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Tech Data Index | Series Introduction

Understanding Metric Sizes & Threads

The size of a metric fastener (screw, bolt, etc.) is specified as diameter, pitch and length, all in millimeters (millimeters is abbreviated "mm"). An exception involves coarse threaded fasteners, in which case only the diameter and length may be listed (coarse threads is then assumed). Consider this size:

M8-1.0 x 20

Here's what the description means...

M = Metric thread designation

8 = Nominal diameter, in millimeters

1.0 = Pitch (distance from thread to thread), in millimeters

20 = Length, in millimeters (see below)

If you normally work with inch fasteners, pitch can be a little confusing--pitch is the distance between adjacent threads. In the example above, a pitch of 1.0 means that from one thread to the next thread, the distance is 1.0 millimeter. Inch fasteners are described as threads per inch, which is sometimes mistakenly called pitch. If you need to know the threads per inch of a metric fastener, first convert pitch from millimeters to inches (multiply by 0.03937), then divide 1 by that number. Or, use the chart below...

Pitch
(mm)
Pitch
(inches)
Threads
per Inch
0.2 0.0079 127.00
0.25 0.0098 101.60
0.3 0.0118 84.67
0.35 0.0138 72.57
0.4 0.0157 63.50
0.45 0.0177 56.44
0.5 0.0197 50.80
0.6 0.0236 42.33
0.7 0.0276 36.29
0.75 0.0295 33.87
0.8 0.0315 31.75
1.0 0.0394 25.40
1.25 0.0492 20.32
1.5 0.0591 16.93
1.75 0.0689 14.51
2.0 0.0787 12.70
2.5 0.0984 10.16
3.0 0.1181 8.47
3.5 0.1378 7.26
4.0 0.1575 6.35
4.5 0.1772 5.64
5.0 0.1969 5.08
5.5 0.2165 4.62
6.0 0.2362 4.23

Metric fasteners are usually classified as either coarse or fine, referring to their thread pitch, although some sizes are available in an extra fine thread. Coarse metric fasteners, when compared to standard "inch" fasteners (those commonly used in North America), tend to be less coarse--their threads are more closely spaced--for a comparable size. Fine metric fasteners can be more or less fine, depending on size. Coarse is the most common metric thread and is assumed when pitch isn't specified. See the chart below:


Size

Series

Thread
Pitch
(inches)
Threads per Inch
M14-2.0 Metric Coarse 0.079 12.7
9/16-12 Inch Coarse 0.083 12
M14-1.5 Metric Fine 0.059 16.9
9/16-18 Inch Fine 0.056 18
M3.5-0.6 Metric Coarse 0.024 42.3
6-32 Inch Coarse 0.031 32
M3.5-0.35 Metric Fine 0.014 72.6
6-40 Inch Fine 0.025 40

The threads of an M14-2.0 screw, which is coarse, are spaced 0.079 inches (2.0 mm). A 9/16-12 screw, which is comparable in diameter and is also coarse, has its threads spaced 0.083 inches, which are farther apart, so there will be fewer threads per unit of measure (we used an inch in the chart above). The same goes for a coarse M3.5-0.6 and a 6-32 screw--the threads of the 6-32 are more widely spaced so, again, it will have fewer threads for the same length. (M3.5-0.6 and M3.5-.6 are exactly the same--the "0" is included to make sure that the decimal point isn't missed.)

When you look at fine M14-1.5 and 9/16-18 fasteners, however, the 9/16-18 is finer--it has more threads per inch. But the M3.5-0.35 has close to double the number of threads per inch of a 6-40, so its threads are very closely spaced (0.014 inches, or 0.35 mm).

The chart below compares various metric sizes to the closest inch equivalent...

