Threads / Shared guide
Read every part
of the callout.
The short size gets you to a reference page. A manufacturing or inspection requirement needs more information.
Metric example: M10 × 1.5
M denotes the metric system, 10 is the nominal diameter in mm, and 1.5 is pitch in mm. Keep the explicit pitch when comparing options such as M10 × 1 and M10 × 1.25. A suffix such as 6H or 6g describes a tolerance class and must be read separately.
Compare metric sizes →Example: 1/4-28 UNF-2B
The short designation identifies the diameter and pitch. A/B distinguish external/internal thread classes. Read the full class and applicable standard on the drawing; this reference does not derive class limits from a base code.
What each result can answer
Nominal size
Diameter and pitch help distinguish nearby designations and compare reference geometry.
Tap-drill reference
Method and theoretical thread percentage choose a chart column. The exact tool and material still determine the permitted hole.
Manufacturing limits
Require the applicable standard edition, class and complete product/drawing requirements. They are not the nominal dimensions.
Identification
A measured part can match more than one system. Check profile, pitch, taper, handedness and the relevant gauge or drawing.
Keep four distinctions clear
#8 is not an 8 mm screw
#8 has a nominal diameter of 0.164 in (4.1656 mm). The number before the dash is a screw size; the number after it is TPI.
Pitch and lead are not always equal
For a single-start thread, lead per revolution equals pitch. The optional tapping-feed arithmetic assumes a single-start thread; it does not cover multi-start work.
A tap drill is not a clearance hole
A pre-tap hole leaves material to form the internal thread. A clearance hole serves a different assembly function and is outside this lookup.
More thread percentage is not a strength calculation
The percentage used here describes theoretical thread height. It does not state engagement length, joint strength or inspection acceptance.
Evidence and boundaries
What has been checked?
Reviewed 2026-10-19. 66 matching chart rows and the metric/UNC/UNF equations were read directly. The other 27 metric/UNC/UNF sizes have equation estimates only; 18 UNEF sizes have no reviewed drill selection. Calculated conversions are labelled; class-specific manufacturing limits are not supplied.
ISO 261 · metric general plan ↗
Public scope only. Nominal sizes are transcribed from the live directory and checked against their designations; paid standard tables were not reviewed.
ISO 965-1:2026 · tolerance principles ↗
Current public listing, published April 2026. No class-specific limits are reproduced from the paid standard.
Guhring · Tap Drill Sizes ↗
PDF page 2: 66 matched metric and UNC/UNF rows, CUT TAPS and FORM TAPS columns (55%, 65%, 75%), sizing equations and drilling-runout note. Page 1: numbered and letter drill diameters. Publication revision is not printed on this chart.
ASME B1.1 · standard scope ↗
Public product page lists B1.1-2024 and its scope. The paid standard tables have not been reviewed here; this reference does not publish class-specific acceptance limits.
Swagelok · thread identification procedure ↗
Measure at more than one axial position to distinguish taper, then check pitch and thread form. This supports the measurement procedure, not identification from a nominal match.
NBS Handbook 28 · 1963 supplement ↗
Historical nominal designation #8 / 0.164 in only. No historical tolerances are used as current acceptance limits.
Metric designations state diameter and pitch in mm. Fractional inch diameters are evaluated exactly; numbered diameters are nominal size values. Inch pitch is 1/TPI in. Metric values use exactly 25.4 mm/in, with display rounding to six decimal places. The reference chart is a general starting point, not a selected tap’s product specification.