Runners and size of shoes - do you go a whole size or half a size up?

[Stumpedasatree]

My feet do swell during a long run and are pretty difficult to buy shoes for. I've just bought a new pair of Asics in half a size larger for road running (I also trail run) but they feel quite roomy when trying on around the house - not sure whether to exchange them for my actual size? It's so hard to predict how they will feel after 10 miles.

My feet are so annoying. They apparently have grown from a size 6 to a 7 over the years too.

I'm a 39-40 in 'normal' shoes.
41 or 42 for running trainers depending on model. and they can change a lot even to the next model update.
the heel should be quite snug and there should a thumb width space in front if the longest toe (a lot like toddler shoe fitting)

general rule of thumb (😂) is that you should have a thumbs width of space between the end of your big tie and end of your trainer.

For me that's 0.5-1 size depending on the brand.

a full size up for road running and my trail shoes are 1.5 sizes up but the brand is notoriously small made

I go 2 sizes up (from a 6 to an 8). I like to wear very cushioned socks and I guess my feet get hot or something! Only ever lost one toe nail and that was after the London Marathon so I can be excused that, I imagine my form was questionable towards the end!