Metric Closest Inch
M1.4-0.3 0-80 (Larger)
M2-0.4 1-64 (Smaller)
M2.5-0.45 3-56 (Larger)
M3-0.5 4-48 (Smaller)
M3.5-0.6 6-40 (Same)
M4-0.7 8-36 (Larger)
M5-0.8 10-32 (Smaller)
M6-1.0 1/4-28 (Larger)
M7-1.0 1/4-28 (Smaller)
M8-1.25 5/16-18 (Smaller)
M10-1.5 3/8-16 (Smaller)
M12-1.75 1/2-13 (Larger)
M14-2.0 9/16-12 (Larger)
M16-2.0 5/8-11 (Smaller)
M18-2.5 3/4-10 (Larger)
M20-2.5 3/4-10 (Smaller)
M22-2.5 7/8-9 (Larger)
M24-3.0 1-8 (Larger)
M30-3.5 1 1/8-7 (Smaller)
M36-4.0 1 3/8-6 (Smaller)

Length is measured the same as inch fasteners, except in millimeters. For button, cheese, hex, pan, socket and low socket heads, measure length from under the head. For flat and oval heads, length includes head height.

Metric fasteners are also manufactured to meet standards set by either DIN - German Institute for Standardization or ISO - International Organization for Standardization. Consequently, they are referred to by the standard number to which they apply. Following is a table of common DIN and ISO numbers and a brief description of the fastener...

No. Description
DIN 84  Slotted Cheese Head Machine Screw
DIN 85  Slotted Pan Head Machine Screw
DIN 94  Cotter Pin
DIN 125A  Flat Washer (without Chamfer)
DIN 125B  Flat Washer (with Chamfer)
DIN 127B  Split Lock Washer
DIN 137B  Spring Wave Washer
DIN 315  Wing Nut
DIN 439B  Machine Screw Hex Nut, Finished Hex Jam Nut
DIN 911  Long Arm Hex Key
DIN 912  Hex Socket Head Cap Screw
DIN 916  Hex Socket Set Screw
DIN 931  Hex Head Cap Screw Partially Threaded
DIN 933  Hex Head Cap Screw Fully Threaded
DIN 934  Finished Hex Nut
DIN 939  Double End Stud
DIN 960  Hex Head Cap Screw Partially Threaded
DIN 961  Hex Head Cap Screw Fully Threaded
DIN 963  Slotted Flat Head Machine Screw
DIN 964  Slotted Oval Head Machine Screw
DIN 965  Phillips Flat Head Machine Screw
DIN 966  Phillips Oval Head Machine Screw
DIN 975  Threaded Rod
DIN 980V  Hex Steel Prevailing Torque Lock Nut
DIN 985  Hex Nylon Insert Lock Nut
DIN 6334  Rod Coupling Nut
DIN 6797A  External Tooth Lock Washer
DIN 6797J  Internal Tooth Lock Washer
DIN 6912  Hex Low Socket Head Cap Screw
DIN 6921  Hex Flange Head Screw
DIN 6923  Hex Flange Nut
ISO 7379  Hex Socket Head Shoulder Screw
ISO 7380  Hex Button Socket Head Cap Screw
DIN 7985  Phillips Pan Head Machine Screw
DIN 7991  Hex Flat Socket Head Cap Screw

Note: If you know the DIN or ISO number of the fastener you're looking for, you can use our search function to locate it quickly.

Remember these tips about metric fasteners:

  • Diameter: the larger the number, the larger the screw or bolt
  • Pitch: the larger the number, the more coarse the thread (the fewer the threads per unit of measure); coarse is the most common metric thread and is assumed when pitch isn't specified
  • Length: the larger the number, the longer the screw or bolt
  • Standard: the DIN or ISO standard number describes the fastener

Below is a partial list of metric sizes (diameter-pitch) Fastener Mart offers (be sure to search our store for other sizes):

Coarse Fine Extra Fine
M1.4-0.3        
M2-0.4        
M2.5-0.45        
M3-0.5        
M3.5-0.6        
M4-0.7        
M5-0.8        
M6-1.0        
M7-1.0        
M8-1.25 M8-1.0    
M10-1.5 M10-1.25 M10-1.0
M12-1.75 M12-1.5 M12-1.25
M14-2.0 M14-1.5    
M16-2.0 M16-1.5    
M18-2.5 M18-1.5    
M20-2.5 M20-1.5    
M22-2.5        
M24-3.0 M24-2.0    
M30-3.5        
M36-4.0        

The differences between inch and metric thread sizes are summarized in these charts...

Thread Size Comparison Chart 1

Thread Size Comparison Chart 2

Tech Data Index | Series Introduction

 

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