Roll width is the whole story
Every consumable in a vacuum bag is sold as a roll of fixed width, and the arithmetic that matters is not area but how the part width divides into that width. A 30 inch part off a 60 inch roll is one strip. A 62 inch part off the same roll is two strips plus an overlap, and the run length doubles even though the area only went up by a factor of two. Cross a roll width and the cost per square foot steps rather than sloping.
That is why this page counts strips first and area second. It also explains the odd result where a longer, narrower part is cheaper to bag than a shorter, wider one of the same area — the narrow one fits the roll and the wide one does not.
The bag is bigger than the part in three directions
Bagging film has to reach across the part, out to the seal line, down over whatever the tool sticks up, and then far enough past the tape to give you something to hold while you press it down. In the default form that turns a 96 by 30 inch part into a 124 by 58 inch sheet of film, and with the four corner pleats counted the bag comes to 51.9 square feet for a 20 square foot part. Nobody who has not laid one down guesses that number correctly.
Pleats add to it again, and they are not optional on anything with a corner in it. Film has no stretch worth counting on, so a bag pulled down over a step either has a pleat in it or bridges the step and leaves the laminate unpressed at exactly the point that most needed pressing. Each pleat here consumes film equal to twice its depth along its length, and two runs of tacky tape down its legs.
Breather runs wider than the laminate
The breather is not just a spacer, it is the path air takes to the port, and a breather that stops at the edge of the laminate can get pinched shut where the film comes down onto the tool. Running it a couple of inches past the part edge keeps the path open. That is what the extension field does, and on a long narrow part it can be what tips the take-off from one strip into two, so it is worth entering rather than ignoring.
Layers are a separate field because the number of breather plies is a decision about the job, not a constant. One is common on a thin flat panel and more get used where there is resin to take up or a long path to the port. The page multiplies the run length by whatever you enter.
What the count deliberately does not do
It counts one part at a time. If you are cutting six identical parts out of one run of film, the nesting will beat this number, sometimes by a lot, because the offcut at the end of each strip gets used by the next part. Treat the roll counts as the pessimistic end of the range and the run lengths as the number to nest against.
And it counts nothing you did not enter. A bag that ended up with an extra pleat, a second port, a patch over a pinhole or a strip of tape across a wrinkle consumed material that is not on this list, because the page has no way to know about it. Consumables on a real job run over the take-off more often than under it.
Questions people ask
How much bagging film does a part actually need?
Far more than the part area. For the 96 by 30 inch part in the form, with a 4 inch seal setback, a 6 inch drop over the tool and a 4 inch flap outside the tape, the flat film comes to 124 by 58 inches — just under 50 square feet for a 20 square foot part. Add four corner pleats at 3 inches deep by 12 inches long and it is another 2, so 51.9 in total. A 60 inch roll covers 58 inches in one strip with two inches to spare, which is closer than it sounds.
When does the bag need a taped seam in it?
Whenever the flat film is wider than the roll. In the default case it is 58 inches against a 60 inch roll, so one strip does it and no seam appears. Widen the part by three inches, or move the seal line out another two, and the film goes past 60 and has to be made from two strips joined with tape — the page then adds a seam line and puts its length into the tacky tape total. It is a normal thing to do and also a normal place for a leak, which is why it gets its own line rather than being buried in the area.
How much tacky tape does one bag take?
For the default part the seal line is 104 by 38 inches, which is 23.7 feet around, and four pleats at 3 inches deep add another 2 feet of legs, for 25.7 feet. That is one 36 foot roll with ten feet left, and the margin disappears the moment the bag needs a taped seam or the seal line moves out. Tacky tape is the consumable people most often come up short on, because the seal line is always bigger than the part and the pleats are always forgotten.
Do I need perforated release film as well as peel ply?
That is a decision about the job and the page treats it as one — there is a yes or no field and it changes the take-off, not a recommendation. Some layups use peel ply alone with the breather straight onto it, some put a perforated film between so the breather takes a controlled amount of resin rather than all of it. Which one you are doing depends on the resin, the fabric and what you want the surface to look like afterwards.
Why is a part slightly wider than my roll so much more expensive to bag?
Because strips are whole numbers. A 58 inch part off a 60 inch roll is one strip; a 62 inch part is two, and the second strip runs the whole length even though it only covers a few inches. On top of that the overlap between strips is consumed twice. The cost per square foot of a bagged part steps up at every roll width, and knowing where those steps sit is often worth more than shaving the area.
Should I trust the roll counts for a production run?
Treat them as the high end. The page counts each part on its own, which is what happens when parts are bagged on separate days and the tail of the roll goes in the bin. If you nest several parts down one run of film the real consumption is lower. The run lengths in yards, rather than the roll counts, are the numbers to nest against.