10-1/2 would be 6.5 mm. See chart below.

Here is a link that might be useful: knitting needle conversion chart

I have a list that says 10-1/2 = 6.5 metric and 3 English. Hope this helps.

Betty

Thanks so much to both of you - just the info I needed
ivamae

I have 10 1/2 needles and use them a lot.

My favorite needles are 10 1/2. Most of the knitted afghans I've made were on 10 1/2. The blanket turns out much lighter than 8-10, and not as loose as 11 & 17.

It's the only needles I found that are "1/2 size". I saw them at Walmart (that evil territory!).

Occasionally needles labelled "10 1/2" are actually 7.0 mm so use a needle gauge and swatch to make sure you get the gauge you want. Some 7 mm needles are labelled as "10.75".