Caliber to MM Conversion Chart for All Cartridges

Many shooters are confused because there are two different systems for measuring the bore diameter for a cartridge: (1) the caliber system measuring the diameter of the bore of the barrel in inches, and (2) the same measurement, but in millimeters. This page will explain the problem as clearly as possible.

The manufacturers of ammunition have done us no favors in making this easy to understand, but I’ve put together this handy resource so you can quickly see the equivalent naming conventions between mm and inches (caliber).

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Rifle Cartridge Caliber to Metric Conversion Table

Rifle Caliber Name	Common Metric Name	Bullet Diameter
.17	4.5mm	.172″ (4.32mm)
.204	5.2mm	.204″ (5.2mm)
.223	5.56mm	.224″ (5.7mm)
.22 (lr)	5.6mm	.223″ (5.66mm)
.22 (not lr)	5.6mm	.224″ (5.7mm)
.243	6mm	.243″ (6.17mm)
.257	6.53mm	.257″ (6.53mm)
.260	6.5mm	.264″ (6.71mm)
.270	6.8mm	.277″ (7.04mm)
.280	7mm	.284″ (7.21mm)
.30	7.62mm	.308″ (7.82mm)
.325	8mm	3.23″ (8.2mm)
.33	8.6mm	.338″ (8.59mm)
.366	9.3mm	.366″ (9.3mm)
.375	9.5mm	.375″ (9.53mm)
.416	10.6mm	.416″ (10.57mm)
.450	11.4mm	.452″ (Bushmaster) or .458″ (Socom)
.50	13mm	.51″ (12.95mm)

If you’re like me, looking at the table above starts to make your head spin. There are some things in that table that just don’t seem right. That’s because the naming conventions for many of the most popular cartridges bend the rules of measuring in order to come up with a nice sounding name.

For example, the good old .308 Winchester is so named because the bullet is .308″ in diameter. Converted to metric, that’s 7.82mm, but most people would refer to it as a 7.62 because of its military distinction. The difference is that one measurement is the distance between the grooves (the cut out portions of the rifling in the barrel, and another measurement uses the measurement to the lands (the raised portion of the rifling). To make matters worse, the .308 is commonly referred to as a .30 caliber cartridge, which isn’t actually correct at all.

As you can see, the naming conventions of cartridges is complicated because ammo manufacturers like using clean, even numbers and to ignore the actual measurements.

Because the table above has to make some generalizations about particular cartridges, I’ve included the table below which lists each rifle cartridge separately so you can see the specific bullet diameter in inches and its exact metric conversion.

Common Handgun Calibers Converted to MM and Inches

Handgun Cartridge	Bullet Diameter in Inches	Bullet Diameter in MM
.22 LR	.223″	5.66mm
.357 Magnum	.357″	9.1mm
.380 ACP	.355″	9mm
.38 Special	.357″	9.1mm
9mm	.355″	9.02mm
.40 S&W	.40″	10mm
.44 Magnum	.429″	10.9mm
.45 ACP	.452″	11.5mm
.50 AE	.50″	12.7mm

Fortunately, understanding the conversions between metric and imperial for handgun calibers is quite a bit easier because there are far fewer common pistol cartridges than there are rifle cartridges.

For the most part, handgun calibers are either 9, 10, 11, or 12mm.

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List of Rifle Cartridges and Their Bullet Diameters in Inches and MM

