What this calculator does — and its lane
This is the tap-drill and thread-size tool for 60° threads: the V-form threads used by ISO metric (M) sizes and by Unified inch sizes (UNC and UNF). Give it a thread — pick a common size or type a major diameter and pitch (or TPI) — and it computes the tap drill hole at your chosen percent of thread, then names the nearest real drill bit so you are not left holding a decimal you cannot find in your index. It also does the jobs that single-number charts leave out: a larger hole for roll-form (thread-forming) taps, clearance holes, the pitch-to-TPI conversion, the basic thread geometry, and three-wire measurement over wires. Every figure is calculated from the public 60° thread formula — nothing is copied from a handbook, a manufacturer or a brand chart. The honest friction with printed charts is that they pin you to one engagement (almost always 75%) and one drill size, and they rarely tell you the bit you actually own; this tool fixes both by letting you set the percentage and showing the nearest number, letter, fractional and metric bit.
Tap drill calculator (any size and % of thread)
The cut-tap (standard tap) hole is the thread’s major diameter minus a slice that depends on the pitch and how much of the full thread you want to leave:
tap drill = major diameter − 0.0130 × (% of thread) × P
inch: P = 1 ÷ TPI | metric: P = pitch in mm
example: 1/4-20 @ 75% = 0.250 − 0.0130 × 75 ÷ 20 = 0.2013 in → #7 (0.201)
The percent of thread is the share of the full thread depth the tap will cut; the rest is clearance so a bolt turns in freely. 75% is the traditional baseline, but it is not sacred — below about 60–70% you lose very little holding strength while cutting tapping torque sharply, which is why so many machinists drop the engagement in stainless, titanium and tough alloy steel to stop snapping taps in the hole. Set whatever percentage you want between 50 and 100 and the hole, and the nearest bit, update as you type.
Drill and tap chart — find the right drill bit
A bare tap-drill decimal is only half the answer; the other half is which bit in your set gets you there. After it computes the hole, this tool searches the standard drill sizes — fractional, numbered wire-gauge (#1–80), letter (A–Z) and metric — and reports the nearest one with its exact decimal and how far over or under it lands. Numbered drills are counter-intuitive (a bigger number is a smaller bit) and the gaps between fractional sizes are exactly why number and letter drills exist, so a drill-and-tap chart that closes that gap saves a lot of squinting. If you only own fractional bits, the result also notes the nearest fraction so you can decide whether a couple of thousandths over is fine for the job.
Roll-form / thread-forming tap drill size
Roll-form taps (also called form or fluteless taps) do not cut a chip — they cold-form the thread by displacing metal, so they need a larger starting hole than a cutting tap of the same thread. Use the form mode for that hole:
roll-form hole = major diameter − 0.0068 × (% of thread) × P
example: M6×1.0 @ 65% = 6 − 0.0068 × 65 × 1.0 = 5.56 mm
Because the metal flows rather than being removed, form taps make stronger threads with no chips to clear — a real advantage in blind and deep holes — but a hole sized for a cutting tap will jam a form tap or tear the thread. The form mode defaults to 65% engagement, which is typical for forming, and flags the result with a reminder to confirm against your own tap before you commit on a part that matters.
Thread pitch and TPI
Pitch and threads-per-inch are two ways of saying the same thing — the distance from one thread to the next:
pitch (in) = 1 ÷ TPI | pitch (mm) = 25.4 ÷ TPI | TPI = 25.4 ÷ pitch (mm)
basic pitch diameter = major − 0.649519 × P
basic minor diameter (internal) = major − 1.082532 × P
thread height H = 0.866025 × P
Metric threads are specified by pitch (M6×1.0 has a 1 mm pitch); inch threads are specified by TPI (1/4-20 has 20 threads per inch, so a 0.050 inch pitch). The pitch/TPI mode converts either way and shows the basic pitch, minor and thread-height geometry for the thread you entered — the same numbers that feed the tap-drill and three-wire calculations. A single-start thread’s lead equals its pitch.
Clearance hole sizes
A clearance hole is for a bolt or screw to pass through, not to be tapped — so it is larger than the major diameter, sized to the fit you want:
close ≈ major × 1.06 | normal ≈ major × 1.12 | loose ≈ major × 1.20
Close is a snug fit for precise location, normal is the everyday choice, and loose gives room for misalignment across multiple holes. The clearance mode gives all three for the chosen size with the nearest drill bit for each. These are typical working clearances, not a tolerance-class limit table — pick the fit that suits the assembly.
Three-wire thread measurement
The three-wire method checks a thread’s pitch diameter by laying three identical wires in the thread groove and measuring across them with a micrometer:
best wire W = 0.57735 × P
measurement over wires M = pitch diameter + 3W − 1.51553 × P
solve: pitch diameter = M − 3W + 1.51553 × P
Enter a thread and the wire mode gives the best (touching-the-flank) wire size and the measurement you should read over three wires if the thread is at its basic pitch diameter. Type in a measurement you actually took and it backs out the real pitch diameter, so you can see how far the thread is from nominal. The 60° constants here apply to both ISO metric and Unified inch threads.
Tap drill size chart (metric and inch)
The chart below is the centrepiece: common ISO metric (coarse and fine) and Unified (UNC and UNF) sizes, each with its major diameter, pitch or TPI, the computed cut-tap drill at your chosen engagement, the nearest standard bit, and a typical clearance hole. Every tap-drill value is calculated live from the formula above — change the engagement in the charts mode and the whole column moves — so it is a thread-size chart and a tap-drill chart at once. Switch the calculator to Charts to see it.
Drill size chart (fractional, number, letter, metric)
The second chart is a plain drill-size reference: fractional, numbered (#1–80), letter (A–Z) and metric drills with their decimal equivalents in both inches and millimetres, sorted by diameter so you can find the nearest bit to any target. It doubles as a decimal-equivalent chart and a quick inch↔mm drill converter. Numbered and letter sizes have no tidy formula — they are the long-established standard gauge sizes — while a fractional drill is just its fraction (7/32 in = 0.2188 in) and a metric drill divided by 25.4 gives its inch value (5.5 mm = 0.2165 in).