M1×0.25 Thread Dimensions and Tap Drill
M1×0.25 is the ISO metric coarse thread for 1 mm fasteners, which is why it is usually written simply as M1. It has a 60° profile with a pitch of 0.25 mm (101.6 threads per inch). ISO 261 lists M1 as a first-choice diameter.
- Tap drill0.75 mm77% thread
- Pitch0.25 mm101.6 TPI
- Major diameter1.000 mm0.0394 in
- Pitch diameter0.838 mm0.0330 in
- Minor diameter (nut)0.729 mm0.0287 in
- Tensile stress area0.460 mm²0.000713 in²
- Clearance hole1.2 mmmedium fit, ISO 273
Thread profile
M1×0.25 basic dimensions
| Dimension | mm | in |
|---|---|---|
| Major diameter d = D | 1.000 | 0.0394 |
| Pitch P | 0.250 | 0.0098 |
| Pitch diameter d2 = D2 | 0.838 | 0.0330 |
| Minor diameter, internal thread D1 | 0.729 | 0.0287 |
| Minor diameter, external thread d3 | 0.693 | 0.0273 |
| Height of fundamental triangle H | 0.217 | 0.0085 |
| Thread height, internal (5H/8) | 0.135 | 0.0053 |
| Thread height, external (17H/24) | 0.153 | 0.0060 |
| Crest flat of external thread (P/8) | 0.031 | 0.0012 |
| Crest flat of internal thread (P/4) | 0.062 | 0.0025 |
| Property | Value |
|---|---|
| Threads per inch | 101.60 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 5.43° |
| Tensile stress area As | 0.460 mm² (0.000713 in²) |
Basic (nominal) sizes per ISO 724, before tolerances. d3 is the root diameter of a bolt with the rounded root that ISO 898-1 uses for the stress area.
M1×0.25 tolerance limits
ISO 965-1 defines no grade 6 tolerances for a 0.25 mm pitch, so there are no 6g/6H limits for M1×0.25. ISO 965-2 specifies tolerance classes 5H for the nut and 6h for the screw on M1, M1.2 and M1.4.
M1×0.25 tap drill size
Tap drill for M1×0.25: 0.75 mm
This leaves about 77% thread in the tapped hole.
The rule for metric tap drills is diameter minus pitch: 1 − 0.25 = 0.75 mm, which is a stocked drill size.
Other drills that work for M1×0.25
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 6H limits | Thread engagement |
|---|---|---|---|---|---|
| #69 (0.0292 in) | 0.74 | 0.0292 | 80% | — | |
| 0.75 mmrecommended | 0.75 | 0.0295 | 77% | — | |
| 0.8 mm | 0.80 | 0.0315 | 62% | — |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for M1
The clearance hole depends only on the 1 mm diameter, not on the pitch. The medium series, 1.2 mm, is the usual choice for general assembly.
| Series | Hole Ø (mm) | Hole Ø (in) | Clearance on Ø (mm) |
|---|---|---|---|
| Fine (H12) | 1.1 | 0.0433 | 0.1 |
| Medium (H13) | 1.2 | 0.0472 | 0.2 |
| Coarse (H14) | 1.3 | 0.0512 | 0.3 |
Hole diameters from ISO 273.
M1×0.25 strength
ISO 898-1 covers coarse threads from M1.6 to M39, so it gives no proof or breaking loads for M1×0.25. For an estimate, multiply the tensile stress area of 0.460 mm² by the stress of the material: for example, 800 MPa gives 0.368 kN.
How the M1×0.25 dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.25 mm
- H = 0.866025 × P = 0.217 mm
- d2 = d − 0.649519 × P = 1 − 0.162 = 0.838 mm
- D1 = d − 1.082532 × P = 1 − 0.271 = 0.729 mm
- d3 = d − 1.226869 × P = 1 − 0.307 = 0.693 mm (external root, 17H/24 deep)
- As = π/4 × ((d2 + d3) ÷ 2)² = π/4 × 0.765² = 0.46 mm²
- Lead angle = arctan(P ÷ (π × d2)) = 5.43°
Related sizes and tools
Sources: ISO 68-1 and ISO 724 (profile and basic dimensions), ISO 261 (diameter and pitch series), ISO 965-1 (tolerances), ISO 273 (clearance holes), ISO 898-1 (mechanical properties) and tap drill tables based on ISO 2306. How this site checks its data. Last reviewed 29 September 2026.