How to Measure a Screw: Diameter, Length & Thread Pitch
Every screw has three numbers you need to identify it: diameter, length, and thread pitch. Put them together and you get a designation like M6 × 1.0 × 25 mm or 1/4-20 × 1". Miss any one, and you’ll end up with the wrong box from the hardware store.
This guide walks through each measurement, shows which rule applies to which head type, and explains what to do if you don’t own a caliper. It takes about 60 seconds per screw once you’ve done it twice.
Tools You Need
- Digital caliper — the single best tool. 0.01 mm resolution, $20–$40.
- Thread pitch gauge — optional but quick. A fan of blades you match against the threads.
- Steel ruler — backup for length if the screw is longer than your caliper jaws.
- A calibration card or printable ruler — for when you’re stuck without tools.
Step 1 — Measuring Diameter
Diameter is measured across the outer crests of the threads, not the smooth shank under the threads or the head. The number you get is called the major diameter.
Read the caliper to two decimal places in mm. Most common metric screws round to whole numbers: 5.98 mm is M6, 4.02 mm is M4. Imperial sizes (US numbered and fractional) use decimal inches: #8 is 0.164", 1/4" is 0.250".
Step 2 — Measuring Length (It Depends on the Head)
This is where most people go wrong. The rule: measure the part of the screw that disappears into the material. That means flat/countersunk heads include the head (because they sit flush), while every other head style measures from under the head to the tip.
If in doubt, ask yourself: what part of the screw ends up inside the material? That is the length.
Step 3 — Measuring Thread Pitch
Thread pitch is the axial distance from one thread crest to the next. Metric screws list pitch in millimeters (e.g., 1.0, 1.25, 1.5). Imperial screws use TPI — Threads Per Inch (e.g., 20, 24, 32).
The fastest method is a thread pitch gauge — slide each blade against the threads until one matches perfectly with no gaps. If you don’t have one:
- Metric: Use a caliper to measure 10 threads, divide by 10. Example: 10 threads span 10 mm → pitch = 1.0 mm.
- Imperial: Put the screw on a ruler, count threads between the 0 and 1 inch mark. That’s your TPI.
Step 4 — Putting It Together
Combine the three numbers into a standard designation:
| System | Format | Example | Reads as |
|---|---|---|---|
| Metric | M[dia] × [pitch] × [length] | M6 × 1.0 × 25 | 6 mm dia, 1 mm pitch, 25 mm long |
| UNC (coarse) | [dia]-[TPI] × [length] | 1/4-20 × 1" | 1/4" dia, 20 TPI, 1" long |
| UNF (fine) | [dia]-[TPI] × [length] | 1/4-28 × 1" | 1/4" dia, 28 TPI, 1" long |
| US numbered | #[gauge]-[TPI] × [length] | #8-32 × 3/4" | 0.164" dia, 32 TPI, 3/4" long |
No Caliper? Use a Calibration Ruler
Our identifier tool has a printable calibration ruler you can use on-screen or on paper. Set your screw against the scale, count the crests, and match to the nearest size. Accuracy lands within ±0.2 mm — good enough to eliminate a shortlist of candidates and find a match.
Identify Your Fastener Now
Use our free identifier tool — pick the head type, drive type, enter your measurements, and get the designation in seconds.
Open Identifier →Recommended Tool
NEIKO 01407A Electronic Digital Caliper
Stainless steel, 0–6" range, 0.01 mm resolution, mm/inch toggle. The single most useful tool for identifying any fastener.
View on Amazon →Frequently Asked Questions
Do I measure the threads or the shaft?
Measure across the outer thread crests (the major diameter). The smooth shank under the threads is smaller and is never what you use to identify a screw.
Why does the screw length depend on the head type?
A countersunk or flat-head screw sits flush with the material, so its length includes the head. Pan, round, socket cap, and hex heads sit on top of the surface, so length is measured from under the head to the tip.
How do I measure thread pitch without a gauge?
Place the screw on a ruler and count threads in one inch for TPI, or measure across 10 threads in millimeters and divide by 10 for metric pitch.
Can I measure a screw with a regular ruler?
Yes, but accuracy suffers below 1 mm resolution. Use our printable calibration ruler for basic measurements, and a digital caliper for anything critical.
What does M6 × 1.0 × 25 mean?
M6 is the metric diameter (6 mm), 1.0 is the thread pitch in millimeters, and 25 is the length in millimeters. So this screw is 6 mm diameter with 1 mm between threads, 25 mm long.