Tap Drill Size Chart

The drill to use before a tap, for UNC and UNF threads from #0 to 2-1/2 in., for ISO metric coarse threads from M1 to M68, and for machine screw sizes with their clearance drills. Drill sizes are given as the fraction, number or letter drill, with its decimal and millimetre equivalents.

Tap drills for UNC and UNF threads, fractional sizes

Threads of American National Form, covering the common UNC and UNF pitches, with the older coarse and fine series included. The drill sizes allow approximately 75 per cent of a full thread. For the small sizes, choosing the larger hole sizes from the hole size limits table reduces tap breakage. Source, Table 3.

Drill dia, mm is converted from the inch value at 1 in. = 25.4 mm. Threads from #0 to 1 in. are also listed below with their clearance drills.

Tap drills and clearance drills for machine screws

Numbered and small fractional machine screws: the tap drill for the thread, and the close fit and free fit clearance drills used when the screw passes through the part. Where a size carries more than one pitch, the values are listed for each pitch in the order shown. Source, Table 4.

Tap drills for ISO metric coarse threads

Recommended and alternative tapping drill sizes for metric coarse pitch threads, in millimetres. The recommended sizes give a theoretical radial engagement of about 81 per cent, the alternative sizes about 70 to 75 per cent. The table is for fluted taps only, a fluteless (cold forming) tap takes a different size hole. Source, Table 6.

Footnotes in the source: (b) for tolerance class 6H and 7H threads only; (c) for tolerance class 7H threads only; the ellipsis marks sizes where no alternative drill is listed.

• As a rule of thumb the tap drill diameter is the nominal diameter minus the pitch, tap drill = major diameter − pitch. The tables above give the recommended values, which build in the thread engagement the standard asks for.
• Tapping is done at the lowest speed of any threading operation because the tap is fully engaged in the cut. Cutting speeds for tapping tool steels are in the cutting speeds and feeds chart.
• Taps are made from high speed steel, and the grades used for them are grouped under high speed steels. Where a thread is formed by rolling instead of cutting, the die material is covered on the thread rolling dies page.
• Hardened steel is tapped only as a repair. The difficulty of tapping rises with the hardness of the material, which follows the machinability ratings of the grades.

Choosing the drill

The tap drill decides the thread engagement. A drill at the top of the range gives a thread that cuts freely and a tap that lasts, at the cost of a slightly weaker thread. A drill under the range overloads the tap and is the usual cause of tap breakage in the small sizes.

Drill sizes list as fractions, numbers and letters on the same chart because the tap drill does not always land on a fraction: a 1/4-20 thread taps with a #7 drill, which is neither. The decimal equivalent column converts any of them to the diameter that goes into the machine.

Related reference pages

Selection guide for high speed steel cutting tools · High speed steel for cutting tools, hobs and broaches

Source: Machinery’s Handbook 28th Edition (Industrial Press), Tapping section: Table 3 (tap drill sizes for threads of American National Form), Table 4 (tap drills and clearance drills for machine screws), Table 6 (British Standard tapping drill sizes for ISO metric coarse pitch series threads, BS 1157). Millimetre equivalents of the inch drills are converted at 1 in. = 25.4 mm.