Identifying screws, bolts and drill bits
One of the most useful jobs for a pocket ruler is matching a loose screw or bolt so you can buy more of the same. A calibrated phone is precise enough to tell standard sizes apart, because they are usually a millimetre or more apart.
Calibrate first. A screen that is 3% off reads an M6 bolt as 5.8 or 6.2 mm, which is still clearly M6, but the same error can blur two wood-screw gauges together. The reference sizes guide lists things you can calibrate against in a shop aisle.
1. Measure the diameter across the threads
A metric bolt or machine screw is named by its major diameter, the width across the outside of the thread. An M6 bolt measures just under 6 mm across the thread crests. Lay it flat on the screen and set the two vertical caliper guides on either side of the threads, not the shank or the head.
Expect a reading slightly under the nominal size. Thread crests are made a little undersize so that bolts fit their nuts, so a reading of 5.8 mm on a bolt is an M6, not something in between. Round to the nearest standard size in the table below.
| Size | Major diameter | Coarse pitch | Threads in 10 mm |
|---|---|---|---|
| M3 | 3 mm | 0.5 mm | 20 |
| M4 | 4 mm | 0.7 mm | about 14 |
| M5 | 5 mm | 0.8 mm | 12.5 |
| M6 | 6 mm | 1.0 mm | 10 |
| M8 | 8 mm | 1.25 mm | 8 |
| M10 | 10 mm | 1.5 mm | about 6.7 |
| M12 | 12 mm | 1.75 mm | about 5.7 |
Coarse pitch is what you get unless a fine thread is specified. Fine threads exist for most sizes, M8 × 1.0 and M10 × 1.25 for example, and are common on vehicles and bicycles.
2. Count the thread pitch
Pitch is the distance from one thread crest to the next. It is too small to measure crest to crest, so measure across several and divide:
- Lay the bolt along the ruler, with the threads facing you.
- Set the green caliper on a crest near the tip.
- Count ten crests along and set the amber caliper on the tenth.
- Divide the reading by ten. 12.5 mm over ten crests is a 1.25 mm pitch: an M8 coarse thread.
Counting gaps rather than crests is the usual mistake. Ten pitches span eleven crests if you count the one you started on, so start from zero.
Imperial bolts are described in threads per inch instead. Count the crests in one inch (25.4 mm) of thread. A 1/4" UNC bolt has 20; a 1/4" UNF has 28. If your count does not land near a whole number of metric pitches, the bolt may be imperial.
3. Measure the length the way it is sold
- Countersunk heads, which sit flush with the surface, are measured overall, from the top of the head to the tip.
- Pan, button, round and hex heads are measured from the underside of the head to the tip.
Screws and bolts are sold in steps, usually 5 or 10 mm in common sizes, so round to the nearest step. A long bolt may not fit on a phone screen; measure the diameter and pitch on screen and the length with something else.
4. Wood-screw gauge numbers
Timber screws in Australia, the US and the UK are often sold by gauge, written 8g, #8 or No. 8, with the length in millimetres or inches: "10g × 50 mm". The gauge describes the shank diameter, and the formula is diameter in inches = 0.060 + 0.013 × gauge.
| Gauge | Inches | Millimetres |
|---|---|---|
| 4g | 0.112" | 2.8 mm |
| 6g | 0.138" | 3.5 mm |
| 8g | 0.164" | 4.2 mm |
| 10g | 0.190" | 4.8 mm |
| 12g | 0.216" | 5.5 mm |
| 14g | 0.242" | 6.1 mm |
Neighbouring gauges are only about 0.66 mm apart. Measure across the threads near the head, where they are full height, take two or three readings at different angles, and average them. If you are torn between two gauges, take the screw with you and compare it against the box.
5. Drill bits
Drill bits are usually stamped with their size on the shank, but the stamp wears off. Measure across the smooth shank. Some larger bits have a reduced shank so they fit a smaller chuck; for those, measure across the cutting end instead.
Metric bits come in 0.5 mm steps in a typical set, and fractional imperial bits in 1/64" or 1/32" steps. Several sizes sit close together: 1/4" is 6.35 mm, between 6.0 and 6.5 mm, and 5/16" is 7.94 mm, almost exactly 8 mm. The inch fractions table has the full list. For a pilot hole a small difference rarely matters; for a dowel or a hinge it does, so check with a physical gauge before drilling something you can't undo.
Where a screen is the wrong tool
A phone screen tells standard sizes apart; it does not replace a pair of vernier or digital calipers when a tolerance matters. Bearings, press fits and anything that has to seal need a real measuring tool. And keep steel threads off the glass: lay a sheet of paper over the screen first if you are worried about scratching it, and recheck your calibration through the paper.