There are a few sets of standard size ranges for boots & shoes - British, American, and European, but they are not absolutely accurate and precise, or directly comparable.
Each manufacturer will have slightly different interpretations of a size, and also there are often differences between different models of boot from the same manufacturer
Thus although your stated size might be a 10, you should try a 10, or 10 1/2, or a 44 or 45, or a 9 1/2, and so on from several makes & models
for some shoes you'll say - no; for some - maybe; for some - yes; and for one or two pairs you'll say ah, yes! definitely! these are the ones
And so on for other sizes
Thus you should use a shoe size as a guide then make the final decision on comfort & fit
But you can't do that with Online purchases, you have to go into a shop, IMHO that's the only sensible way
Also, your feet get bigger as you get older; typically from age 50 to 55 you might need a size bigger. I found this out the hard way, with some boots I'd worn for years round the Lakes & elsewhere, then I tried doing the Ribble Way in them and had to drop out after 2 days, in agony
I used to take about a 10 1/2; now I'm in my 60s and take an 11 1/2 to 12. Like I said, go by the fit, not the numbers
Having said that, if there's any doubt, go for a slightly larger size. As stated your feet will swell as you do some walking, plus you can always wear a thicker sock or lace them a bit tighter. But if boots & shoes are too tight, there's not much you can do about it - except suffer