How do I work out how much ribbon I need to go round a full circle skirt?

[KatyMac]

It's floor length for my DD (about 5ft2)

It isn't here or I'd measure it

Damn I need to buy it tonight

Circumference of a circle: pi x (radius squared).

Measuring from centre of circle, not waistband.

I think

Oh, for radius, take measurement from waist to floor.

Add a bit to be sure.

Get 7 or 8 meters to be certain, six might be enough but would be annoying to miss out.

Isn't that the area?

d x pi is the circumference I think assuming the radius is about 4ft? (maybe???)

8*3.142 is about 25 ft so 10 yards should do it I guess

Thanks (I wasn't thinking maths at all)

Oh dear that is quite a difference

Circ is 2 pi r, which I don't think is the same as d pi, is it?

Diameter = 2 x Radius (I think?)

so 2 Pi r is the same as pi D (I hope)

Yep, I'm wrong! 2piR sounds right (and the same as piD)

But 2 pi r is 2 x pi, then x r. So not same as pi d? Am I deranged?

Pi d would be 2 r pi, wouldn't it?

Is 2 x 3 x 4 the same as 4 x 3 x 2?

With mulitiplication the order doesn't really matter

I think it does in division

Maybe?

Argh. Hang on. Say pi is 3 (for sake or arg) and r is 5, then pi d is 30 and so is 2pi r. God, I'm tired and dim. So why was I taught 2 pi r? Why not just pi d?

Because they are so similar
2 pi r
&
pi r (squared....a little 2 in the air)

So it's easier to learn them

D'you reckon? I think pi d is easier. Bah. Would you believe I have maths a level?

And pi d makes it easier not to mix them up. Damn you, Mr Jefferson. Calculus I can do, apparently, but not timeses and percentages...

circumference is (2 pi r) so radius is the length from waist band to floor. allow 1m for radius, so buy 6m.

Me too

But I was thinking some weird "craft" way of working it out rather than maths

I bought 10 black satin ribbon will always come in

btw, pi x d = 2 pi r as 2 r =d. and order in multiplication doesn't matter

and ten Will be loads :)

just to be really awkward - is it really a full circle - or is it more (some of those skirts are)?

TBH stealth I haven't a clue; it's a prom dress we have added godets too (should that be to or too? not sure)

So it could be anything

I hadn't thought of that

Yes yes yes, I've already owned up to maths idiocy, madame. No need to rub it in. (tetchy) (dim)