Hoop House Calculator estimates film size, hoop pipes, base boards, purlins, volume, and fan CFM using film area = film width × film length for a semicircle hoop house with overhang and buying buffer.
Bows, Film, and Pipe: What Actually Goes Into the Numbers
Most first-time hoop house builders get the plastic wrong — not the type, but the quantity. They measure the arc over one bow, multiply by the length of the tunnel, and end up buying a roll that’s four feet too narrow and ten feet too short to wrap both end walls. The film ends at the last hoop with nothing left to bury. This calculator works out all five material categories at once, from the arc length of each individual bow down to the CFM rating of the exhaust fan you’ll need to ventilate the finished structure.
The inputs are deliberately minimal: the footprint of the structure (length and width), how far apart you’re spacing the bows, how much plastic you want hanging past the last hoop on each side, and how many lengthwise purlins you’re running. Everything else is derived geometry.
How Each Output Is Calculated
Hoop Count and Arc Length
The calculator assumes a pure semi-circle cross-section. The radius is half the structure width, and the arc length of each bow is simply π times that radius — the half-circumference of a circle with that diameter. For a 10-foot-wide tunnel, each bow needs π × 5 = 15.71 feet of pipe to bend.
Hoop count uses a ceiling function on the total length divided by spacing, then adds one. The reason for that +1 is structural, not arithmetic: you need a bow at both the front and back of the tunnel. Spacing defines the distance between bows, and bays are always one fewer than bows. A 20-foot tunnel at 4-foot spacing has 5 bays — but 6 bows. The code reflects this exactly as ceil(L / S) + 1.
The Film Sheet
Two dimensions define the minimum sheet size. The width of the sheet runs up one side, over the peak, and down the other — that’s the arc length of one hoop — plus the overhang allowance on each side for tucking or burying into a baseplate channel. Film width = π(W/2) + 2 × Overhang.
The length of the sheet is where most people’s intuition breaks down. It’s not just the tunnel length. The calculation is: L + W + 2 × Overhang. The structure width appears here because this is the over-the-top pull-down method — a single piece of film covers both end walls by being pulled down over them. Since the tunnel height equals W/2, covering both ends requires 2 × (W/2) = W of additional film. Add the overhang allowance for both ends and you have the full sheet length. A 20 × 10 tunnel with 3 ft of overhang needs 20 + 10 + 6 = 36 feet of film, not 20.
Area is the product of those two dimensions, and the “Recommended Buy” figure adds a 10% buffer automatically.
Base Framing and Purlins
Base perimeter is straightforward: 2L + 2W — the lumber or ground stakes that anchor the hoop feet around the full footprint. Purlin pipe is the purlin count you select multiplied by the tunnel length. One center ridge purlin on a 20-foot tunnel needs 20 feet of pipe. Three purlins (center plus two side runs) need 60 feet. Five need 100 feet. The structural total shown is these two figures combined.
Changing the purlin setting does not change the hoop count, the arc length, or any of the film dimensions. It only affects how much lengthwise pipe you’ll need to order.
Volume and Ventilation
Internal air volume is calculated as a semi-cylinder: ½ × π × r² × L. The exhaust fan figure targets one complete air exchange per minute — a standard rule of thumb for passively managed high tunnels. In US customary mode, the fan capacity in CFM equals the volume in cubic feet directly (1 cu ft/min per cu ft of space). In metric mode, the output converts to cubic meters per hour (CMH), which is the common metric fan rating unit, by multiplying by 60.
The End-Wall Trap: Why the Width Shows Up in the Length Formula
This is the single most counterintuitive result the calculator produces, and it catches experienced builders who have ordered film for smaller structures before. On a 30-foot-wide commercial tunnel, the end walls are 15 feet tall. Covering both of them with the pull-down method adds a full 30 feet to the sheet length before you even account for overhang. Order a 100-foot roll for a 96-foot tunnel and you’ll be stapling scraps to the end walls. The correct length for that same structure with 3 feet of overhang is 96 + 30 + 6 = 132 feet.
