Wrenches Inches Metric Us Conversion Comparison

Overview

Wrench and spanner sizes differ between the U.S. customary (SAE fractional-inch) and metric systems. This reference maps common SAE sizes to their metric equivalents and shows how the two systems overlap. When a metric fastener is closest to an SAE size, or vice versa, the table highlights which wrench can substitute — a practical need in mixed-standard workshops and maintenance environments.

SAE to Metric Wrench Size Conversion

The table below lists common SAE (U.S.) wrench sizes alongside their metric equivalents. Fractional-inch values are converted to millimetres for direct comparison. Rows where only a metric size exists (no direct SAE equivalent) indicate common metric-only fasteners.

Quick Inch-to-Millimetre Converter

Use this calculator to convert any fractional or decimal-inch wrench size to millimetres.

Quick Millimetre-to-Inch Converter

Restored Original Source Tables

The following tables are restored from the original source page to preserve the complete reference data. The cached source also includes shared search/layout form inputs and empty UI tables; those artifacts are not engineering wrench data. Functional conversion coverage is preserved with the inch-to-millimetre and millimetre-to-inch calculators.

Wrench Spanners - Inches vs. mm

Original Source Images

The following original source images are preserved to avoid losing visual reference material. When an image contains chart or tabular data, its extracted values are represented in the page tables, calculators, or interactive charts; remaining images are retained as visual source references.

Engineering Notes

• Close-fit substitutions. For example, a 5/16 in SAE wrench (7.94 mm) is very close to an 8 mm metric wrench (difference of only 0.06 mm). In many applications these can be used interchangeably, but always verify tolerance before relying on a substitute on high-torque or precision fasteners.
• Rounding. Fractional-inch sizes do not map to round metric numbers. When selecting a replacement, choose the next-larger metric size if the exact equivalent is unavailable to avoid rounding damage to fastener flats.
• SAE and metric fasteners should not share wrenches in torque-critical assemblies. Even a small size discrepancy can round corners and reduce the applied torque.
• Common overlap zones. Sizes from roughly 7 mm to 10 mm have the closest SAE-metric equivalents. Larger sizes tend to diverge more, making substitutions impractical.

References

• Original Source — Engineering ToolBox