Bolt Size Identifier: Identify Any Bolt in 60 Seconds
The bolt in your hand needs a replacement. It doesn’t have to be a mystery. This article walks you through every input the identifier wants, explains the naming conventions, shows you how to read grade markings, and warns about the “dangerous lookalikes” — metric and imperial bolts that look identical but will strip threads on first contact.
How the Bolt Identifier Works
You answer five questions. The tool returns one answer: a standard designation plus a shopping link.
Bolt Naming Conventions
There are three systems you’ll run into, and the identifier supports all three:
| System | Example | What it means |
|---|---|---|
| Metric (ISO) | M10 × 1.5 × 40 | 10 mm dia, 1.5 mm pitch, 40 mm long |
| UNC (coarse) | 3/8-16 × 1-1/2" | 3/8" dia, 16 TPI, 1.5" long |
| UNF (fine) | 3/8-24 × 1-1/2" | 3/8" dia, 24 TPI, 1.5" long |
| Numbered (US) | #10-32 × 1" | 0.190" dia, 32 TPI, 1" long |
Reading Grade Markings
Grade markings live on the top face of the bolt head. Imperial bolts use radial lines. Metric bolts use a two-digit number.
Special Bolt Types
Beyond the common hex and socket cap, these special types need their own head identification:
- Carriage — smooth dome head with a square shoulder that locks into wood. No wrench surface.
- Flange — hex head with an integrated washer face that distributes clamping load.
- Eye bolt — ring-shaped head for lifting or anchoring cables and chains.
- Lag — coarse wood-screw threads under a hex head; a heavy-duty wood screw disguised as a bolt.
The Dangerous Lookalikes
Some metric and imperial sizes differ by less than half a millimeter. The caliper tells the truth, not your eye.
| Metric | mm | Imperial | mm | Δ |
|---|---|---|---|---|
| M6 | 6.00 | 1/4" | 6.35 | 0.35 mm |
| M8 | 8.00 | 5/16" | 7.94 | 0.06 mm DANGER |
| M10 | 10.00 | 3/8" | 9.53 | 0.47 mm |
| M12 | 12.00 | 1/2" | 12.70 | 0.70 mm |
| M14 | 14.00 | 9/16" | 14.29 | 0.29 mm |
When You Have No Caliper
Use the Calibration Ruler panel on our homepage — hold a credit card to the screen to verify scale, then lay the bolt alongside the ruler. You’ll get within ±0.2 mm, which is tight enough to identify most bolts when combined with thread count.
Identify Your Bolt Now
Pick the head, enter measurements, read your grade stamp — the tool returns a standards-compliant designation with a “closest alternative” warning for lookalikes.
Open Identifier →Recommended Tool
NEIKO Digital Caliper + Thread Pitch Gauge
Combines a 0–6" digital caliper with a 52-blade metric/imperial pitch gauge. Enough to identify 99% of bolts you will ever meet.
View on Amazon →Frequently Asked Questions
What size bolt is this?
Measure the diameter across the thread crests, length from under the head, and the pitch in mm or TPI. Enter those into our bolt identifier along with head type and grade marking to get the full designation.
How do I identify a metric vs SAE bolt?
Metric bolts carry a numeric head stamp (8.8, 10.9, 12.9) and use pitch in mm. SAE bolts use radial line markings (Grade 2/5/8) and TPI. When in doubt, measure the diameter — 6 mm vs 1/4" (6.35 mm) are different enough to see on a caliper.
What do numbers like 8.8 mean on a metric bolt?
It is the property class. The first digit is nominal tensile strength in hundreds of MPa (8.8 = 800 MPa). The second digit is the yield-to-tensile ratio (0.8 = 80%). Higher numbers = stronger bolt.
Are M8 and 5/16 bolts interchangeable?
No. M8 is 8.0 mm, 5/16 is 7.94 mm — barely 0.06 mm apart — but their thread pitches (1.25 mm vs 18 TPI) are completely different. Threading them together will strip one side or both.
Can the identifier handle carriage and eye bolts?
Yes. The head type selector includes hex, socket cap, carriage, flange, eye, lag, and shoulder bolts, each with the correct anatomy for measurement.
How accurate does my diameter measurement need to be?
Within 0.1 mm for metric and 0.005 inch for imperial. Most digital calipers meet this easily. Rounding too aggressively can cross a boundary between a 6 mm and 1/4" bolt.