Coarse vs Fine Threads
Every common bolt diameter comes in a coarse pitch and at least one fine pitch. The fine thread is shallower, so the bolt keeps a larger core and is stronger in tension; the coarse thread is deeper, faster to assemble and harder to damage. Coarse is the default, and fine is chosen for a reason.
The same diameter at four pitches
Metric: coarse against fine
| Coarse | Fine | Thread height (mm) | Stress area (mm²) | Gain | Lead angle |
|---|---|---|---|---|---|
| M8×1.25 | M8×1 | 0.677 / 0.541 | 36.6 / 39.2 | +7% | 3.17° / 2.48° |
| M10×1.5 | M10×1.25 | 0.812 / 0.677 | 58 / 61.2 | +6% | 3.03° / 2.48° |
| M12×1.75 | M12×1.25 | 0.947 / 0.677 | 84.3 / 92.1 | +9% | 2.94° / 2.04° |
| M12×1.75 | M12×1.5 | 0.947 / 0.812 | 84.3 / 88.1 | +5% | 2.94° / 2.48° |
| M16×2 | M16×1.5 | 1.083 / 0.812 | 157 / 167 | +7% | 2.48° / 1.82° |
| M20×2.5 | M20×1.5 | 1.353 / 0.812 | 245 / 272 | +11% | 2.48° / 1.44° |
| M24×3 | M24×2 | 1.624 / 1.083 | 353 / 384 | +9% | 2.48° / 1.61° |
Inch: UNC against UNF
| UNC | UNF | Thread height (in) | Stress area (in²) | Gain | Lead angle |
|---|---|---|---|---|---|
| #10-24 UNC | #10-32 UNF | 0.0226 / 0.0169 | 0.0175 / 0.0200 | +14% | 4.65° / 3.35° |
| 1/4-20 UNC | 1/4-28 UNF | 0.0271 / 0.0193 | 0.0318 / 0.0364 | +14% | 4.18° / 2.87° |
| 5/16-18 UNC | 5/16-24 UNF | 0.0301 / 0.0226 | 0.0524 / 0.0580 | +11% | 3.66° / 2.66° |
| 3/8-16 UNC | 3/8-24 UNF | 0.0338 / 0.0226 | 0.0775 / 0.0878 | +13% | 3.40° / 2.18° |
| 1/2-13 UNC | 1/2-20 UNF | 0.0416 / 0.0271 | 0.1419 / 0.1599 | +13% | 3.11° / 1.95° |
| 5/8-11 UNC | 5/8-18 UNF | 0.0492 / 0.0301 | 0.226 / 0.256 | +13% | 2.93° / 1.72° |
| 3/4-10 UNC | 3/4-16 UNF | 0.0541 / 0.0338 | 0.334 / 0.373 | +12% | 2.66° / 1.61° |
| 1-8 UNC | 1-12 UNF | 0.0677 / 0.0451 | 0.606 / 0.663 | +9% | 2.48° / 1.61° |
Each cell gives coarse / fine. Gain is the increase in tensile stress area from coarse to fine.
What the fine thread gains
- Tensile strength of the bolt. The stress area is 4 to 15% larger, and the bolt carries that much more load at the same material strength.
- Resistance to loosening. The lead angle is smaller, so less of the clamp load acts to turn the nut back.
- Finer adjustment. One turn moves the nut a shorter distance, which suits adjusters and bearing preload.
- Thin walls. A shallow thread can be cut in a tube or a thin section that a coarse thread would weaken or break through.
What the coarse thread gains
- Stripping strength in soft material. The thicker thread ridge carries more shear, so coarse threads are preferred for tapped holes in aluminium, cast iron, brass and plastics.
- Tolerance of damage. A nick, a burr, dirt or a thick coating such as hot-dip galvanizing is a smaller fraction of a deep thread.
- Speed. Fewer turns to run a nut down, and less risk of cross-threading when starting it.
- Less galling. Stainless steel and titanium fasteners seize less readily on a coarse pitch.
- Availability. Coarse is what is stocked everywhere, in every strength class.
Choosing
Use the coarse thread unless one of the advantages of the fine thread is needed. Typical reasons to go fine are a highly loaded bolt in a steel or hardened nut thread, a joint exposed to vibration, a thin-walled part, or an adjustment. Whatever the pitch, both parts must match: a fine nut does not go on a coarse bolt.
Common questions
Are fine threads stronger than coarse threads?
The bolt is, by about 4 to 15% in tension, because its core is larger. The thread in the nut or tapped hole is weaker in shear per unit length, which matters in soft materials.
Do fine threads hold better under vibration?
Yes, somewhat. The smaller lead angle reduces the tendency of the clamp load to unscrew the nut. A locking feature is still needed where vibration is severe.
Does the tap drill change with the pitch?
Yes. A finer pitch needs a larger tap drill: M10×1.5 uses 8.5 mm, M10×1.25 uses 8.8 mm and M10×1 uses 9 mm.
Other charts and tools
Sources: ISO 724, ISO 898-1 and ASME B1.1 (dimensions and stress areas). How this site checks its data. Last reviewed 29 September 2026.