Hoop House Calculator
Enter your hoop house dimensions, choose a structure type and covering material, and this calculator instantly returns the total surface area to cover, the heat energy you need to keep it warm on the coldest night, the heater capacity required, and the material cost. Works for Quonset tunnels, arched-roof greenhouses, gabled greenhouses, and lean-to structures, in both metric and imperial units.
What is a hoop house?
A hoop house (also called a poly tunnel, caterpillar tunnel, or hoophouse) is an unheated or minimally heated structure built from a series of curved metal or PVC pipes arched over a growing bed and covered with a layer of greenhouse-grade plastic film. Unlike a full greenhouse, most hoop houses rely on the passive heat absorbed from sunlight and the insulating effect of the covering to extend the growing season by four to eight weeks at each end. They are used by market gardeners, homesteaders, and small farms to grow tender crops in cooler weather, overwinter hardy greens, or start seedlings earlier than outdoor conditions would allow.
How to use this calculator
Select your preferred units (metric or imperial) and structure type. Enter the width (the span from one base rail to the other), the total length, and - for all types except a pure Quonset - the sidewall height and gable or arch rise. Then choose your covering material, enter the coldest expected outside temperature, the minimum inside growing temperature you want to maintain, and your heater efficiency. The calculator returns the total surface area to order, the number of hoops, the conductive heat loss through the skin at those design temperatures, the heater output needed, and the material cost if you enter a price per unit area. The heat chart shows how heater demand changes across a range of outside temperatures.
How hoop house surface area is calculated
Each structure type uses a different geometric model. A Quonset hoop house has a semicircular cross-section, so one arch equals half a circumference (PI times the radius). The total roof area is that arc length multiplied by the tunnel length, and the two end walls are together equal to one full circle of the same radius. An arched-roof structure adds two rectangular sidewalls and rectangular end-wall panels below the arch. A gabled roof uses the Pythagorean theorem to find the slope length from half-width and peak rise, then multiplies by length for each of the two roof panes, plus triangular gable ends and sidewalls. A lean-to uses the same slope calculation on the full width and adds three wall panels. All areas are summed to give the total square footage you must order.
Heating and heat-loss calculations
The standard formula for conductive heat loss through a building envelope is Q = U x A x Delta-T, where U is the thermal transmittance of your covering material in watts per square metre per degree Celsius (W per m²K), A is the total surface area in square metres, and Delta-T is the difference between the inside and outside temperatures in Celsius. The result is the rate of heat loss in watts. To find the heater output needed, divide that figure by the heater efficiency (for example, 0.80 for 80%). Keep in mind this formula covers only conductive losses through the skin; ventilation, infiltration through unsealed edges, and ground losses are additional. A common rule of thumb is to add 10-20% to the calculated capacity as a safety margin.
Common covering material U-values
| Material | U-value (W/m²K) | Relative insulation |
|---|---|---|
| 6 mil polyethylene (single) | 6.81 | Low |
| 4 mil polyethylene (single) | 7.50 | Very low |
| Inflated double-layer poly | 4.00 | Moderate |
| Woven polyethylene (11 mil) | 5.20 | Low |
| 4 mm polycarbonate twin-wall | 4.10 | Moderate |
| 6 mm polycarbonate twin-wall | 3.50 | Good |
| 8 mm polycarbonate twin-wall | 3.00 | Good |
| Glass - single pane | 5.80 | Low |
| Glass - double pane | 3.00 | Good |
| Acrylic panels | 4.50 | Moderate |
| Fiberglass panels | 4.00 | Moderate |
U-value (W per m² per K): lower = better insulation, less heat loss in winter.
Frequently asked questions
What is the difference between a Quonset and an arched-roof hoop house?
A Quonset (or Gothic arch) hoop house has a semicircular cross-section that goes all the way to the ground - there are no vertical sidewalls. An arched-roof hoop house has vertical sidewalls of a set height and the arch begins at the top of those walls. The sidewalls give you more usable growing space near the edges and make it easier to attach roll-up or zippered side vents, but they add surface area and therefore both material cost and heat loss.
How much plastic covering do I need for a hoop house?
Use the surface area this calculator returns and add 10-15% for overlaps, ground burial, and waste. For a typical 14 ft x 24 ft Quonset hoop house the total arc area (roof plus two end caps) is roughly 400-450 square feet. Order plastic in widths that match or exceed the arc length of a single hoop so you have a single-piece run from ground to ground along the length.
What size heater do I need for a hoop house?
The heater capacity depends on three things: the total surface area of your covering, the U-value (heat conductance) of the material, and the temperature difference you need to maintain. Use the formula Q = U x A x DeltaT to get the heat loss rate, then divide by your heater efficiency. For a 14 x 24 ft single-layer polyethylene hoop house with a 40-degree design delta, expect roughly 20-35 kW (70,000-120,000 BTU/h). Actual propane heaters in that range are widely available for row-crop greenhouses.
Does this calculator account for infiltration and wind?
No - it calculates only conductive heat loss through the covering (the dominant factor for well-sealed structures). Infiltration through edges, ground-contact losses, and wind-driven penetration can add 10-25% to the total. A common practice is to size your heater at 1.25 times the calculated output to cover those unknowns.
How far apart should hoop house hoops be spaced?
Most market gardeners use 4-foot (1.2 m) spacing for PVC pipe hoops and 6-foot (1.8 m) spacing for heavier metal conduit or galvanized electrical metallic tubing. Shorter spacing resists snow load better and is recommended in climates with significant snowfall. Longer spacing reduces material and cost for mild climates where wind is the main concern.
What is the best covering material for a cold-climate hoop house?
For maximum heat retention, an inflated double-layer polyethylene system or 6-8 mm twin-wall polycarbonate gives the lowest U-values in a flexible or semi-rigid covering. The air gap in both systems acts as an insulating barrier. Single-layer 6-mil poly is the most popular and affordable option and works well with supplemental heat, but a double layer can cut heating costs by 30-50%.