPY/PPM/DPMO and thresholds for CTQ; implement SPC, first-article inspection, a response SLA and RCA/CAPA processes, a batch quarantine policy, and regular updates of work instructions.

Common mistakes

  • Confusing "defective units" with the "number of defects"
  • Inconsistent denominator; counting rework as "good"
  • No MSA (Gage R&R) or batch traceability
  • Different rules across lines; ignoring defects found after shipment

Questions for discussion

  • Which defect tops the Pareto chart, and what is the potential for reducing scrap?
  • What effect will a 30% reduction in setup have on startup defects?
  • Where is the weak link in inspection: MSA, frequency, or sampling point?
  • What Cp/Cpk does the process have for critical CTQ?

Questions and answers

How do you calculate the defect rate?

Defect rate = number of defective units / total units produced in the period × 100. Also calculate it in money at cost.

What is PPM?

Parts Per Million — the number of defects per million units: a 2.5% defect rate is 25,000 PPM. It is used when defects are very rare.

How do you reduce the defect rate?

Find root causes with a Pareto chart, control quality at early operations, standardise processes and train staff.