Measure the threads, not the head
The M-size or numbered screw size refers to the nominal outside diameter of the threaded section. The head can be much wider.
On-screen fastener gauge
What size is my
screw or bolt?
Place the threaded shaft against the guides below and choose the closest match. Nothing to upload. Nothing to install.
Runs in your browser
Takes seconds
No account
Screen scale estimated
Good for a quick closest-match check. Keep browser zoom at 100%.
1
Hold the threaded shaft over each gap. The best match is the one whose inner edges line up with the outside of the threads.
You're finding the closest nominal diameter. Thread pitch is a separate measurement.
The simple way
No calipers? Start here.
BoltCheck turns your screen into a quick comparison gauge for common fastener diameters. It's designed to answer the first question most people have: “Is this roughly an M5, M6, 1/4-inch, or something else?”
How to use it
Three things to get right
Look for the closest fit
Center the shaft between a pair of guide lines. If the thread crests visually touch both inner edges, that size is a strong candidate.
Confirm pitch before buying
Two fasteners can share the same diameter but have different thread pitches. Use this tool for diameter first, then confirm pitch if compatibility matters.
Quick reference
What does “M6” mean?
In the ISO metric system, the number after M is the nominal outside thread diameter in millimetres. So an M6 fastener has a nominal major diameter of about 6 mm.
The complete specification may also include thread pitch and length, such as M6 × 1.0 × 30.
FAQ
Before you head to the hardware store
Can a website know the exact physical size of my screen?
Not reliably on every browser and device. BoltCheck starts with a browser-based estimate so you can measure immediately. If you need a more accurate physical scale, use “Improve accuracy.”
Why does browser zoom matter?
Zoom can change how large CSS pixels appear physically. For an uncalibrated comparison, 100% browser zoom gives the most predictable result. A saved manual calibration is tied to the display scale used when you calibrated.
Does BoltCheck identify thread pitch?
Not in this first tool. It compares nominal outside diameter. Thread pitch must also match when choosing a compatible nut, tapped hole, or replacement fastener.
Is this accurate enough for engineering work?
No. It is a convenient identification aid, not a replacement for calipers, a physical thread gauge, or manufacturer specifications when exact measurements matter.
Ready?
Open the gauge