Comparing PVC imperial nominal sizes (ASTM / NPS system used in the U.S.) to PVC metric nominal sizes (ISO / DIN system used internationally).

The Core Difference

So:

• A 1/2" SCH40 PVC pipe actually has OD ≈ 21.34 mm,
• while “20 mm PVC” metric pipe has OD = 20 mm — close, but not the same, so fittings don’t interchange.

Comparison Chart: Imperial (ASTM/NPS) vs Metric (ISO/DIN PVC)

Key Takeaways

• They look close but aren’t interchangeable — a 25 mm pipe will not fit a ¾″ Schedule 40 fitting.
• Metric pipe/fittings are standardized by OD, while U.S. PVC Schedule 40/80 uses NPS, meaning OD is fixed by standard, and ID varies with wall thickness.
• When converting systems, always use adaptors or transition unions made specifically for Imperial↔Metric.

Schedule 40 / Schedule 80 PVC Imperial ↔ Metric Comparison Table

Quick Reference Rules

• OD stays constant across schedules — only wall thickness and ID vary.
• Metric series steps up in roughly 5 mm or 10 mm increments; nearest equivalent given above.
• Do not mix systems directly — use a metric-to-imperial transition fitting if joining the two.

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