Hoop House Calculator
Size a hoop house or high tunnel: covered area, volume, heat loss and heater output — plus the hoops, pipe and greenhouse film it takes to build it.
Give it the shape, then the climate
Width, length and height set the structure and the glazing; the covering and the design night temperature set the heater.
How the hoop house calculator works
A hoop house calculator determines the materials and dimensions needed to build a hoop house. Enter the structure’s length, width, hoop spacing, and hoop dimensions to calculate the number of hoops and covering requirements. Material needs vary by hoop spacing, greenhouse size, frame design, and covering dimensions.
Those same measurements turn into everything else. Width, length and peak height fix the arc of a single hoop; the arc fixes the glazing area, the pipe, the plastic and — with the covering and the design night temperature — the heater.
The arc is where estimates go wrong, because it is always longer than people expect. A 20 ft wide half-round tunnel is 31 ft 5 in over the top, so its curved skin is more than half again its footprint. That skin is the surface losing heat all winter, which is why a modest change in covering does more for the heating bill than any change in shape.
Width and peak height give the radius, and the radius gives the arc over the top.
Arc × length is the curved glazing; the two end walls close it in and the arch area × length is the air.
U-factor and the temperature difference turn area into BTU per hour, and volume into infiltration.
The formulas involved
The hoop arc
R = w²/8h + h/2 · arc = 2Rθ
A circular arch through a span w and a rise h. For a half-round house it collapses to the familiar arc = πw/2.
Glazed surface
SA = arc × L + 2 × Aarch
The curved skin ground to ground, plus both end walls — the area that loses heat.
Conduction loss
Qc = SA × U × TD
Heat through the covering, in BTU per hour. TD is the night temperature you hold minus the design outside temperature.
Infiltration loss
Qi = 0.02 × V × C × TD
Heat carried out by air leaks, where C is air changes per hour. The heater has to cover both.
Why the covering matters more than the shape: conduction is usually 80% or more of the total, and it scales directly with the U-factor. Going from single poly at 1.15 to inflated double poly at 0.7 cuts the conduction loss by 39% without moving a single hoop.
Method reference: the surface-area, heat-loss and heater-sizing sequence follows Omni Calculator's Hoop House Calculator.
Where the heat goes
U-factors and what they cost
The U-factor is the heat that passes through a square foot of covering for every degree of difference, every hour. Conduction loss for a 20 × 48 ft half-round tunnel — 1,822 ft² of glazing — at a 50 °F temperature difference:
| Covering | U-factor | Loss at TD 50 °F | Notes |
|---|---|---|---|
| Single layer — poly, glass or polycarbonate | 1.15 | 104,800 BTU/hr | The default hoop house; cheapest to build, dearest to heat. |
| Double polyethylene, inflated | 0.70 | 63,800 BTU/hr | An air blower and a second layer cut the loss by 39%. |
| Twin-wall polycarbonate or acrylic | 0.60 | 54,700 BTU/hr | Rigid, long-lived, and usually reserved for end walls on a tunnel. |
Infiltration is on top of this: at one air change per hour the same house leaks about another 7,500 BTU/hr. Air changes run from 0.5 to 1 in a new house up to 2 or 3 in a poorly maintained one — and an exposed site adds 10% to 15% again.
Heat loss reference: the U-factors, the infiltration formula and the air-change ranges are from John W. Bartok Jr., emeritus extension professor at the University of Connecticut, in Determining greenhouse heat loss.
The frame
Spacing, pipe and purlins
A hoop house is a structure before it is a greenhouse. These are the figures extension engineers give for a tunnel that has to carry snow and stand up to wind.
| Tunnel width | Hoop spacing | Frame pipe | Purlins | Ground posts |
|---|---|---|---|---|
| Under 26 ft | 4 ft on center or tighter | 1.66 in × 14 gauge | 3 minimum | Driven 24–30 in deep |
| 26 ft and wider | 4 ft on center or tighter | 1.90 in × 14 gauge | 5 minimum | Driven 24–30 in deep |
| Any width | 4 ft on center or tighter | 2 in square × 16 gauge | as above | Extra anchors ~10 ft apart in wind |
Snow country: where a tunnel stays covered through winter, a 2×4 post under the ridge every third or fourth hoop keeps the frame from folding under a wet snow load. Diagonal corner bracing does the same job against racking — an 80 mph wind pushes on the skin at roughly 16 pounds per square foot.
Frame reference: UConn Integrated Pest Management, Engineering High Tunnels for Better Performance; ground post depth and hoop assembly also in West Virginia University's Design and Construction of High Tunnels.
