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# Geometry/Calculating a sprial's primiter (i.e. rolled carpet)

Tom wrote at 2008-12-17 16:45:18
Hey Dan,

The way I do it, seems to work pretty well within a foot or two which is all I need.

Here's how I do it;

1) measure the in side diameter of the roll.  For figuring purposes say it is 4"

2) Then measure the rough outside diameter given compression and let's say it is 24"

3) Subtract the interior diameter from the exterior and that gives us 20" or  the average layer diameter.

4) Multiply that by Pi = 3.14 and that give us 62.8" or the average length of each layer.

5) Multiply that by the number of turns, or layers, and that will give you the approximate length.  Say there are 12 turns x 62.8" = 753.6" or 62.8' of length.

This is a quick way that I have found is pretty accurate.  Hope this makes sense.

Thanks

Tom

Mike wrote at 2012-11-16 02:50:24
That is awesome, I once worked in a carpet warehouse where I developed a similar formula that got me "close" to the length of carpet on a rolled tube. I was sitting here thinking: "I wonder if anyone has an actual formula for that thing I was trying to figure out years ago as a young teen" lol. Nice work, I also enjoy strange but useful math.

Geometry

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