Thread Calculator

The major diameter is the largest diameter of the screw thread, measured from crest to crest. The pitch diameter is the theoretical cylinder diameter where the thread thickness equals the space between threads — it is used for thread tolerance and gauge measurements. The minor (root) diameter is the smallest diameter, measured from root to root, and determines the tensile stress area of the thread.

Thread pitch is the distance between adjacent thread crests, measured in mm for metric threads. A finer pitch (smaller number) means more threads per unit length, which provides better resistance to loosening under vibration and allows finer adjustment. A coarser pitch (larger number) is faster to assemble and more tolerant of minor damage. UNC (Unified National Coarse) uses coarser pitches suitable for general-purpose fastening.

A tap drill is the drill bit used to create the pre-drilled hole before cutting internal threads with a tap. The tap drill size is smaller than the major diameter, leaving material for the thread crests to be cut. The sizes shown in this tool target approximately 75% thread engagement, which is the industry-standard balance between thread strength and ease of tapping. Using too small a drill increases risk of tap breakage; too large reduces thread engagement.

UNC stands for Unified National Coarse — the standard coarse-thread series used for general fastening in North America. UNF (Unified National Fine) has a finer pitch and is used where greater precision, higher thread strength per unit length, or finer adjustment is needed. This tool covers UNC threads only. For UNF thread data, consult ASME B1.1 standard tables.

The "M" prefix indicates ISO metric thread. The number after M is the nominal major diameter in millimeters (e.g., M8 = 8 mm outer diameter). The number after × is the pitch in millimeters (e.g., ×1.25 means 1.25 mm between thread crests). M8×1.25 is the standard coarse pitch for M8. Metric threads with finer pitches (e.g., M8×1.0) would be classified as metric fine threads.

The tool displays all dimensions in millimeters for consistency and international usability. Metric measurements allow direct comparison between metric and imperial threads and are easier to use with digital calipers calibrated in mm. The tap drill size for imperial threads is shown with both the traditional letter/number drill designation and its equivalent in mm.

Thread engagement refers to the percentage of full thread height that contacts between the bolt and nut threads. 75% engagement (achieved with the listed tap drill sizes) provides approximately 75% of the maximum possible thread strength while requiring much less torque to tap than a 100% thread. For most applications in steel, 75% is more than sufficient. For softer materials (aluminum, plastic) you may want higher engagement, which requires a slightly smaller tap drill.

For CNC thread milling or turning, you need the pitch to set the thread helix, the major diameter for the tool path outer boundary, and the pitch diameter for tolerance specifications. Use the minor diameter to verify the pre-drill depth clears the thread root. For external threads (bolts), the major diameter equals the nominal thread diameter. For internal threads (nuts/tapped holes), the minor diameter is your pre-drill target.