Thread Size Chart
Thread Type
Data Sources: Thread diameters, TPI/pitch, and tap drill sizes are cross-checked against
American Thread Standards and
Metric Thread Standards
on sizes.com, which cover sizes up to 1" (American) and 30 mm (metric); larger sizes use standard machinist references instead.
No ASME/ISO standard we could access directly tabulates tap-drill selection for UN threads (ASME B1.1 defines thread geometry
— including the basic minor diameter D1 in Tables 6/7 — but no drill-selection table, and ASME B94.9's tap drill table
covers pipe taps only), so tap drills above 1" are computed from that same B1.1 geometry: each one sits at a constant offset above
its size's basic minor diameter D1, and that offset is identical for every size sharing the same TPI — confirmed against the
four 12-TPI UNF sizes from 1" to 1-1/2" (including the already-verified 1" row), which all sit exactly 0.0277" above D1, and
similarly for the 7-TPI and 6-TPI UNC pairs (0.0140" and 0.0085").
Clearance drill sizes are read directly from primary standards: ASME B18.2.8-1999 Table 2 (Close Fit / H12) for inch sizes #0–1-1/2", which is the full range that standard covers per its own Scope section — two sizes (#12 and 9/16") aren't individually tabulated there and are computed the same way the standard derives the rest of the table: nominal diameter plus the Close Fit allowance from Table 1's size bracket, rounded to the nearest standard drill. Inch sizes 1-3/4" and 2" fall outside B18.2.8's stated scope (it explicitly covers "#0 through 1.5 in"); their values extend the standard's own last allowance bracket (1-3/8", 1-1/2": Close Fit = major diameter + 1/16") rather than a tabulated figure. Metric clearance sizes are read from the "fine" series (H12) of ISO 273:1979 Table (dimensions in mm), which ASME B18.2.8 itself cites as the source its own metric table agrees with; this covers every metric size on this chart except M2.2, whose value (major diameter + 0.2 mm) is the same allowance ISO 273 uses for both its immediate neighbors, M2 and M2.5 — interpolated from the standard rather than tabulated directly, since M2.2 itself is a non-preferred size neither ISO 273 nor B18.2.8 lists.
Clearance drill sizes are read directly from primary standards: ASME B18.2.8-1999 Table 2 (Close Fit / H12) for inch sizes #0–1-1/2", which is the full range that standard covers per its own Scope section — two sizes (#12 and 9/16") aren't individually tabulated there and are computed the same way the standard derives the rest of the table: nominal diameter plus the Close Fit allowance from Table 1's size bracket, rounded to the nearest standard drill. Inch sizes 1-3/4" and 2" fall outside B18.2.8's stated scope (it explicitly covers "#0 through 1.5 in"); their values extend the standard's own last allowance bracket (1-3/8", 1-1/2": Close Fit = major diameter + 1/16") rather than a tabulated figure. Metric clearance sizes are read from the "fine" series (H12) of ISO 273:1979 Table (dimensions in mm), which ASME B18.2.8 itself cites as the source its own metric table agrees with; this covers every metric size on this chart except M2.2, whose value (major diameter + 0.2 mm) is the same allowance ISO 273 uses for both its immediate neighbors, M2 and M2.5 — interpolated from the standard rather than tabulated directly, since M2.2 itself is a non-preferred size neither ISO 273 nor B18.2.8 lists.