Thread Size Chart — pitch, pitch diameter, tap drill
Pitch, pitch diameter, minor diameter and tap drill size for ISO metric coarse and fine threads, UNC/UNF and pipe threads (G, R/Rc, NPT)
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Pitch, pitch diameter, minor diameter, tap drill and clearance hole for metric, UNC/UNF, pipe (G, R/Rc) and NPT threads in one chart; a row shows a profile drawing with all dimensions. For choosing a tap drill or checking a drawing.
How to use
- On "Thread chart" choose a "Thread family" such as "Metric coarse", "UNC" or "Pipe G · R/Rc".
- Type a designation such as M8, 10x1.25, 1/4 or G1/2 into "Search"; switch "Display unit" between mm and inch.
- Press a row for the profile drawing with pitch, pitch diameter, minor diameters, tap drill and clearance hole, or "Print" the chart.
- On "Tap drill percent" enter a thread and a drill diameter, or a target percent, to get the other.
How it works
- 60° threads follow the ISO 68-1 basic profile: d2 = d − 0.649519·P, d3 = d − 1.226869·P, D1 = d − 1.082532·P. Pipe threads use the Whitworth 55° profile (d2 = d − 0.640327·P, d1 = d − 1.280654·P).
- Tap drills: metric by the d − P rule (about 77 % thread), UN from tables that agree (about 75 %); the common G and Rc values come from one source. Percent of thread = (d − drill) ÷ (0.01299 × P).
- Pitches and tabulated drills are the same two-source values as the drill size chart tool; diameters are basic sizes without tolerances. Clearance holes: ISO 273, M1.6 to M30.
Questions and answers
- What are the pitch and tap drill of an M8 thread?
- Pitch 1.25 mm, pitch diameter 7.188 mm, internal minor diameter 6.647 mm. The d − P rule gives a 6.75 mm tap drill; 6.8 mm is the usual stock size.
- What is the pitch diameter d2?
- The diameter of an imaginary cylinder where the thread ridge is as wide as the groove: d − 0.649519 × P for metric threads, the reference size for thread gauges.
- Why is the major diameter of G 1/2 not 12.7 mm?
- Pipe thread sizes are names derived from the pipe bore. G 1/2 (R/Rc 1/2) has a major diameter of 20.955 mm and a pitch of 1.814 mm (14 threads per inch).
- What percent of thread should a tap drill give?
- Steel is usually tapped at 60 to 75 %, aluminium and brass at 75 to 85 %. Below 50 % the threads are weak.