Article — Hoop House Calculator
Hoop house calculator: pipe length, plastic film, and interior volume
A hoop house calculator takes length, base width, peak height, and hoop spacing, and returns the arc radius, the length of pipe per hoop, the total pipe needed, the plastic film area with overlap, the foundation perimeter, and the interior volume. For a 20 ft long, 12 ft wide, 6.5 ft tall tunnel with 4 ft hoop spacing: 6 hoops, 18.3 ft of pipe per hoop, 366 ft² of arc plastic plus 128 ft² of end walls, and 1,280 ft³ of interior space.
Hoop houses (also called high tunnels) extend the growing season 4 to 6 weeks on each end, and let cold-hardy crops keep growing through winter in mild climates. The build is straightforward — PVC or EMT pipe bent into arcs, plastic film stretched over the top, end walls framed in lumber. The math is geometry: arcs, chord lengths, and surface areas. Get the geometry right and you order the right amount of material on the first trip to the hardware store.
Hoop house arc geometry
The hoop is a circular arc spanning the base width with a peak height. Given chord W (base) and sagitta H (peak), the radius is R = (W² + 4H²) / (8H). The central angle is θ = 2 arcsin(W / 2R). Arc length per hoop is R × θ in radians.
R = (W² + 4H²) / (8H) arc radiusθ = 2 arcsin(W / 2R) central angleL_arc = R × θ per hoop, radN = floor(L / s) + 1 hoops by spacingFor typical residential geometry (12 ft base, 6.5 ft peak), the radius is about 6 ft and the arc length per hoop is about 18 ft — a single 20-foot pipe with a small offcut. Wider tunnels with low peaks need much larger radii and longer arcs; narrow tunnels with tall peaks approach a semicircle (arc length ≈ π × W / 2).
Hoop spacing and snow load
The standard spacing is 4 ft (1.2 m) on center. Closer spacing of 3 ft is mandatory north of the snow line because each hoop carries less snow load that way. Wider spacing of 5 to 6 ft saves material in mild climates but the plastic billows between hoops in wind and tears at the attachment points.
Engineering rule of thumb: a 1.5-inch Schedule 40 PVC hoop at 4 ft spacing carries about 15 lb/ft² of vertical load — enough for 6 to 8 inches of wet snow. Sites with regular heavy snow need 3 ft spacing, 1.5-inch EMT (electrical conduit, much stiffer than PVC), and end-wall diagonals to resist wind.
PVC vs EMT for hoop house frames
PVC is the cheap option: 1.5-inch Schedule 40 pipe costs $2 to $3 per linear foot, bends by hand into a 6 ft arc, and is easy to drill for attachment. It lasts 5 to 10 years before UV degradation makes it brittle. PVC works well for tunnels under 14 ft wide in mild climates.
The USDA NRCS High Tunnel System Initiative has cost-shared more than 25,000 high tunnels since 2009 because the research showed dramatic gains in crop quality and growing season for vegetable producers. A typical 30 × 96 ft commercial high tunnel can produce $30,000 to $50,000 of vegetables per year on an 80-day spring-to-fall season — numbers a hoop house at residential scale tracks proportionally.
EMT (electrical metallic tubing) is the better-engineered option: 1.5-inch EMT costs $4 to $6 per linear foot, requires a pipe bender (a $50 hand tool or a $300 pneumatic bender), and lasts 20+ years. EMT is the right choice for tunnels over 14 ft wide, sites with heavy snow, and any structure intended as a multi-decade investment.
Plastic film grades for hoop houses
Plastic film is the recurring cost in hoop house ownership. Construction-grade 4 mil polyethylene from the hardware store lasts one growing season because UV breaks it down quickly — not worth the labor to install for anything but emergency cold-snap covers.
Greenhouse-grade 6 mil UV-stabilised film lasts 3 to 4 years and costs $0.10 to $0.15 per ft². The IR/AC version adds an infrared-reflective layer that holds 10 to 15% more heat overnight — meaningful for late-fall and early-spring growing. Top-grade 8 mil woven film lasts 10+ years but costs $0.40 per ft² and is harder to stretch tight without specialty tooling.
End walls and ventilation
End walls take more material than the casual builder expects. Each end of a hoop house has a curved top section (the arc above the chord), framed-in vertical sections for doors and vents, and plastic film with overlap allowance. For a 12 × 6.5 ft hoop, each end wall is about 64 ft² of plastic plus 25 to 35 ft of 2×4 framing lumber.
- Curved-top section area = 0.5 × R² × (θ − sin θ) per end wall
- Roll-up sides = 2 ft tall ventilation along both long sides, hand-cranked
- End-wall door = standard 3 ft wide for tools, 6 ft wide for tractor access
- Gable vent = at least 5% of floor area for cross-ventilation
- Diagonal braces = 2 per end wall, prevents twist under wind load
- Baseboard = 2×6 pressure-treated lumber bolted to ground stakes
Anchoring the hoop house frame
An unanchored hoop house becomes airborne in the first windstorm. Two anchoring methods are common. Ground posts: drive 4-foot lengths of rebar 18 inches into the ground at each hoop location, slip PVC over the exposed rebar end. Baseboards: bolt the hoop ends to a 2×6 pressure-treated wood frame that is staked or buried.
For semi-permanent installs, pour 12-inch by 24-inch concrete piers at each hoop location and embed a rebar stub for the hoop to slip over. The piers add $50 to $100 in concrete cost but turn a temporary structure into one that survives 60 mph winds. USDA NRCS cost-share programs require this level of anchoring for tunnels they fund.
Season extension with a hoop house
A single-layer 6 mil hoop house runs 15 to 30 °F warmer than outside during the day, even in winter sun. Overnight, the temperature inside drops to within 2 to 5 °F of the outside low. That gives 4 to 6 weeks of season extension on each end in temperate zones — tomatoes go in mid-April instead of mid-May, and harvest runs until early November instead of mid-September.
Double-cover (hoop house plus row cover floating over the crops) adds another 5 to 10 °F overnight. In USDA zones 5 to 7, this is enough to harvest kale, spinach, lettuce, carrots, and arugula through January. Above zone 4, even double-cover does not prevent freezing on the coldest nights, but the structure protects against wind damage and lets the soil thaw quickly in spring.
Hoop house material costs
A 12 × 20 ft (240 ft²) hoop house with PVC frame and 6 mil greenhouse film costs roughly $600 to $1,000 in materials: $200 for PVC pipe, $100 for ground anchors, $80 for greenhouse film, $200 for end-wall lumber and hardware, $100 for clips and batten boards.
Adding electric or propane heat to a single-layer hoop house wastes most of the heat to the outside. The plastic film has a U-value around 6.5 W/m²K — about 5× worse than a single-pane window. Heating makes sense only with double-walled inflated film (U ≈ 4) and only when growing high-value crops like seedling transplants. For season extension, the passive solar gain alone does the work.
Going to EMT frame and 8 mil woven film doubles the cost but quadruples the lifespan, dropping the per-year cost. Spreadsheet the math over 10 years before committing to PVC-and-cheap-film for a permanent install.