Formula for sizing wide band rings

[Keia]

Hello everyone. Hope you're all enjoying the sunshine as much as possible =)
I'm struggling with sizing up wide band rings - as I plan on doing a line of spinner style rings I really need to crack this issue.
Does anyone know of a formula for what size of ring should be made when someone says their finger size is N for example?

Here's an example of my problem. I made myself a wide banded ring to fit my exact size which is N. So, I made the ring a size N exactly then after it was finished found that due to the wideness of the band (it was 8mm wide) it was actually far too small. I worked it out that a normal ring sizer will only give a true ring size if the ring is around 2mm wide - anything after that wideness the ring becomes smaller on the finger.
So, does anyone have a formula should someone ask me for a wide banded ring in their size - which size should the ring actually be made to? What advice would be easiest to work with to give my potential customers on sizing up for wide banded rings?

Sorry for all the questions, I'm in such a pickle over it and really hope some of you lovely people can help. I did try a forums search on here but I'm not finding anything relevant to my problem. It's probably here already somewhere but I'm missing it =S

[mizgeorge]

I generally add one and half to two ring sizes for an extra wide band, depending on the shape of the finger it's to go on - some people have slimmer fingers with large knuckles, and they need less extra added, where those with more triangular shaped fingers tend to need the full two sizes. For spinner rings, you can generally add a little less, as the flare helps them fit more easily.

You can get a wide band ring sizer, but I found these were still not quite wide enough for things like spinner rings, so I ended up making a set of flared rings in copper in a size range from L to U. It didn't take long and meant I could size much more accurately. I'll even send these out for people to try - I've yet to have one not returned, and if they didn't it wouldn't be a very big deal as it's pretty easy to re-make a couple if necessary.

[Dennis]

Hi Sian,

As a rough guide, a wide ring should be half below and half above the line on your ring stick. Even so you can't be quite sure unless you do what George suggests. Regards, Dennis

[Keia]

That's brilliant, thankyou both for taking the time to reply. Sending out the copper "blanks" to customers to ensure it's the right size sounds like the best option - and very practical! Thanks =)