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SHOP REFERENCE / MEASURING

How to Measure a Socket or Fastener

To pick a socket or wrench for an external hex bolt head or nut, measure straight across two opposite flat faces. That head width is separate from the bolt’s thread diameter.

Open the Socket Finder tool
THE KEY MEASUREMENTACROSS FLATS

Two parallel flat sides determine a standard hex socket’s nominal opening. Corners do not.

WHERE TO MEASURE

Flats, not corners

Measuring a hex head across opposite flat sidesA hex head with the correct across-flats distance shown between two horizontal opposite flat faces. The corner-to-corner dimension is marked as the wrong measurement for a wrench opening.Across flatsCorner-to-corner is wider

Why corner-to-corner is wrong

A hex head’s points stick out farther than its parallel flat faces. A corner-to-corner reading therefore overstates the wrench or socket opening and may lead you to choose a loose tool.

Hold the caliper jaws on the flat faces, away from a chamfer, damaged edge or raised marking.

STEP BY STEP / 01

Measure the hex head or nut

  1. Expose and clean the flats.Remove dirt or heavy paint. Find two opposite, parallel faces of the outside hex.
  2. Measure across them.With calipers, close the outside jaws gently and squarely against those faces. Zero the calipers first and read in mm or inches.
  3. If using a ruler, look straight on.Place the zero mark at one flat and read to the opposite flat; avoid reading diagonally. A ruler gives only a rough result, especially on small heads.
  4. Compare sizes and test the fit.Use the nearest marked socket or wrench that fully seats with minimal play. If an SAE and metric size both appear close, compare both tools on the actual fastener before applying force.

READINGS IN PRACTICE

Metric or SAE?

13 mm across flats

Try a 13 mm socket on a standard external hex head. A standard M8 hex bolt is one possible example; measure its threads separately to identify the bolt.

9/16 in. across flats

Try a 9/16 in. socket. A standard US 3/8 in. hex bolt commonly has this head size, but head styles can differ.

About 19 mm across flats

A 19 mm opening and a 3/4 in. opening (19.05 mm) are very close. A rough ruler reading cannot reliably tell which system made the fastener.

Unknown socket marking

Check for a stamped size first. If unreadable, use the caliper’s inside jaws to measure between opposite internal flats near the mouth; compare with marked sockets and test the fit.

Convert an inch reading to millimeters by multiplying by 25.4. Measurement alone may not reveal whether a near-match is metric or SAE, because manufacturing tolerances, wear and paint affect the reading. Do not force a loose socket; use the best fitting correctly marked tool.

Compare metric and SAE socket openings → See bolt thread to head-size examples →

References

Bolt Depot’s fastener measuring guidance distinguishes thread diameter from hex head width across flats. Mitutoyo’s caliper guidance describes light contact and correct alignment. NIST defines 1 inch as exactly 25.4 mm. Metric and US standard hex bolt head charts support the examples. This guide is for external hex heads and matching hex socket openings; internal Allen and Torx fasteners require different measurements.