The other half of the climate
Cooling is the harder problem
Growers size heaters carefully and then lose the crop to a 105 °F afternoon in March. A hoop house is a solar collector with no thermal mass to speak of, and on a clear day it gains heat far faster than it loses it.
8 CFM per ft²
Summer exhaust fan capacity is eight times the floor area — 7,680 CFM for a 20 × 48 ft house.
15–20% for vents
Natural ventilation wants roof and sidewall openings each at 15% to 20% of the floor area — which is what roll-up sides deliver.
2 CFM per ft²
Horizontal air flow fans for winter circulation are sized at twice the floor area, and they earn their keep against condensation.
Roll-up sides pay twice: they ventilate without electricity and they let the house run through shoulder seasons unattended. On tunnels without them, a single sunny day in February can cook a crop that survived the whole winter.
Ventilation reference: the 8 CFM, 2 CFM and 15–20% figures are from Basic greenhouse engineering calculations, also by John W. Bartok Jr.
Where estimates go wrong
Three things the arithmetic cannot see
The formulas are exact for a still, dark night. The weather is neither.
Wind strips heat
An exposed site adds 10% to 15% to the infiltration loss, and a loose cover that flaps adds far more than that. A windbreak is cheaper than the fuel.
Snow does not read tables
A wet snow load is the usual way a tunnel ends. Spacing hoops wider than 4 ft to save pipe is a false economy that shows up in one bad storm.
The sun overshoots
Heat loss sets the heater; solar gain sets everything else. A house sized perfectly for a January night can still hit 100 °F in March by mid-morning.
Weather reference: UVM Extension's guide to preparing high tunnels for extreme weather covers what to do before the storm rather than after it.
Interesting fact
The frame outlives five covers
A properly built tunnel frame lasts about 20 years. The 6 mil greenhouse polyethylene over it lasts about four. That ratio quietly shapes how tunnels are designed and financed: the structure is capital, the skin is a consumable, and the wiggle wire channel that makes recovering a one-day job is worth more over the life of the house than almost any other fitting on it. It also explains the advice to pull plastic on a calm day above 60 °F — film put on warm shrinks tight as it cools, and a tight cover is one that does not flog itself to death in the first winter gale.
Source: West Virginia University Extension, Design and Construction of High Tunnels.
Buying it
Kit, bender or bought hoops
Film comes in fixed roll widths, so the arithmetic rarely decides the order — the next roll up does. Frame pipe is the opposite: it is cut and bent to whatever the arc demands.
What to check before ordering
- Arc length, not width, against the film roll widths available
- Enough film length for both end walls, which are often forgotten
- Channel and wiggle wire for every edge you intend to fasten
- Purlin count and ridge bracing for the snow you actually get
Conditions on the day
Cover on a calm day at 60 °F or warmer, with four or five people. Film pulled in the cold goes on slack and stays slack, and film pulled in wind goes on torn. Ground posts want to be driven, not dug and backfilled, and the first hoop should be checked for square and plumb before the other twelve copy it.
Worked example: a 20 × 48 ft half-round tunnel · arc = π × 20 ÷ 2 = 31.4 ft · glazing = 31.4 × 48 + 2 × 157 = 1,822 ft² · at U 1.15 and TD 50 °F, conduction = 104,800 BTU/hr · volume 7,540 ft³ at 1 air change adds 7,500 · so a 115,000 BTU/hr heater output, and 13 hoops at 4 ft on center.
Common mistakes to avoid
Buying film to the width of the house. A 20 ft house needs a sheet over 35 ft wide once the arc and the burying allowance are counted. Width is the one number the film is never sold by.
Sizing the heater on floor area. Heat leaves through the skin, and the skin of an arched house is far larger than its footprint. Floor area is for planting, not for BTUs.
Stretching the hoop spacing. Going from 4 ft to 6 ft saves a third of the pipe and costs the whole tunnel in the first heavy snow.
Planning for heat but not for heat waves. Ventilation capacity is the number that decides whether a spring crop survives; size it before the heater.
Frequently Asked Questions
How do you calculate the size of a hoop house?
Start with the arc of one hoop, because every other figure depends on it. A half-round hoop is half the circumference of a circle whose diameter is the width of the house, so the arc measured over the top from ground to ground is π × width ÷ 2 — 31.4 ft on a 20 ft house, bent to a 10 ft radius. Multiply that arc by the length for the curved glazing area, add the two end walls, and you have the surface that loses heat and the plastic you have to buy. Floor area is simply width × length, and the volume is the cross-section area of the arch multiplied by the length. Raising the peak height above half the width gives a gothic arch instead: a longer arc, a tighter radius at the ridge, and more material for the same footprint.
