What is the conversion for UK and US crochet hooks to mm?
For hook sizes, it's always best to refer to the size of the hook in mm, if possible. This should be standard across all countries. Metric sizes are used in many countries including Australia and New Zealand. UK sizes are generally used in the UK and Canada. US sizes are generally used only in the US.

The equivalents (when applicable) are below. Note that some sites give different size conversions for the same hook and not all mm sizes are available in every system, which is why some sizes are listed under more than one mm size.

Crochet Hook Conversions:

mm size ----- US -------- UK   

2.00 mm ---- 0 ------ 14  
2.25 mm --- 1/B ----- 13  
2.50 mm ---- 2 ------ 12  
2.75 mm --- 2/C ----- 11/12  
3.00 mm ---- 3 ------ 11  
3.25 mm --- 3/D ----- 10  
3.50 mm --- 4/E ----- 9  
3.75 mm --- 5/F ----- 9  
4.00 mm --- 6/G ----- 8  
4.50 mm ---- 7 ------ 7  
5.00 mm --- 8/H ----- 6  
5.50 mm --- 9/I ----- 5  
6.00 mm --- 10/J ---- 4  
6.50 mm -- 10½/K ---- 3  
7.00 mm --- n/a ----- 2  
7.50 mm --- n/a ----- 1
8.00 mm --- 11/L ---- 0  
9.00 mm -- 13/M/N --- 00  
10.0 mm -- 15/N/P --- 000  
15.0 mm --- P/Q ----- n/a
16.0 mm ---- Q ------ n/a
19.0 mm ---- S ------ n/a