What this calculator does
A metric thread is described by two numbers, such as M8 x 1.25: the major diameter (8 mm) and the thread pitch (1.25 mm, the distance from one thread peak to the next). From those two figures, the standard ISO metric thread form defines several other dimensions that matter for fitting nuts, bolts and tapped holes together correctly.
The two dimensions people most often need beyond the basic size are the minor diameter, which sets how much material remains at the root of the thread, and the tap drill size, the hole diameter to drill before cutting an internal thread with a tap. Getting the tap drill wrong is one of the most common practical mistakes in home workshop threading.
The formula
The pitch diameter, minor diameter and tap drill size are all derived from the major diameter and pitch using the standard ISO 68-1 metric thread proportions: pitch diameter subtracts 0.6495 times the pitch from the major diameter, minor diameter subtracts 1.0825 times the pitch, and the recommended tap drill size is approximately the major diameter minus the pitch.
| Term | Meaning |
|---|---|
| Major diameter | The largest diameter of the thread, measured across the outer peaks, and the number that names the thread size (the "8" in M8). |
| Pitch | The distance from one thread peak to the next, in millimetres. |
| Pitch diameter | The diameter measured where the thread groove and ridge are equal width, used in precision thread fits. |
| Minor diameter | The smallest diameter of the thread, measured at the root of the groove. |
| Tap drill size | The hole diameter to drill before cutting an internal thread with a tap, chosen to leave the correct amount of material for the tap to cut. |
The inputs explained
| Field | What to enter |
|---|---|
| Major diameter (nominal size, e.g. 8 for M8) (mm) | The nominal thread size, the number after the M in a designation like M8 (enter 8 for M8). |
| Thread pitch (mm) | The thread pitch in millimetres. Standard metric sizes have a defined coarse pitch (for example M8 is normally 1.25 mm) but fine-pitch variants exist for many sizes. |
When to use it
Drilling a hole to tap a thread
The tap drill size tells you what diameter to drill before cutting an internal thread, so the tap has the right amount of material to cut into without breaking or producing an oversized thread.
Checking a used or unlabelled bolt
Measuring the major diameter and counting threads over a fixed distance (or using a thread-pitch calculator) gives you the pitch, which this calculator then turns into the full set of standard dimensions to confirm the size.
Specifying a custom-machined thread
Working from just a nominal size and pitch, this calculator gives the minor and pitch diameters needed to check a part against a technical drawing or standard.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How do the thread dimensions change across major diameters at a fixed 1.25 mm pitch?
A range of major diameters, all at the same 1.25 mm pitch (the standard coarse pitch for M8), to isolate what changing the size alone does.
| Major diameter | Minor diameter | Recommended tap drill size |
|---|---|---|
| M3 | 1.647 mm | 1.750 mm |
| M4 | 2.647 mm | 2.750 mm |
| M5 | 3.647 mm | 3.750 mm |
| M6 | 4.647 mm | 4.750 mm |
| M8 | 6.647 mm | 6.750 mm |
| M10 | 8.647 mm | 8.750 mm |
How does pitch alone change the thread dimensions for an M10 bolt?
Major diameter held at 10 mm, comparing the standard coarse pitch against common fine-pitch alternatives.
| Pitch | Pitch diameter | Recommended tap drill size |
|---|---|---|
| 0.75 mm | 9.513 mm | 9.250 mm |
| 1.00 mm | 9.351 mm | 9.000 mm |
| 1.25 mm | 9.188 mm | 8.750 mm |
| 1.50 mm | 9.026 mm | 8.500 mm |
Questions
What is the difference between this and the thread pitch calculator?
The thread pitch calculator only converts a pitch in millimetres to threads per inch (TPI) and back. This calculator starts from the major diameter and pitch and works out the full set of standard thread dimensions: pitch diameter, minor diameter and tap drill size.
Why does the tap drill size matter?
Drilling too small makes the tap harder to turn and more likely to break, and can produce an oversized thread as the tap forces its way through. Drilling too large leaves too little material, producing a weak thread that can strip under load.
What is the difference between coarse and fine pitch?
A coarse-pitch thread has a larger pitch (fewer, deeper threads per given length) and is the standard default for most metric sizes. A fine-pitch thread has a smaller pitch, giving a larger minor diameter, more thread engagement and finer adjustment, at the cost of being slower to cut and drive.
Are these dimensions exact for every metric thread standard?
The formulas used here are the standard ISO 68-1 metric thread form proportions, which cover the great majority of standard metric fasteners. Specialised or non-standard threads may use different tolerance classes or proportions, so check a manufacturer's specification for critical or safety-related work.
To convert just the pitch to threads per inch or back, see the Thread Pitch Calculator.