Tap and drill size chart: metric and SAE
A tap cuts an internal thread into a hole you drill first, and the hole has to be smaller than the tap itself, small enough that the tap has material to cut but large enough that it does not jam and snap. That is the whole game, and the tap drill chart is the lookup that settles it. This page covers the common ground: metric coarse taps from M3 to M12 and SAE taps from 6-32 to 1/2-13, with the decimal equivalent of every drill in inches and millimeters. Every row is cross-checked against at least two independent references, named below each table.
Tap drill calculator: type a tap size, get the drill
Tap size lookup
Works with metric and SAE notation: M6x1.0, M8, 1/4-20, #8-32, 10-24. Metric sizes without a pitch default to coarse.
The lookup runs entirely in your browser from the same sourced table printed below; it adds no values of its own. If your tap is not in the common range, the tables and the formula section cover how to calculate the hole instead of guessing.
Metric tap and drill chart (M3 to M12, coarse)
The metric rule is one line: drill size = major diameter minus pitch. M8 x 1.25 means 8.0 minus 1.25 = 6.75 mm, rounded to the standard 6.8 mm drill. That lands near 75 percent thread engagement, the general-purpose default. The values below are cross-checked between LittleMachineShop's tap and clearance drill chart (the shop reference most machinists keep at the bench), the Mech Forged tap drill chart, and the ConcreteCalculate tap drill chart's stated-calculation tables. All three agree on every row except M12, where LittleMachineShop rounds up and the others round down; both sizes are given.
| Tap (coarse) | Pitch | Tap drill | Decimal (in) | Closest imperial drill |
|---|---|---|---|---|
| M3 x 0.5 | 0.5 mm | 2.5 mm | 0.0984 | #39 (0.0995 in) |
| M4 x 0.7 | 0.7 mm | 3.3 mm | 0.1299 | #30 (0.1285 in) |
| M5 x 0.8 | 0.8 mm | 4.2 mm | 0.1654 | #19 (0.1660 in) |
| M6 x 1.0 | 1.0 mm | 5.0 mm | 0.1969 | #8 (0.1990 in) |
| M8 x 1.25 | 1.25 mm | 6.8 mm | 0.2677 | H (0.2660 in) |
| M10 x 1.5 | 1.5 mm | 8.5 mm | 0.3346 | R (0.3390 in) |
| M12 x 1.75 | 1.75 mm | 10.2 mm (LMS: 10.3) | 0.4016 | 13/32 (0.4062 in) |
Sources per row: LittleMachineShop, Mech Forged, ConcreteCalculate (links above). The M12 calculated hole is 10.25 mm; charts round to 10.2 or 10.3, and either drills an acceptable thread.
SAE tap and drill chart (6-32 to 1/2-13)
Inch taps are named by major diameter and threads per inch, and the calculation behind the chart is drill = major diameter minus (0.01299 x percent thread / TPI). At the usual 75 percent engagement that gives 0.2013 in for 1/4-20, and the nearest standard drill is the number 7 at 0.2010 in. Every drill below is confirmed by both LittleMachineShop and Mech Forged, with Power Tools Insider agreeing on all UNC rows it lists.
| Tap | Series | TPI | Tap drill | Decimal (in) | Decimal (mm) |
|---|---|---|---|---|---|
| #4-40 | UNC | 40 | #43 | 0.0890 | 2.26 |
| #6-32 | UNC | 32 | #36 | 0.1065 | 2.70 |
| #8-32 | UNC | 32 | #29 | 0.1360 | 3.45 |
| #10-24 | UNC | 24 | #25 | 0.1495 | 3.80 |
| #10-32 | UNF | 32 | #21 | 0.1590 | 4.04 |
| 1/4-20 | UNC | 20 | #7 | 0.2010 | 5.11 |
| 1/4-28 | UNF | 28 | #3 | 0.2130 | 5.41 |
| 5/16-18 | UNC | 18 | F | 0.2570 | 6.53 |
| 5/16-24 | UNF | 24 | I | 0.2720 | 6.91 |
| 3/8-16 | UNC | 16 | 5/16 | 0.3125 | 7.94 |
| 3/8-24 | UNF | 24 | Q | 0.3320 | 8.43 |
| 7/16-14 | UNC | 14 | U | 0.3680 | 9.35 |
| 1/2-13 | UNC | 13 | 27/64 | 0.4219 | 10.72 |
| 1/2-20 | UNF | 20 | 29/64 | 0.4531 | 11.51 |
Sources per row: LittleMachineShop + Mech Forged; UNC rows also Power Tools Insider. UNC is the coarse series you want by default; UNF (fine) rows are included because fine-thread taps come up in automotive and machine work and use a different drill at the same nominal diameter.
