Millimeters are very small. For this reason, they are perfect for measuring jewelry parts. For example, the gluing surfaces on most flat pad posts are 3mm, 4mm, 6mm, 8mm, and 10mm. If those were converted to inches, they would be (in order) approximately 1/8 inch, 5/32 inch, 1/4 inch, 5/16 inch, and 3/8 inch.
I'm guessing that even if you've always hated the metric system, that row of millimeter sizes looks cleaner than that row of inch fractions. In case you've never gotten along with millimeters, or in case you need to do some conversions and never got along with algebra, this page contains easy conversions back and forth between inches and millimeters.
Estimates & Approximations  (without a calculator)
If you have the inches, and need to know the millimeters:
In order of increasing size...
 1/16 inch = almost 2mm
 1/8 inch = just barely over 3mm
 1/4 inch = just barely over 6mm
 3/8 inch = almost 10mm
 1/2 inch = almost 13mm
 5/8 inch = 16mm
 3/4 inch = 19mm
 7/8 inch = just barely over 22mm
 1 inch = 25.4mm
If you have the millimeters, and need to know the inches:
 1mm = just over 1/32 inch
 2mm = just over 1/16 inch
 3mm = almost 1/8 inch
 4mm = 5/32 inch (= a bit over 1/8 inch)
 5mm = just over 3/16 inch
 6mm = almost 1/4 inch
 7mm = almost 9/32 inch (= a bit over 1/4 inch)
 8mm = 5/16 inch
 9mm = almost 3/8 inch
 10mm = just over 3/8 inch
 11mm = almost 7/16 inch
 12mm = almost 15/32 inch (= almost 1/2 inch)
 13mm = just over 1/2 inch
 14mm = almost 9/16 inch
 15mm = almost 19/32 inch
 16mm = 5/8 inch
 17mm = almost 11/16 inch
 18mm = just over 11/16 inch
 19mm = 3/4 inch
 20mm = 25/32 inch
 21mm = just over 13/16 inch
 22mm = almost 7/8 inch
 23mm = 29/32 inch
 24mm = just over 15/16 inch
 25mm = almost 1 inch
Calculator Conversions
If you have inches, and want to know how many millimeters:
Step one: Convert any inch fractions to decimals.
Example: 1/4 inch = 1 divided by 4 = .25 inch
Example: 1/16 inch = 1 divided by 16 = .0625 inch
For amounts over an inch, you want to first convert the fractional part, then add the whole inches part:
Example: 21/4 inch = 1 divided by 4, plus 2 = 2.25 inch
Example: 15/8 inch = 5 divided by 8, plus 1 = 1.625 inch
Step two: Now multiply that decimal by 25.4
For each of the above examples, the complete steps would be:
1/4 inch = 1 divided by 4, x 25.4 = 6.35mm (6mm)
1/16 inch = 1 divided by 16, x 25.4 = 1.5875 (1.6mm)
21/4 inch = 1 divided by 4, plus 2, x 25.4 = 57.15 (57mm)
15/8 inch = 5 divided by 8, plus 1, x 25.4 = 41.275 (41mm)
If you have millimeters (or centimeters), and you want inches:
Step one: Divide your millimeter amount by 25.4
(or divide your centimeter amount by 2.54)
Example 1: 2mm divided by 25.4 = .07874 inches
Example 2: 6mm divided by 25.4 = .2362 inches
Example 3: 30mm divided by 25.4 = 1.1811 inches
Example 4: 4cm divided by 2.54 = 1.5748 inches
If you are trying to figure out how many 6mm beads you need for a 20" necklace, you can use this decimal value directly. From Example 2 above, you know that a 6mm bead = .2362 inches.
So, 20 inches (per necklace) divided by .2362 inches (per bead) = 84.67 beads per necklace. Since you probably need to reserve a bit of space for your bead tips and a clasp, I would round down to 80 beads.
If, however, you want fractions of an inch (1/16, 1/4, 1/2), rather than a decimal, try this:
Step two: Converting your decimal to a useable fraction:
I prefer to round to the nearest 1/16 of an inch. To round to the nearest 1/16 of an inch, multiply your decimal by 16. The resulting number is how many 1/16's of an inch you have.
Using the numbers from Step 1 above:
.07874 times 16 = 1.26, which rounds to 1, so about 1/16 inch.
.2362 times 16 = 3.779, or rounds to 4, so about 4/16. 4/16 = 1/4 inch
For amounts over an inch, it is easiest to remove the whole inches, and add them back in after you convert the portion after the decimal point: 1.1811 Removing the 1 gives you .1811 times 16 = 2.9, which rounds to 3, so about 3/16 inch... don't forget to add the 1 back on, so this is really 13/16 inch.
1.5748 Removing the 1 gives you .5748 times 16 = 9.1968, which rounds to 9, so about 9/16 inch... don't forget to add the 1 back on, so this is really 19/16 inch.
For jewelry purposes, I don't recomend rounding to larger fractions, such as 1/4, or even 1/8 inch. If you are not making jewelry, or your needs are not very exact, then you can round to a larger fraction such as 1/4. To round to the nearest 1/4 of an inch, simply replace all the 16's in Step 2's examples with 4's.
Using the numbers from step 1, that I also used in step 2, you would get...
.07874 times 4 = .315, which rounds to 0, or 0/4 of an inch (not a very useful number!)
.2362 times 4 = .9448, which rounds to 1, or 1/4 of an inch (which happens to be accurate)
Miscellaneous Numbers For Conversions
To Convert...
 inches to millimeters, multiply by 25.4.
 inches to centimeters, multiply by 2.54
 millimeters to inches, divide by 25.4
 centimeters to inches, divide by 2.54
 meters to feet, multiply by 3.28
 feet to meters, divide by 3.28
 millimeters to centimeters, multiply by 10
 centimeters to meters, multiply by 100
