FIND THE SIZE. COMPARE THE OPTIONS
Drill & Tap Reference
Look up metric, fractional, number and letter drill sizes, or choose a thread to calculate a starting-hole size for cutting or forming taps. Compare nearby sizes and see both inch and millimetre measurements.
Straight threads / metric + Unified
Find your tap drill.
Search the thread marked on your drawing, then choose its exact size and pitch. Values follow the selected thread: inches first for inch threads, millimetres first for metric threads, with the other units in brackets. Compare the calculated pre-hole with nearby nominal drills. Open “Adjust the calculation” for forming taps or a different target percentage.
Adjust the calculation
Uses Guhring’s conventional thread-percentage calculation. This is not strength percentage, axial engagement length or a tolerance class. The selectable range is a calculator input range, not a recommendation for every application.
Calculated pre-hole
Closest nominal size
Compare nearby sizes.
Nominal sizes, not a stock guarantee.
Thread series and tolerance classes
UNC, UNF and UNEF describe Unified pitch series. 8UN, 12UN and 16UN are constant-pitch series. A screw number such as #10 is a thread size; it is not number drill #10.
1B · 2B · 3B
Internal Unified classes. 1B has a wider manufacturing tolerance, 2B is general purpose, and 3B has closer tolerances. External mates use A designations. A closer class does not mean an interference fit.
4H · 5H · 6H · 7H · 6G
Metric internal grade/position combinations. The number describes tolerance grade. H has zero fundamental deviation; G has a positive one. Uppercase internal G is different from lowercase external g. ISO 965-2 uses 5H for M1–M1.4 and 6H for M1.6–M100 in its listed scope.
A drill alone does not establish the thread class. Finished minor and pitch diameters must meet the drawing’s limits. H/D tap limits describe tool tolerances and are not interchangeable with finished thread classes. This tool does not calculate class acceptance limits.
Browse the thread catalogue
Calculated cutting-tap targets at 75%, not a table of certified class limits. Choose a row to load it above.
How the tap-drill calculation works
Let D be the nominal major diameter in millimetres, P the pitch in millimetres and C the desired percentage entered as a whole percentage: 75 for 75%.
Cutting taphole = D − (C × P ÷ 76.98)
Forming taphole = D − (C × P ÷ 147.06)
For inch threads, pitch is 25.4 ÷ TPI. Calculations use millimetres internally and exactly 25.4 mm per inch. The inch and metric versions printed by manufacturers use rounded constants, so very small differences can occur.
Example: M8 × 1.25 at 75% gives a calculated cutting pre-hole of about 6.782 mm. A 6.8 mm nominal drill gives about 73.9% by this convention. The closest nominal size is a comparison, not a verified hole tolerance or a guarantee of thread strength.
Guhring also gives the general cutting rule D − P and forming rule D − P/2. Those are convenient starting rules, not the same as choosing an exact percentage. Forming must use a suitable material and the particular tap maker’s hole-size limits.
Metric / fractional / number / letter
Put every size in perspective.
Enter the designation on a drill, or a diameter you need to compare. See its metric and inch values, then compare exact, smaller and larger nominal sizes. A close size is not necessarily a suitable substitute.
Use # for number drills. Fractions are inches. A bare decimal uses the input units below.
Nominal diameter comparison
“Exact” refers to equal nominal diameters. Displayed decimals are rounded; nearby sizes are not automatically interchangeable.
What are wire-number and letter drills?
Number drills are often called wire-gauge drills. They are not AWG electrical-wire sizes. In this collection, #107 is the smallest and #1 the largest. #80–#1 is the common range; #107–#81 is a documented micro extension. Letters A–Z increase in diameter.
The fractional list is a mathematical 1/64-inch grid through 4 inches. The metric list uses 0.1 mm increments to 100 mm, plus 0.05 mm increments below 3 mm. These grids aid conversion and do not promise that every size is stocked. Shank size is not cutting diameter.
Scope, calculation methods and sources
This collection covers listed straight 60° metric and Unified combinations through M100 / 4 inches, plus custom calculations within that diameter range. Metric pitches are a manufacturer-listed selection. Constant-pitch entries are selected combinations, not an exhaustive standards database. Pipe (NPT/NPTF/BSP), STI inserts, Acme/trapezoidal, buttress and UNJ/MJ threads need separate data.
The adjustable calculations use Guhring’s approximate metric formulas: cutting pre-hole = D − (% × P) / 76.98; forming pre-hole = D − (% × P) / 147.06. D and P are in millimetres. Conversion uses exactly 25.4 mm per inch. Displayed results are rounded; use the actual produced hole and the tap maker’s limits for the job.
- Guhring Decimal Equivalents — drill sizes and popular tap drill sizes
- Fastenal Metalworking Decimal Equivalents Chart, poster 9701309
- Viking Drill and Tool — hosted tool diameter chart
- AFT Fasteners — drill diameter reference
- Yamawa — nominal metric sizes and listed pitches
- NIST / NBS Handbook H28 — Unified nominal thread series
- Guhring — Common formulas for tapping
- Learn It! — mathematical conversion grids · Exactly 25.4 mm per inch; grid availability is not a stock claim.
- NIST — exact inch-to-millimetre definition
- OSG — tap-drill calculation and pitch-diameter guide
- Guhring — pre-holes for thread forming
- ISO 965-2:2024 — scope and listed tolerance classes
- Guhring — tap limits and thread classes
Reference build V1.1.2 · checked 2 November 2026. Nominal facts are separated from calculated estimates. Historical NIST data is used for size/series facts, not current dimensional acceptance limits.