How can I determine the volume a bag will hold given its flat dimensions? Say a sandbag 20 x 36"? Is there a formula to estimate volumes of flat bags?
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Re: flat bag volume
This seems to be a good time to mention a new calculator I added to the site. It calculates the volume of a rectangular object.
http://www.onlineconversion.com/object_volume_box.htm
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what is the formula for calculating polythene bag volume
[QUOTE=Robert Fogt]what is the formula for calculating polythene bag volume and the conversion of area to volume
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Re: flat bag volume
With all due respect, Robert Fogt does not seem to have the imagination needed to generate a formula to find the volume of a flat bag. Perhaps he never put a sandwich in a baggie. We know you need 3 dimensions to generate a volume. But flat bags are defined by 2 dimensions only. Nonetheless, they get filled all the time. If it was easy, I’d do it myself, but the math gets pretty complex. Let’s start by defining terms. Bags are usually given as Length (L) by Width (W), with the opening on the W side. That makes the circumference (C) of the opening 2W. The diameter of that opening, 2W/Pi, would be the maximum depth of our flat bag. We could use L x 2W/Pi, but both ends are flat. I suspect the formula would be something like (L/W x constant (K)) x 2W/Pi x L. We could, I suppose, fill up a bunch of various sizes of bags and see if a pattern emerges, but a mathematical formula seems much cleaner, don’t you think? Any thoughts? Anyone?
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Re: flat bag volume
I hate being wrong, but have to admit it when I am. My first shot at a formula to estimate the volume of a flat bag seriously UNDERestimated the actual values I was able to obtain experimentally. I still think it should be close for very long bags, but for bags where the length and width are at all close, it's not too good. I'm still open to suggestions. Does anyone know people who work with flat bags? They must know something we don't...
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Re: flat bag volume
I derived a formula based on drcruzr's idea of cylinder approximation. Two assumptions:
1. Circumference of cylinder = 2w and
2. Area of cylinder = area of bag. This will overestimate the actual volume of the bag since some of the bag material is wasted due to crumpling at the corners.
Volume = Wsquared/Pi * (L  W/Pi)
instead of...
V = ((Wsquared/Pi) x (LW)) + (1/2 or 1/4 or 1/8 x Wcubed/Pi).
I found an article on wikipedia about what's called the "paper bag problem." It gives an approximate formula. I have a 29 x 17 cm bag. (drum roll) Here are the results.
Cylinder approx: 2.17 L
Wikipedia formula: 1.98 L
Actual volume of bag: 2.15 +/ 0.05 L
I'll be doing this experiment again to confirm this. I use bags at work that are made out of a plastic that does not stretch significantly. Also, I sometimes use square bags which presumably would not follow this formula as well.
SK
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Re: flat bag volume
This is an interesting question. After reading everyone's answers and thinking of bags of seed, I would say a bag would have the cylinder shape except for the top inch and bottom inch for a 50 pound bag of seed.
You would really have to pack in the sand to get the cylinder shape and you may not get the top sewed shut.
When the bag is laying on it's side, it's base area appears as an oval excluding the top and bottom inch.
RB
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Re: flat bag volume
Originally posted by UnregisteredI derived a formula based on drcruzr's idea of cylinder approximation. Two assumptions:
1. Circumference of cylinder = 2w and
2. Area of cylinder = area of bag. This will overestimate the actual volume of the bag since some of the bag material is wasted due to crumpling at the corners.
Volume = Wsquared/Pi * (L  W/Pi)
instead of...
V = ((Wsquared/Pi) x (LW)) + (1/2 or 1/4 or 1/8 x Wcubed/Pi).
I found an article on wikipedia about what's called the "paper bag problem." It gives an approximate formula. I have a 29 x 17 cm bag. (drum roll) Here are the results.
Cylinder approx: 2.17 L
Wikipedia formula: 1.98 L
Actual volume of bag: 2.15 +/ 0.05 L
I'll be doing this experiment again to confirm this. I use bags at work that are made out of a plastic that does not stretch significantly. Also, I sometimes use square bags which presumably would not follow this formula as well.
SK
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Re: flat bag volume
Originally posted by UnregisteredCan you post how you got the Wiki equation (en[dot]wikipedia[dot]org/wiki/Paper_bag_problem)to work? I keep getting 2319.0673136863, and I don't know if that's cubic liters or what!
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