What size heater does a hoop house need?
Add the conduction loss through the glazing to the air infiltration loss, then buy a heater whose output rating meets or exceeds the total. Conduction is surface area × U-factor × temperature difference; infiltration is 0.02 × volume × air changes per hour × the same temperature difference. A 20 × 48 ft single-poly tunnel held at 50 °F against a 0 °F design night loses about 112,000 BTU per hour, so a 115,000 BTU/hr heater — and that figure falls by more than a third if the house is double-poly inflated. Running cost is usually what decides the covering: a greenhouse skin is among the cheapest parts of the build and the dearest part of the winter, which is why many growers heat only enough for season extension rather than trying to hold a summer crop through January.
How much plastic do you need to cover a hoop house?
Take the hoop arc and add roughly 2 ft on each side for burying at the base, wiggle wire or a hip board, then add about 2 ft at each end of the length. A 20 ft wide half-round tunnel with a 31.4 ft arc therefore wants a sheet about 36 ft wide, which is bought as the next standard roll size up — the calculator rounds it there for you. Greenhouse polyethylene is made in fixed widths, so it is nearly always the roll size rather than the arithmetic that settles the quantity you order, and going one width up costs far less than a cover that lands 6 in short. Budget separately for the end walls and the door, which are cut from the same material.
How far apart should hoops be on a high tunnel?
No more than 4 ft on center, which is the spacing extension engineers recommend for the frame to carry snow and wind. Each bow is bent from metal tubing — 1.66 in pipe on tunnels under 26 ft wide, 1.90 in above that — and drops onto ground posts driven 24 to 30 inches deep, with extra anchors roughly every 10 ft on an exposed site. Tunnels under 26 ft wide take at least three purlins along the ribs and wider ones take five, and in snow country a 2×4 post under the ridge every third or fourth bow is cheap insurance against a collapse. PVC will do for a low garden tunnel but flexes badly under a wet snow load; galvanized conduit or purpose-made steel is the material for anything you mean to keep. Widening the spacing to save on pipe is the single most common way a tunnel ends up flat in the spring.
How much ventilation does a hoop house need?
For fan ventilation the rule is 8 CFM per square foot of floor area, so a 20 × 48 ft house needs roughly 7,700 CFM of exhaust capacity for summer. Roll-up sides do the same job passively, and where vents are used each of the roof and sidewall openings should be 15% to 20% of the floor area. A door at each end that can be propped open earns more than its size suggests, because it gives the house a through-draught on a still day. Horizontal air flow fans for winter circulation are sized much smaller, at about 2 CFM per square foot, and they pay for themselves by keeping condensation off the crop rather than by cooling it.
How long does hoop house plastic last?
About four years for 6 mil greenhouse-grade polyethylene, against roughly 20 years for a frame that is properly built and maintained. That mismatch is worth planning for: the cover is a consumable and the largest recurring cost of a season extension structure, and it is the item most likely to decide when the tunnel is out of production. Film should be pulled on a calm day at 60 °F or warmer so it stretches into place and tightens as it cools, which also makes it far less likely to flap itself to pieces in the first winter. A cover tensioned hard against the ridge and fastened properly at the base will outlast a slack one by a season or more, whatever the label claims.
Before you place the order
- Arc measured over the top, not across the base
- Film width rounded up to a roll size that is actually sold
- Design outside temperature taken from local winter data
- Hoop spacing at 4 ft or tighter, whatever the kit suggests
- Ventilation sized alongside the heater, not after it
- Heater output rating checked, not its input rating
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Disclaimer
This calculator models a hoop house as a circular arch of constant cross-section. A gothic frame, a kneewall, an attached headhouse or roll-up sides will change the glazing area, and a house built on sloping ground will change everything. Treat the figures as a sound starting point for ordering, not as a substitute for the supplier's own take-off.
Heat loss is calculated for still, dark conditions at the design temperature you enter. It does not model wind speed, solar gain, thermal screens, root-zone heating or the humidity a crop adds. Real fuel use over a season depends on all of them, and a heater is normally selected with headroom over the calculated loss.
Frame spacing, purlin counts and post depths are general extension guidance, not an engineered design. Snow and wind loads vary enormously by site and by code, and a tunnel in a heavy snow region should be built to a manufacturer's or engineer's specification for that load.
Last updated
Formulas, U-factors, and references checked on this date.