7/16 Tap Drill Size Depends on the Thread Pitch

For a conventional cutting tap, use a letter U drill for 7/16-14 UNC or a 25/64-inch drill for 7/16-20 UNF. “7/16” alone is not enough to select the bit: read the threads-per-inch marking on the tap before drilling.

Thread marking	Thread series	Recommended cutting-tap drill	Diameter in inches	Metric equivalent*
7/16-14	UNC, coarse	Letter U	0.3680	9.3472 mm
7/16-20	UNF, fine	25/64 inch	0.390625	9.921875 mm

These pairings appear in the IRWIN HANSON tap-and-drill chart and are corroborated by Imperial Supplies’ chart. Metric values are calculated conversions, not separate recommended drill sizes.

Read the whole tap marking

In 7/16-14, the first number is the nominal thread diameter; 14 means 14 threads per inch. A 7/16-20 tap has the same nominal diameter but finer threads, so it uses a larger starting hole.

Do not drill a 7/16-inch hole for either of these cutting taps. That bit is 0.4375 inch—the nominal thread diameter—and removes the material needed to cut the internal thread profile.

Also check whether the tap is a cutting tap or forming tap. The two sizes above apply to cutting taps, not automatically to every tool marked 7/16.

Can you substitute a fractional bit for letter U?

A fractional-only set may lack the U drill. Its nearby sizes are not equivalent:

Bit	Nominal diameter	Difference from U
23/64 inch	0.359375 inch	0.008625 inch smaller
Letter U	0.3680 inch	—
3/8 inch	0.3750 inch	0.0070 inch larger

The smaller 23/64-inch hole makes the tap remove more material; the larger 3/8-inch hole reduces thread height. In general, undersized holes increase cutting resistance and tap-breakage risk, while excessive hole size can weaken the resulting threads, as tap manufacturer Jarvis explains.

For an ordinary chart-based 7/16-14 job, get the U bit rather than treating either fraction as a direct replacement. If the tap manufacturer or job specification permits a different hole size for the material and required thread, follow that data. A near match in diameter is not by itself approval to substitute.

Forming taps need different hole data

Forming taps displace material instead of cutting it away. For example, OSG’s 7/16-14 H12 HY-PRO NRT forming tap specifies a tap-drill range of 0.401071–0.408357 inch, substantially larger than letter U.

That range belongs to the listed OSG tool; it is not a universal forming-tap recommendation. Use the hole-size range for your exact tap and application.

If you instead meant which tap uses a 7/16-inch drill, that is a different lookup: see the 1/4-18 NPT tap-drill guide.