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Ditto here Cliff.

Like that Acorn, but should the result be a bmi or calories or boyh?

I like this one as a good example. It tends more towards the British values (I've no idea my height in inches but I've no idea...

I like Acorns range sliders.

I did a search on Stack Overflow and came up with a very simple form. Weight & Height only.

Glad you found that Acorn, I'd forgotten all about that ;)