When the current nappies start to get a little too difficult to close, then it’s time to move up a size. Also the weight of your baby can determine which size nappy they wear.

When it was getting a little snug around her tummy and thighs

We looked at the length of the nappy and if there was an increase in the number of accidents.

Use the weight range as a guide and also the fit – you’ll know if it seems to small / not quite covering properly. They may start to leak etc

I tried different brands until I found the one I liked the best. I looked at comfort, price, sticky tab efficiency, absorbency, if bub dud or didn’t have a reaction to them. Lots of different things. Doesn’t take long to find a favourite

yeah just when it was getting harder to do up. it is easy to tell.

when the nappy was getting to tight, the tabs wouldn’t do up anymore I would change to the next size

I mainly focused on where the tabs did up, too far apart made baby’s tummy bulge a little and they can irritate the skin where they start touching. The height of the nappy up baby’s back, if you can see a plumber’s line then they are too small haha.
It’s definitely different for each baby. Hope that helps