If you’re using separate end-wall panels rather than the over-the-top method, this calculator’s film length will overestimate what you need for the main body — you would subtract W from the length figure. The tool’s hero output explicitly labels the sheet dimensions as the “over-the-top pull-down” method, so read that note before placing a film order on a structure where the ends are handled separately.
Worked Example: 48 × 14 Market Garden Tunnel
A grower in zone 6 wants a single-bay high tunnel for extending the tomato season. The footprint is 48 feet long by 14 feet wide, bows at 4-foot centers, 3 feet of overhang on all sides, and a 3-purlin setup for the center ridge plus two side stabilizers.
Radius is 7 feet. Arc length per bow: π × 7 = 21.99 ft, rounded to 22 feet. Bow count: ceil(48 ÷ 4) + 1 = 12 + 1 = 13 bows. Total hoop pipe: 13 × 22 = 286 feet.
Film width: 22 + (2 × 3) = 28 feet. Film length: 48 + 14 + (2 × 3) = 68 feet. Sheet size: 28 × 68. Film area: 1,904 sq ft, with the recommended buy quantity coming out to 2,094 sq ft after the 10% buffer.
Base perimeter: (2 × 48) + (2 × 14) = 124 feet of baseboard lumber. Purlin pipe: 3 × 48 = 144 feet. Structural total: 268 feet. Interior volume: ½ × π × 49 × 48 = 3,694 cu ft. Ventilation target: 3,694 CFM — a single well-sized roll-up side and two 48-inch circulation fans covers this comfortably for passive-assisted ventilation.
Frequently Asked Questions
Why does the calculator add one extra hoop beyond what the spacing math suggests?
Spacing defines the gap between bows, and gaps are always one fewer than bows. A 20-foot tunnel at 4-foot spacing creates 5 spaces — but those 5 spaces are framed by 6 bows, one at each end and four in between. Omit that +1 and you build the last bay without a bow to anchor it. The formula ceil(L ÷ S) + 1 is correct for this reason.
The overhang input changes both the film width and the film length. Is that right?
Yes. The overhang value applies twice in each direction. For width, it adds to each side of the arc for ground burial or baseplate clamping. For length, it adds to each end of the tunnel so there’s material to pull down and secure at the front and rear. Setting overhang to 3 feet adds 6 feet to both the width and the length of the required sheet. Setting it to zero gives you the absolute minimum bare coverage with nothing to secure — fine for planning rough estimates, not for actual construction.
I changed the purlin count and the film and hoop numbers didn’t change. Why?
Purlins are longitudinal framing members — they run the length of the tunnel and stabilize the bows, but they don’t change the cross-sectional geometry or the number of bows required. The purlin dropdown only controls how much lengthwise pipe goes into the “Base & Purlin Framing” total. Switching from 1 to 3 purlins on a 48-foot tunnel adds 2 × 48 = 96 feet to your pipe order. It has no effect on hoop count, arc length, or film dimensions.
What happens to the ventilation figure when I switch to metric?
In US mode, the fan capacity in CFM equals the interior volume in cubic feet — because 1 air exchange per minute means moving that many cubic feet every minute. In metric mode, the interior volume is in cubic meters and the fan rating convention changes to cubic meters per hour (CMH), which is the standard metric unit on most fan spec sheets. The tool multiplies the metric volume by 60 to convert from per-minute to per-hour. A 36 cu m tunnel needs a 2,160 CMH fan, not a 36 CMH fan.
My structure has straight sidewalls before the arch starts. Will these numbers be correct?
No — and the tool’s own alert note says as much. The formulas assume the cross-section is a perfect semi-circle starting at ground level. A gothic arch profile or a structure with vertical knee walls before the curve begins has a longer arc per bow and needs more plastic per unit of width. If your design includes knee walls, treat the arc-only portion separately and add the straight sidewall material on top of what the calculator gives you.