SAE to Metric Wrench Calculator

Convert SAE fractional wrench and socket sizes to metric (mm) equivalents, or find the closest standard SAE tool match for metric fasteners.

SAE to Metric Wrench Calculator

Select Standard SAE Size

Or Enter Custom Fraction or Decimal

inch

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Enter Metric Size

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Fitment Comparison

Closest Tool

Exact Size Conversion: 0.00

Size Gap / Clearance: 0.00

Fitment Rating: N/A

The SAE and Metric Math Formulas

In mechanical engineering, converting between fractional-inch SAE (Society of Automotive Engineers) and metric (millimeter) fasteners is a matter of scaling based on the international definition of the inch:

Constant: 1 inch = Exactly 25.4 millimeters

1. Converting SAE to Metric (Decimal Equivalent)

To convert an SAE size to millimeters, resolve the fraction to a decimal first, and then multiply by 25.4:

Decimal Inches = Numerator ÷ DenominatorExact Metric Size (mm) = Decimal Inches × 25.4

2. Converting Metric to SAE (Decimal Inches)

To reverse the process and find the exact decimal inch value of a metric fastener:

Exact SAE Size (inches) = Metric Size (mm) ÷ 25.4

SAE to Metric Tool Conversion Chart

This comparison chart features standard toolbox sizes, aligning imperial fractional wrench widths with their metric millimeter equivalent. The Gap indicates the physical difference between the tool and fastener flats.

SAE Size (Inches)	Exact Metric (mm)	Closest Metric Wrench	Fitment Gap (mm)	Fitment Evaluation
5/16"	7.94 mm	8 mm	0.06 mm	Excellent (Safe Match)
3/8"	9.53 mm	10 mm	0.47 mm	Poor Fit (Caution / Do Not Torque)
7/16"	11.11 mm	11 mm	0.11 mm	Excellent (Safe Match)
1/2"	12.70 mm	13 mm	0.30 mm	Caution (Usable in low-torque settings)
9/16"	14.29 mm	14 mm	0.29 mm	Caution (Tight but usable)
5/8"	15.88 mm	16 mm	0.12 mm	Excellent (Safe Match)
11/16"	17.46 mm	17 mm	0.46 mm	Poor Fit (High risk of rounding)
3/4"	19.05 mm	19 mm	0.05 mm	Excellent (Safe Match)
13/16"	20.64 mm	21 mm	0.36 mm	Caution (Usable in emergency)
7/8"	22.23 mm	22 mm	0.23 mm	Excellent (Safe Match)
15/16"	23.81 mm	24 mm	0.19 mm	Excellent (Safe Match)
1"	25.40 mm	25 mm	0.40 mm	Poor Fit (Avoid using 25mm wrench)

Practical Mechanic and Shop Guidelines

Using the wrong wrench on a hex-head bolt can easily round off the corners, making it extremely difficult to remove. The guidelines below can help keep your fasteners in good condition:

1. Understanding Clearance Limits

Standard hand tools are manufactured with a tiny clearance window relative to the fastener size to allow the wrench to slide over the bolt head easily. A clearance difference of less than 0.15 mm is tight enough to safely transmit high torque. A gap between 0.15 mm and 0.38 mm can work for low-torque assembly but is risky on seized, rusted, or stubborn bolts. A gap over 0.38 mm is likely to cause damage under load.

2. Tool Systems: SAE vs. Metric

SAE (Imperial): Sized in fractions of an inch (e.g., 1/2", 9/16"). This is standard for older American-made cars and machinery.
Metric: Sized in exact millimeters (e.g., 10mm, 13mm). This is the global automotive standard, found on European and Japanese cars, as well as modern American vehicles.

3. Tool Substitutes to Avoid

There is a common misconception that a 10mm wrench can be used interchangeably with a 3/8" wrench. While a 10mm socket can sometimes turn a loose 3/8" bolt, the 0.47 mm gap is too wide for high-torque applications. Under load, a 10mm socket is likely to strip the corners of a 3/8" bolt head. By contrast, a 19mm wrench is a near-exact match for a 3/4" bolt (with a gap of only 0.05 mm), making it a safe substitute in almost any scenario.

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