How to measure screw size: diameter, length and thread pitch

A screw size is three numbers: diameter, thread pitch and length. Measure all three and you can identify a lost fastener and find its match in any kit on this site. Here is the method we use at the bench to measure screws and bolts, with the metric and imperial charts the numbers come from.

Step 1: measure the diameter

Set calipers across the outer thread, crest to crest. That is the major diameter, and it is the number in the screw size: an M4 screw measures about 3.9 to 4.0 millimeters, a 1/4 inch bolt about 0.245 inch. For a US gauge number (#4, #6, #8, #10, #12) the number is a designation, not a dimension, so use the chart below to convert. If you measure the hole or the nut instead, you get the minor diameter, which is smaller; add the thread depth, or simply try the next size up from the kit. A fraction on a bolt (5/16, 3/8) is the same major diameter written the imperial way: 5/16 is 0.3125 inch, 3/8 is 0.375 inch.

Step 2: measure the length

Length is measured from the point where the head sits on the surface to the tip. For a socket head, pan head, round head, button head, hex head or hex washer head that means from the bottom of the head to the tip; for a flat countersunk head, from the top of the head, because the whole head sinks into the material. An oval head is measured from the widest point of the head. Measure to the tip, not to the end of the thread, and on a self-drilling screw include the drill point. A 30 mm socket head screw is 30 mm under the head; its total length with the head is more, and that total is not the size.

Step 3: measure the thread pitch or threads per inch

Metric threads are described by pitch, the distance between two adjacent crests in millimeters: 0.5 mm for M3, 0.8 mm for M5, 1.0 mm for M6. Imperial threads count threads per inch (TPI): 20 for 1/4-20, 32 for #6-32. A thread gauge reads either directly; without one, lay the screw on a ruler and count ten threads, then divide the distance by ten. Coarse is the common pitch for each size and the one in every Hapric kit; fine thread (M6 × 0.75, 1/4-28) is rarer, and a fine screw will not fit a coarse nut however correct the diameter.

Metric screw size chart

Major diameter, coarse pitch and the hex key a socket head takes:

• M2: 2.0 mm, 0.40 mm pitch, 1.5 mm key
• M2.5: 2.5 mm, 0.45 mm pitch, 2 mm key
• M3: 3.0 mm, 0.50 mm pitch, 2.5 mm key
• M4: 4.0 mm, 0.70 mm pitch, 3 mm key
• M5: 5.0 mm, 0.80 mm pitch, 4 mm key
• M6: 6.0 mm, 1.00 mm pitch, 5 mm key
• M8: 8.0 mm, 1.25 mm pitch, 6 mm key
• M10: 10.0 mm, 1.50 mm pitch, 8 mm key

The screw size is written diameter × length: M3 × 16 is a 3 mm screw, 16 mm under the head. Pitch is left out when it is the coarse standard.

Imperial screw size chart

Gauge or fraction, decimal major diameter, coarse TPI:

• #4: 0.112 inch, 40 TPI
• #5: 0.125 inch, 40 TPI
• #6: 0.138 inch, 32 TPI
• #8: 0.164 inch, 32 TPI
• #10: 0.190 inch, 24 TPI
• #12: 0.216 inch, 24 TPI
• 1/4 inch: 0.250 inch, 20 TPI
• 5/16 inch: 0.3125 inch, 18 TPI
• 3/8 inch: 0.375 inch, 16 TPI

How to tell a #6 from a #8: the #8 measures about 0.16 inch across the outer thread, the #6 about 0.14; in millimeters that is 4.2 against 3.5, a difference calipers show at once. A #10 is 0.19 inch, almost 5 mm, and a #12 at 0.216 inch is easy to confuse with a 1/4 inch bolt until you count the threads: 24 against 20.

Matching the screw to a kit

Measured 3.0 mm across, 0.5 mm pitch, 16 mm under the head: that is M3 × 16, in our 600-piece M3 case and the master kit. Measured 0.164 inch and 32 TPI: #8-32, a SAE machine screw, which is not in the metric kits but whose set screw cousins live in the 25-size set screw boxes. Measured 6 mm across with a flange under the head: M6 flange bolt, in the 384-piece kit. When the calipers and the chart disagree, trust the thread count; when in doubt, send us the three numbers and we will name the compartment.