Bolt Size Chart

A bolt designation like 1/4-20 packs two numbers into one label: the nominal diameter (1/4 inch) and the thread count (20 threads per inch, TPI). This chart covers the most common imperial bolt sizes from #4 up to 1 inch, with each size's decimal diameter, standard coarse thread pitch, and closest metric equivalent.

The metric column shows the closest common metric bolt size, not an exact match. Imperial and metric threads use different pitch standards, so a bolt and nut from the two systems will not reliably interchange even when the diameters look close.

The number before the dash is the nominal diameter, either a fraction (1/4, 3/8, 1/2) or a screw number (#4, #6, #8, #10) for sizes below 1/4". The number after the dash is threads per inch (TPI) for the standard coarse (UNC) series. A bolt marked 1/4-28 instead of 1/4-20 is the finer UNF thread for the same 1/4" diameter; see the imperial thread size chart for the full UNC and UNF pitch listing. For help choosing which fractional drill bit sizes to buy for tapping and clearance holes, see the drill bit buying guide.

Cutting internal threads for any bolt on this chart requires drilling a pilot hole first, known as the tap drill. The correct tap drill diameter depends on both the bolt's nominal size and its thread pitch. See the tap and clearance drill chart for the matching drill size for each bolt in this table, or the wrench and socket size chart to find the tool that fits each bolt head.