Cartridge	Caliber	Bullet Frontal Area (in2)	Avg Muzzle Energy (ft-lbs)
.17 Hornet	0.172	0.023	622
.17 WSM	0.172	0.023	398
.17 HMR	0.172	0.023	240
.204 Ruger	0.204	0.033	1,325
.22lr	0.223	0.039	133
.220 Swift	0.224	0.039	1,766
.224 Valkyrie	0.224	0.039	1,519
.22-250	0.224	0.039	1,654
.22 Nosler	0.224	0.039	1,613
.22 Creedmoor	0.224	0.039	1,769
.22 WMR	0.224	0.039	276
.223 / 5.56	0.224	0.039	1,499
.22 Hornet	0.224	0.039	680
.222 Remington	0.224	0.039	1,165
.243 Winchester	0.243	0.046	1,958
6mm BR	0.243	0.046	1,712
.240 Weatherby Magnum	0.243	0.046	2,099
6mm Remington	0.243	0.046	1,953
6mm Creedmoor	0.243	0.046	2,125
.25-06 Remington	0.257	0.052	2,201
.257 Weatherby Magnum	0.257	0.052	2,675
6.5 Creedmoor	0.264	0.055	2,231
6.5 PRC	0.264	0.055	2,780
.260 Remington	0.264	0.055	2,273
6.5-300 Weatherby Mag	0.264	0.055	3,395
.26 Nosler	0.264	0.055	3,125
6.5-284 Norma Match	0.264	0.055	2,462
6.5 x 55 Swedish Mauser	0.264	0.055	1,983
.264 Winchester Magnum	0.264	0.055	2,766
6.5 Weatherby RPM	0.264	0.055	3,098
6.5 Grendel	0.264	0.055	1,447
.270 Winchester	0.277	0.060	2,862
.270 WSM	0.277	0.060	3,072
.270 Weatherby Magnum	0.277	0.060	3,176
6.8 Western	0.277	0.060	3,011
6.8 Remington SPC	0.277	0.060	1,624
.27 Nosler	0.277	0.060	3,513
7mm-08 Remington	0.284	0.063	2,528
7mm Rem Mag	0.284	0.063	3,122
.28 Nosler	0.284	0.063	3,678
.280 Ackley Improved	0.284	0.063	2,912
.280 Remington	0.284	0.063	2,873
7mm Weatherby Magnum	0.284	0.063	3,482
7 SAUM	0.284	0.063	3,004
7 STW	0.284	0.063	3,458
7mm Mauser	0.284	0.063	2,330
7 WSM	0.284	0.063	3,255
.308 Winchester	0.308	0.075	2,784
.300 Winchester Magnum	0.308	0.075	3,827
.30-06 Springfield	0.308	0.075	3,179
.300 WSM	0.308	0.075	3,718
.300 Weatherby Magnum	0.308	0.075	4,092
.300 PRC	0.308	0.075	4,246
.300 RUM	0.308	0.075	4,135
.30-30 Winchester	0.308	0.075	1,942
.300 Blackout	0.308	0.075	998
7.62 x 39mm	0.308	0.075	1,608
.30 Nosler	0.308	0.075	4,111
.300 Ruger (RCM)	0.308	0.075	2,948
.30-378 Weatherby Mag	0.308	0.075	4,666
.325 WSM	0.323	0.082	3,596
.338 Win Mag	0.338	0.090	4,164
.338 Lapua Magnum	0.338	0.090	4,851
.338 Federal	0.338	0.090	3,340
.340 Weatherby Magnum	0.338	0.090	4,674
.33 Nosler	0.338	0.090	4,799
.338-378 Weatherby Mag	0.338	0.090	5,035
.338 RUM	0.338	0.090	4,694
.350 Legend	0.357	0.100	1,907
.35 Whelen	0.358	0.101	3,932
9.3 x 62mm Mauser	0.366	0.105	4,017
.375 H&H	0.375	0.110	4,560
.375 Ruger	0.375	0.110	4,780
.378 Weatherby Magnum	0.375	0.110	6,004
.416 Ruger	0.416	0.136	5,498
.416 Remington Magnum	0.416	0.136	5,123
.416 Rigby	0.416	0.136	5,166
.444 Marlin	0.429	0.145	3,067
.450 Bushmaster	0.452	0.160	2,810
.45-70 Govt	0.458	0.165	3,138
.458 Win Mag	0.458	0.165	5,063
.50 BMG	0.51	0.204	12,600