What Size Bolt Do I Need? (Measure It or Snap It)

What size bolt is it? Start with the diameter

Calipers will read a hair under the nominal size. Most diameters have an inch and a metric candidate, and the thread pitch tells them apart.

Full thread and wrench data for every size is on the bolt size chart.

Check it with a wrench

If a wrench fits the head snugly, it narrows things down fast.

Flange-head and many vehicle bolts don't always follow these head sizes, so confirm with the threads.

How to measure a bolt, step by step

1. Clean the threads. Dirt and rust throw off the reading.
2. Diameter: close calipers on the thread crests, not the root. Note inch and mm.
3. Pitch:
   • With a thread gauge, try leaves until one sits in the threads with no light showing.
   • Without one, count threads over 1/2 in and double it (inch), or measure across 10 threads and divide by 10 (mm).
   • Or try a nut you know fits that size. If it binds after a turn or two, the thread is different.
4. Length: from the underside of the head to the tip. For flat (countersunk) heads, measure overall.
5. Grade: look at the head. Three radial lines = SAE Grade 5, six = Grade 8, a number like 8.8 or 10.9 = metric class. Replace it with the same.

Choosing a new bolt

• Diameter and thread: match the hole or nut exactly. Close is not good enough.
• Length: add up the parts, washers and nut. The bolt should end at least flush with the outside of the nut. In a tapped hole, don't let it bottom out.
• Grade: same as the original. If you don't know, ask the equipment maker.
• Finish: match the original's coating or material (zinc-plated, galvanized, stainless), especially outdoors.

Common mistakes

• 3/8-16 vs M10 x 1.5. A classic mix-up: 9.5 vs 10 mm, and the pitches are close (1.59 vs 1.5 mm).
• 5/16-18 vs M8 x 1.25. Almost the same diameter. The pitch differs.
• Measuring the thread root. That reads small and points you to the wrong size.
• Reading the wrench size as the bolt size. A 9/16 in wrench fits a 3/8 in bolt.
• Measuring a flat-head bolt from under the head. Measure it overall.

Skip the calipers: snap it

Snap the old bolt next to a ruler and Mechy tells you the size, thread and length. If the thread has to be exact, confirm it with a thread gauge or a nut that fits. Identify a bolt from a photo.