Here is a bit of guidance on my belt sizing and measurements. Traditionally the saddlery trade works in inches but you can use the Metric size guide to convert this easily.
Adult Classic, Wye Valley & Marshalsay belts have 7 holes spaced 1″ (25mm) apart.
Plaited, Laced & all Ranger belts have 5 holes spaced 1″ (25mm) apart.
The size indicates the measurement from the buckle, (but not including it) to the tightest (first) & loosest (last) holes. My sizing is based on the belt being usually fastened around the middle holes, but this is entirely your choice if you prefer a longer or shorter overlap, adjust the size ordered as required!
Use the guide below to choose the best size for your needs. 
Please contact me if you'd like any variation to these standard sizes.

Adult belt sizing guide
Order your
Waist/Trouser
size 
Classic, Wye Valley &
Marshalsay belts  
7 holes
(min-max)
Plaited, Laced &
Ranger belts 
5 holes
(min-max)
26″/66cm
24″-30″
26″-30″
28″/71cm
26″-32″
28″-32″
30″/76cm
28″-34″
30″-34″
32″/81cm
30″-36″ 32″-36″
34″/86cm
32″-38″ 34″-38″
36″/91cm
34″-40″ 36″-40″
38″/96cm
36″-42″ 38″-42″
40″/101cm
38″-44″ 40″-44″
42″/106cm 40″-46″ 42″-46″
44″/111cm 42″-48″ 44″-48″
46″/116cm 44″-50″ 46″-50″
48″/122cm 46″-52″ 48″-52″
50″/127cm 48″-54″ 50″-54″