bore size on a Bb trumpet

[Paul Cote]

how can I tell the bore size weather its large bore or med bore thanks paul

[Dale Proctor]

@Paul-Cote
Some trumpets have it stamped on the valve block. If you can find the specs on your trumpet, the bore size is sometimes given in mm or inch measurements. If you have calipers, you can measure the inside diameter of the second valve slide tubing (not the slide receiver). How that measurement relates to a bore size designated by small, medium, medium large, large, or extra large depends on the measurement range the manufacturer chooses. For example, the same bore measurement on horns from two different makers may be called medium large by one and large by the other. There are no hard and fast rules on what’s what, just general ranges of what constitutes a bore designation.

[J. Jericho]

Here's a short essay on bore size written by someone who has helped trumpet players at all skill levels: https://bbtrumpet.com/blogs/Equipment/trumpet-bore-sizes