What shirt collar size for a 6 year-old boy?

[Azure]

I want to order some school shirts for DS1 (6 last week) from the John Lewis website but they are offered by collar size rather than age, e.g. 12.5", 13", 13.5", 14". As I am at work and have no ready access to a child or their clothing, does anyone have a clue? Many thanks.

Oops, finally found the info on the website. None offered in the right size though, typically.

12.5", but I think that you might need chest size as well - sometimes 12.5 covers 5-9 yos but obv 9yo shirts are bigger with longer arms.

With Trutex shirts 11.5 is Age 5-6; 12 is 7-8 and 12.5 if 9-10.

DS2, who is 5, but tall (115cm) and skinny has the 11.5, but they are roomy. You might be best with a 12 - especially if they wear ties, as the tight collar annoys DS2. The only problem is that bigger also = longer and DS has to tuck it into his trousers and ends up with loads of fabric....

DS1 6 wears 11.5 from John Lewis, I have some 12's but they are far to big.

Ds1 (6) must have a thick neck then. He is currently wearing a Trutex size 12.5, but was in a Jonelle one yesterday. I think that DH is now a 16" so it may run in families.

Fab - thanks for all your help. I'm not used to clothing sizes other than by age, but the John Lewis ones seemed most suitable (right colour etc).