Getting the right ring size is very important for titanium rings.  They cannot be sized as easily as ordinary gold rings, so more consideration in getting the correct size the first time is required.  The best way to get the correct ring size is simply to send me a ring that you know fits.  I can match the size very exactly.  The next best option is to go to a jeweler and try on professional metal sizer rings.  I can work to quarter sizes, so try to determine the ring size that fits best.  It should have some resistance going over the knuckle and should feel comfortable on a finger.  Bear in mind that fingers swell in the summer and shrink in the winter.  Other factors such as exercise, being in the sun, and water weight gain should be factored in as well.  They might make a quarter size difference.  It also pays to get fitted at different times of the day and different places.   Extremely wide rings like 15mm ones feel tighter than normal width rings, so it might be prudent to add a quarter size in that case.  Please never resort to paper sizers, string, plastic sizers, measuring of fingers, guessing based on other fingers, or other methods which can be several ring sizes off.  A quarter ring size is usually around .007", which is the thickness of 2 sheets of paper, so anything other than professional metal sizing rings is bound to be off in size.  I can stretch most rings a quarter to a half size, but I cannot go down in size.  For rings going overseas or very large rings, it pays to include an internal diameter measurement just to be sure the standards are the same.

Here's a chart of the sizes I use:

 
US SIZE INCH ID MM ID BRITISH   US SIZE INCH ID MM ID BRITISH   US SIZE INCH ID MM ID BRITISH   US SIZE INCH ID MM ID
3.5 .569 14.45 G 6.75 .674 17.13 N 1/2 10 .781 19.84 T 1/2 13.25 .888 22.56
3.75 .577 14.65 H 7 .683 17.35 O 10.25 .788 20.02 U 13.5 .900 22.86
4 .585 14.86 H 1/2 7.25 .687 17.45 O 1/2 10.5 .797 20.20 U 1/2 13.75 .906 23.01
4.25 .592 15.04 I 7.5 .699 17.75 P 10.75 .804 20.44 V 14 .912 23.16
4.5 .601 15.27 I 1/2 7.75 .707 17.97 P 1/2 11 .814 20.68 V 1/2 14.25 .920 23.37
4.75 .611 15.53 J 8 .716 18.19 Q 11.25 .821 20.85 W 14.5 .928 23.57
5 .618 15.70 J 1/2 8.25 .722 18.35 Q 1/2 11.5 .830 21.08 X 14.75 .936 23.77
5.25 .626 15.90 K 8.5 .730 18.53 R 11.75 .836 21.24 X 1/2 15 .945 24.00
5.5 .634 16.10 L 8.75 .736 18.69 R 1/2 12 .846 21.49 Y 15.25 .953 24.21
5.75 .642 16.30 L 1/2 9 .748 18.89 S 12.25 .854 21.69 Y 1/2 15.5 .961 24.41
6 .650 16.51 M 9.25 .757 19.22 - 12.5 .862 21.89 Z 15.75 .969 24.61
6.25 .658 16.71 M 1/2 9.5 .764 19.41 S 1/2 12.75 .870 22.10 16 .978 24.84
6.5 .666 16.92 N 9.75 .772 19.62 T 13 .879 22.33 16.25 .986 25.04