Metric or inch bolt? Check these two measurements
A practical way to separate close metric and Unified inch fasteners without relying on the wrench that fits the head.
GaugeFox reference desk
Bolt, thread, drill, and pipe size guides.
Start with the surface you can actually measure. Use the charts to compare bolt and thread sizes, tap drills, clearance holes, drill labels, tool openings, washers, and pipe—then confirm the second measurement that prevents the most common wrong purchase.
Fasteners
Read
M6 bolt reference: pitch, wrench, Allen key, and length
The useful measurements for identifying a common M6 fastener, including the fine-thread alternative and head-style differences.
1/4-20 bolt reference: diameter, TPI, wrench, and Allen key
A bench reference for the common 1/4-20 UNC thread and the sizes most often confused with it.
How to measure thread pitch without an off-by-one error
A clear method for metric pitch, inch TPI, and checking several thread crests on a physical or on-screen gauge.
Where fastener length starts: under the head or over it?
The practical length convention for external hex, socket cap, button, and flat countersunk fasteners.
Metric, UNC, and UNF thread chart for common fasteners
Common thread diameters, Metric pitch, and Unified threads per inch for comparing nearby bolt and screw sizes.
Tools
What bolt or nut size fits this wrench?
A correct way to use an across-flats wrench measurement without pretending one head width proves one thread size.
Allen key size guide for socket-head screws
Common metric and inch internal-hex drive sizes, separated by head profile so a button head is not mistaken for a socket-head cap screw.
Tap drill size chart for common Metric, UNC, and UNF threads
Common tap-drill sizes with their actual diameters, nearby Metric or Inch alternatives, and the reason a tap drill is not a clearance hole.
Clearance hole size chart: close, normal, and free fits
ISO 273 clearance-hole sizes for common metric bolts, with a clear separation between close, normal, free, and tap-drill holes.
Drill bit size chart: Metric, Fractional, Number, and Letter
Actual diameters for common Metric, Fractional, Number, and Letter drill labels, plus a practical way to compare nearby sizes.
Washers
Washer size guide: measure the hole, then identify the series
Why a washer hole is larger than its named fastener and why inch narrow and wide washers can produce two correct openings.
Pipe
Nominal pipe size is not the outside diameter
A plain-language NPS, IPS, CTS, and NPT reference for measuring the part that is actually available.
How to size pipe by measuring around it
A reliable circumference method for pipe that is installed, larger than a caliper, or too wide for the phone screen.
How to identify a male NPT fitting without measuring the wrong part
Where to place a tapered male thread, what TPI can confirm, and why a female fitting body is not a sizing surface.
Accuracy
Phone-screen measuring: calibration and viewing-angle checks
How to calibrate with a real card, avoid parallax on larger parts, and know when a measurement deserves a second pass.
What these tables are for
They are identification references, not inspection certificates. Dimensions and common tool pairings narrow a part to likely standards. Markings, material, condition, and a no-force fit with a known mating part still matter.
See how GaugeFox measures