The best way to get your belt size is to measure a current belt from the end of the leather where it folds around the buckle to the most commonly used hole.
Typically it measures around 3" larger than your jeans size, but that doesn't mean you should just look at your tag, add three, and place an order. There are a few other factors to consider.
Thicker leather fits tighter, so if you’re going from a thin department store belt to a much thicker belt, you’ll want to order 1-2" larger than your thin belt.
The rise of your jeans can mess with the 3" rule. If you wear your jeans particularly high or low, it’ll throw things off, so it’s always best to measure when possible.
Older belts will curve and wrinkle, so do your best to lay it out flat and measure the full length.
Some belt leathers will be very stiff when new, which will make them feel tighter. As they soften up they will contour your body more closely and loosen up. While this isn't technically stretching, it does effect the fit.
Some leathers stretch more than others. Chromexcel is particularly stretchy, but I do my best to minimize this by pre-stretching (yanking the hell out of) the belts before I size them up. I find with my own CXL belts that they grow by around an inch over the first year and then stop.
Almost every incorrectly sized belt is too small. If you're unsure, ordering larger is much safer than ordering smaller.
If you need specific advice, get in touch and I'll do my best.