Metric to SAE Calculator — Wrench & Socket Size Converter
Enter a metric wrench or socket size in millimeters to find the nearest SAE (Society of Automotive Engineers, fractional-inch) size, the deviation in mm, and the adjacent SAE sizes above and below.
mm
3/8" = 9.525 mm
- 1
Metric size
101 inch = 25.4 mm exactly - 2
Size in inches
10 ÷ 25.4 = 0.39370
How does this calculator work?
1 inch = 25.4 mm exactly. To find the nearest SAE wrench size, divide the metric mm by 25.4 and compare to standard 1/16-inch fractions. Common near-matches: 10 mm ≈ 3/8", 13 mm ≈ 1/2", 17 mm ≈ 11/16", 19 mm ≈ 3/4". A deviation over 0.5 mm risks rounding fastener heads.
Formula
How this is calculated
SAE (Society of Automotive Engineers) tool sizes are expressed in fractions of an inch, commonly in 1/16-inch increments (e.g. 3/8", 7/16", 1/2", 9/16"…). Metric tools are sized in whole millimeters. Because 1 inch = exactly 25.4 mm, the gaps between SAE sizes do not align with whole-millimeter steps, so most metric sizes have no exact SAE match — there is always some deviation.
To find the nearest SAE size, convert the metric millimeter value to inches (mm ÷ 25.4), then compare to the list of standard fractional SAE sizes and select the one with the smallest absolute difference. The deviation tells you how much smaller or larger the nearest SAE is versus the metric size in millimeters; positive deviation means the SAE is slightly smaller than the metric fastener (the tool will be loose), and negative means slightly larger (the tool may slip).
In practice a deviation of ≤ 0.5 mm may work in a pinch but risks rounding fastener heads — always use the correct metric tool where available. Common nearly-equivalent pairs include 8 mm ≈ 5/16" (+0.325 mm), 10 mm ≈ 3/8" (−0.025 mm — almost exact), 13 mm ≈ 1/2" (−0.3 mm), 17 mm ≈ 11/16" (+0.538 mm), and 19 mm ≈ 3/4" (−0.05 mm).
Frequently asked questions
3/8" = 9.525 mm — a deviation of only 0.475 mm from 10 mm, making it one of the closest metric-to-SAE matches. In a pinch a 3/8" SAE wrench can sometimes be used on a 10 mm fastener, but always prefer the correct metric tool.
1/2" = 12.7 mm is the nearest SAE size (deviation −0.3 mm). 13 mm is slightly larger than 1/2", so a 1/2" socket fits but is marginally loose. Some manufacturers stock a 13/32" or rely on 1/2" for 13 mm fasteners.
Only if the nearest SAE size fits snugly — typically a deviation under 0.5 mm. A loose fit risks rounding the fastener head. For critical fasteners (e.g., wheel bolts, engine components) always use the correct metric tool to avoid damage or injury.
Also known as
TG we-Calculate Editorial Team. (2026). Metric to SAE Calculator — Wrench & Socket Size Converter [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/metric-to-sae-calculator
TG we-Calculate Editorial Team. "Metric to SAE Calculator — Wrench & Socket Size Converter." TG we-Calculate. 2026. https://we-calculate.com/calculator/metric-to-sae-calculator.
TG we-Calculate Editorial Team, "Metric to SAE Calculator — Wrench & Socket Size Converter," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/metric-to-sae-calculator
@misc{wecalculate_metric_to_sae_calculator, title = {Metric to SAE Calculator — Wrench & Socket Size Converter}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/metric-to-sae-calculator}}, year = {2026}, note = {TG we-Calculate} }
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