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Identifying the size of my Bolt or Screw

With so many thread types across metric and imperial it can be confusing, even to us experts. It can look like M5 but why doesn’t a M5 nut wind on? Is there a way to tell if I don’t have a thread tool. You can most of the time with ideally a digital vernier, next best is a ruler or tape measure. Start off by measuring the outside of the thread, then click on the dropdown and select the range your measurement falls within. This will give you a selection of possibilities. You need then to identify the thread type. if you fastener has a marking on the head, refer to the bolt identification chart, once identified, and now you have narrowed down the size, you know need to work out the thread. Is a metric or imperial, is it coarse or fine?

For accuracy we recommend using a vernier. Measurements are intended to be used as a guide only.

Outside diameter of the thread

Metric Coarse

M1.6 x 0.35p

Metric Fine

Metric Extra Fine

UNC

UNF

#0-80

BSW

BSF

BA

Screw Gauge

Select a size to view outside diameter measurements

You know the size screw but want to cross check the diameter. Select the size from the dropdown to see the closest match.

Screw Type	Thread Type	Outside Diameter of Thread

Thread Pitch Chart

A reference chart to view the thread pitch for metric coarse, metric fine and extra fine and imperial fasteners UNC (unified national coarse) and UNF (unified national fine) and includes hard to find fasteners like BSW (British standard whitworth), BSF (British standard fine) and BA (British association). Laid out in diameter order with the smallest first, you can easily see the closest similar size across each different thread type working from left to right. This helps decide what fastener you have or if you need to substitute it for one that is available. Please note this chart has been complied to be a helpful resource and does not mean we carry the size or thread type in stock.

Measuring Threads by Pitch
Measuring Threads Per Inch (TPI)

METRIC THREADS
Pitch (mm)
Size	Coarse	Fine	Extra Fine
M1.6	0.35
M2	0.40

M2.5	0.45

M3	0.50

M3.5	0.60
M4	0.70

M5	0.80

M6	1.00

M7	1.00
M8	1.25	1.00
M10	1.50	1.25	1.00

M12	1.75	1.50	1.25
M14	2.00	1.50
M16	2.00	1.50

M18	2.50	1.50
M20	2.50	1.50
M22	2.50	1.50
M24	3.00	2.00	1.50
M27	3.00	2.00
M30	3.50	2.00
M33	3.50	2.00
M36	4.00	3.00
M39	4.00	3.00
M42	4.50
M45	4.50
M48	5.00
M52	5.00
M56	5.50
M60	5.50
M64	6.00
M72	6.00
M80	6.00

M90	6.00

M100	6.00

AMERICAN INCH THREADS
Threads per Inch (TPI)
Size	UNC	UNF	UN8
#0		80
#1	64	72
#2	56	64
#3	48	56
#4	40	48
#5	40	44

#6	32	40
#8	32	36

#10	24	32
#12	24	28

#1/4”	20	28

5/16”	18	24
3/8”	16	24
7/16”	14	20
1/2”	13	20
9/16”	12	18
5/8”	11	18


3/4”	10	16
7/8”	9	14
1”	8	14/12	8
1-1/8”	7	12	8
1-1/4”	7	12	8
1-3/8”	6	12	8
1-1/2”	6	12	8
1-5/8”			8

1-3/4”	5		8
1-7/8”	5		8
2”	4.5		8
2-1/4”	4.5		8

2-1/2”	4		8
2-3/4”	4		8
3”	4		8
3-1/4”	4		8
3-1/2”	4		8
3-3/4”	4		8
4”	4		8

BRITISH INCH THREADS
Threads per Inch (TPI)
Size	BSW	BSF	BA


8BA			59.1

6BA			47.9
1/8”	40
5BA			43
4BA			38.5
3BA			34.8
2BA			31.4
3/16”	24	32	32
1BA			28.2
0BA			25.4
1/4”	20	26
9/32”		26
5/16”	18	22
3/8”	16	20
7/16”	14	18
1/2”	12	16
9/16”	12	16
5/8”	11	14
11/16”		14

3/4”	10	12
7/8”	9	11
1”	8	10
1-1/8”	7	9
1-1/4”	7	9
1-3/8”	7	8
1-1/2”	6	8
1-5/8”	5	8

1-3/4”	5	7

2”	4.5	7
2-1/4”	4	6

2-1/2”	4	6
2-3/4”	3.5	6
3”	3.5	5
3-1/4”	3.25	5
3-1/2”	3.25	4.5
3-3/4”	3	4.5
4”	3	4.5