Bolt Length Calculator
Enter what the bolt goes through and whether it ends in a nut or a threaded hole, and get the bolt length to buy, rounded up to a standard size.
How to work out bolt length
- Grip: measure the total thickness of everything the bolt clamps, with a ruler or calipers.
- Add washers: about 1/16 to 1/8 inch (1.5–3 mm) each, more for wide fender washers.
- Through-bolt: add the nut height, plus at least two threads past the nut.
- Tapped hole: add the thread engagement, 1 to 2 bolt diameters depending on the material.
- Round up to the next standard length. Rounding down leaves the nut short of threads.
Nut heights and washer thickness
| Bolt | Hex nut height | Flat washer | Bolt | Hex nut height | Washer (DIN 125) |
|---|---|---|---|---|---|
| 1/4-20 | 0.219 in | 0.065 in (SAE) | M5 | 4.7 mm | 1 mm |
| 5/16-18 | 0.266 in | 0.065 in (SAE) | M6 | 5.2 mm | 1.6 mm |
| 3/8-16 | 0.328 in | 0.065 in (SAE) | M8 | 6.8 mm | 1.6 mm |
| 7/16-14 | 0.375 in | 0.065 in (SAE) | M10 | 8.4 mm | 2 mm |
| 1/2-13 | 0.438 in | 0.095 in (SAE) | M12 | 10.8 mm | 2.5 mm |
| 5/8-11 | 0.547 in | 0.095 in (SAE) | M16 | 14.8 mm | 3 mm |
| 3/4-10 | 0.641 in | 0.134 in (SAE) | M20 | 18 mm | 3 mm |
Nylon-insert lock nuts are taller than plain hex nuts: measure yours and enter its height above. Washer sizes for every bolt are in the washer size chart.
Standard bolt lengths
Inch: 1/4 inch steps up to 3 inches (1/2, 5/8, 3/4, 1, 1 1/4 …), then 1/2 inch steps to 6 inches, then whole inches. Metric: 8, 10, 12, 16, 20, 25 mm, then 5 mm steps to 80 mm and 10 mm steps above that. Hardware stores may not stock every size.
Frequently asked questions
How do I know what length bolt I need?
Add up everything the bolt passes through: the parts, any washers and the nut, then add at least two threads so the bolt sticks out past the nut. For two 3/4 inch boards with a 1/2 inch bolt, two washers and a nut: 1.5 + 0.19 + 0.44 + 0.15 = 2.28 in, so buy a 2 1/2 inch bolt.
How is bolt length measured?
From under the head to the tip for hex, socket and carriage bolts. Flat (countersunk) heads are the exception: they are measured from the top of the head, because the head sits inside the part.
How far should a bolt stick out past the nut?
At least two full threads, so the nut grips the whole thread form and any lock nut insert engages. More than about a bolt diameter past the nut is wasted length that can snag or rust.
How deep should a bolt go into a tapped hole?
At least one bolt diameter in steel (a 1/2 inch bolt engages 1/2 inch of thread), about 1.5 diameters in cast iron and 2 diameters in aluminum or softer metals. Make the hole deeper than the bolt reaches so it doesn't bottom out.
Not sure of the bolt's size? Use the thread identifier, then check the bolt torque chart for how tight to make it.