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The tarp size your load or stack actually needs
A tarp is bought flat and used draped: the flat width must cover the load and hang down both sides. Enter your load, get the arithmetic, and see which published formats cover it. For a stack of round bales, it gives the width that reaches the widest span of the bottom row, next to four vendor charts for the same stack. The tool never invents a format: when nothing published fits, it says custom, and the dimension sheet goes on the quote.
Your loadYour stack
The drop must reach at least this far down to protect the bottom of the load.
Measure a bale: the vendor charts assume the true bale size, not the baler setting.
Arithmetic on your numbers, published formats from dated US listings, nothing else.
What the job needs
Fill in the three dimensions to see the result.
Published formats against this need
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How the arithmetic works, in the open
Flat width needed = load width + 2 x drop, where the drop is at least your load height. The length must cover the load; on lumber tarps the end flap covers the front face, which is why published lumber formats state their flap separately. That is the whole formula; there is no hidden coefficient.
The flatbed tarp size guide explains how overall width, drop and end flaps differ, with a worked example and links to the published lumber and steel formats.
Coils are the exception: a coil tarp is shaped to the coil, so the published cube format matters less than your coil dimensions (width, diameter). State them on the quote and the dimension sheet does the rest.
Hay stacks: one published rule, and charts that disagree
Extension work on round bales stacked in a pyramid gives one rule and no figure: "The tarp should cover the widest span of the bottom tier of bales." (N1 · Ohio State University Extension, Hay Storage Considerations, factsheet 21-96 (2021)) On a round bale, the widest span is at mid-height. One vendor says the same in plain terms: hay tarps should only reach about halfway to the bottom of the bale on the side of the stack, not all the way to the ground. (N2 · BTL Liners, hay tarp sizes guide, read 2026-10-01)
The tool follows that rule on a tight pyramid of equal bales: the tarp runs up both flanks of the stack, each (n - 1) bale diameters long, n being the bales on the bottom row, and around three arcs, the two corner bales and the top one, which add up to half a bale's circumference. Width = D x (π/2 + 2 x (n - 1)), from mid-height to mid-height. Real stacks settle and bales flatten, so the distance measured over your own stack decides.
Four vendor charts give a width for the same stacks, and they disagree: the tool shows each one next to the arithmetic and never averages them. Length: measure the stack and add 2 ft, so the tarp folds over the stack ends. (N2 · Inland Tarp and Liner round bale selection guides and Celina Enterprises hay tarp calculator, read 2026-10-01) Listed hay tarp sizes are cut sizes: the finished tarp is about 6 inches smaller once hemmed, 3 to 5 percent at one vendor and up to 3 percent at another. (N2 · Tarp Supply and TarpsPlus listings, read 2026-10-01)
Vendor charts, read 2026-10-01: Inland Tarp and Liner, Bull Flex; Inland Tarp and Liner, Performer; Carolina Tarps; Celina Enterprises. Named without a link: they are direct competitors.