Number and letter drills are the odd middle tier of the imperial system: number drills run #1 (0.228 in) down to #80, letter drills A (0.234 in) to Z (0.413 in), and together they fill the gaps between fractional sizes. A set that stops at 1/16 through 1/2 in fractions cannot tap most small inch threads, which is why the number series matters here.
Why tap drill charts disagree
Published charts are not wrong when they differ by one drill size; they are answering different questions. Three knobs move the answer:
- Thread engagement. 75 percent is the general-purpose default for aluminum, brass, and plastics. LittleMachineShop's own chart adds a second column for steel, stainless, and iron at about 50 percent thread, one drill size larger, because full-depth threads in tough material multiply tapping torque and snapped taps. ConcreteCalculate covers the same trade-off at 50 to 85 percent.
- Tap type. Everything above is for cutting taps. Forming (roll) taps displace metal instead of cutting it and need a larger hole: about major diameter minus half the pitch for metric. M6 x 1.0 forming wants roughly 5.5 mm, not 5.0 mm.
- Rounding. The calculated hole rarely lands on a real drill (M12 x 1.75 calculates to 10.25 mm), so charts round to stock sizes, and they do not all round the same direction.
Practical reading: in soft material, use the tables above as printed. In steel, step up one drill size from the table and accept a shallower thread, and always use cutting fluid. If a tap fights you hard in the first half turn, stop and open the hole; that resistance is the whole chart disagreement compressed into one moment.
How the math works, with an example
For inch threads: drill = major diameter minus (0.01299 x percent / TPI). Worked example, 1/4-20 at 75 percent: 0.250 minus (0.01299 x 75 / 20) = 0.250 minus 0.0487 = 0.2013 in, so the number 7 drill at 0.2010 in. For metric: drill = major diameter minus pitch, which is the same statement in simpler clothes. If you are tapping something outside the tables (a fine pitch, a 5/8-11, an M16), the formula plus a decimal-equivalent drill chart gets you there; do not round down more than a hair, because an undersized hole is what breaks taps.
Printable chart and PDF
This page prints clean: the tables are plain HTML with no interactive dependencies, so your browser's Print dialog (and its Save as PDF option) produces a one or two page shop reference with the sources intact. We deliberately do not ship a separate PDF that can drift out of sync with the checked tables; the printed page is the document.
Taps in one paragraph
Taper taps have a long lead for starting straight in through holes, plug taps are the general-purpose middle ground, and bottoming taps have almost no lead for threading blind holes to the bottom. Spiral-point taps push chips forward and suit through holes in a drill press; spiral-flute taps pull chips back out and suit blind holes. The drill size is the same for all of them; only the forming taps described above break that rule. Speed matters as much as size: hand-tap speed in steel is slow, a third of what you would drill at, and our drill speed chart covers the drilling side by material and diameter.
Quick answers
- What drill for a 1/4-20 tap? #7 (0.2010 in). The most-tapped thread in North America.
- What drill for M6 x 1.0? 5.0 mm, or a #8 (0.199 in) in an imperial index.
- What drill for 1/2-13? 27/64 in (0.4219 in).
- What drill for 10-24 vs 10-32? #25 vs #21. Same nominal diameter, different thread, different drill.
- Steel or aluminum, same drill? Start with the table size for aluminum and brass; step one size up for steel and stainless to spare the tap.
- Tap drill vs clearance drill? A tap drill leaves material for threads; a clearance drill lets a bolt slide through freely. They are opposites and not interchangeable. For screws into wood rather than threads into metal, see the pilot hole chart and the countersink size chart for seating flat heads flush; for fasteners already rounded or snapped, the bolt and screw extractor size chart picks up where drilling stops.
Related guides
Also on this site: the pilot hole chart for wood screws, lag screws, and metric wood screws, drill bits by material for which bit should be doing this drilling, the drill speed chart for RPM by diameter, and screwdriver bit types for the driving end of the fastener world.