A hoop house is one of the few structures a grower can lay out on paper and put up in a weekend, and the whole build hinges on a single number: how long each bent bow has to be. Get that wrong and every hoop is either short of the ground or standing proud of the peak height you planned. This hoop house calculator takes the three dimensions you actually measure on site — the width between ground posts, the height you want at the ridge, and how long the house runs — and returns the arc length of one bow, how many bows the run needs, how many ground posts to drive, and the size of poly sheet to order.
Arb Digital builds free calculators for the trades and for small growers because the arithmetic behind a tunnel is completely knowable and yet almost nobody does it before buying pipe. Everything on this page is geometry. It sizes nothing structurally: it will not tell you whether your hoop spacing carries the snow that falls on your county, whether the gauge of pipe you bought is stiff enough, or whether the ground anchors hold in a gust. Those are load questions, and they belong to your local code official, your kit manufacturer's engineer, or a licensed engineer if you are building from scratch.
What This Hoop House Calculator Does
The calculator models a bow as a circular arc that passes through the two ground posts and rises to your chosen peak height at the centre. That is a good match for the quonset profile you get when you bend a stick of top rail or electrical conduit in a hoop bender, and it is an honest approximation for a kit bow that is nominally circular. From that arc it derives four things at once.
The hero number is the developed length of one bow — the distance along the curve from ground to ground. That is the length of pipe you cut before bending, ignoring the small allowance a bender consumes. Below it, the grid gives the number of bows a house of your length needs at your chosen spacing, the total footage of bow pipe that adds up to, the finished sheet of poly needed to skin the arch, and the count of ground posts.
Two further figures come out of the same geometry and are printed in the summary line: the cross-sectional area under the arch, which is the area of poly or polycarbonate each end wall takes, and the enclosed air volume, which is the starting point for any ventilation or heater sizing you do later. The square footage calculator handles irregular beds inside.
How to Use It
- Enter the width across the base — the distance between the outside faces of the two ground post lines, not the width of the bed inside.
- Enter the peak height you want at the centre of the arch, measured from grade. On a caterpillar tunnel this is often close to half the width; on a low tunnel it is far less.
- Enter the length of the house and the hoop spacing you intend to build to. The calculator places a hoop at each end and fills the run between them.
- Set the side tuck and end allowance — the extra poly beyond the frame that gets buried, wrapped over a hip board, or clamped into wiggle-wire channel.
- Give the stock pipe length your supplier sells and how many purlin runs you plan, and read the total sticks of pipe the frame consumes.
The Formula and How It Is Calculated
Call the base width W and the peak height h. The half-width is a = W ÷ 2. A circular arc through both base points with rise h has radius:
R = (a² + h²) ÷ (2h)
The arc subtends a half-angle θ at the centre of that circle, found from cos θ = (R − h) ÷ R, and the developed arc length is 2Rθ with θ in radians. The cosine form is used rather than the more familiar sine form because it stays correct when the rise exceeds the half-width — a tall gothic-leaning profile where the arc passes beyond a half circle. The sine form silently breaks there.
The cross-sectional area under the arch is the area of the circular segment, R²(θ − sin θ · cos θ), and the enclosed volume is that area multiplied by the length. The number of bows is the number of spacings that fit in the length, plus one for the closing end hoop, so a 48 ft house at 4 ft centres takes twelve bays and thirteen bows. Ground posts are two per bow. Cover sheet width is the arc length plus the side tuck at both sides; cover sheet length is the house length plus the end allowance at both ends.
Worked example, using the defaults: a 20 ft wide house with an 8 ft peak gives a = 10 and R = (100 + 64) ÷ 16 = 10.25 ft. Then cos θ = 2.25 ÷ 10.25 = 0.2195, θ = 1.3496 rad, and the arc is 2 × 10.25 × 1.3496 = 27.67 ft of pipe per bow. Over a 48 ft run at 4 ft centres that is thirteen bows and about 359.7 ft of bow pipe, twenty-six ground posts, and a sheet roughly 30.7 ft by 52 ft. The segment area works out at 119.3 sq ft, so each end wall is about that much sheeting and the house encloses roughly 5,727 cubic feet.
Why the Peak Height You Choose Changes Everything
Height is the input people guess at, and it has the largest leverage of the three. Raising the peak of a 20 ft house from 7 ft to 9 ft adds only two feet of headroom but adds nearly three and a half feet to every bow, which across thirteen bows is another two full sticks of pipe. It also steepens the shoulder, which is generally good for shedding snow and bad for wind exposure, and it increases the enclosed volume faster than the footprint, which changes how quickly the house heats in the morning and how much air a fan has to move to turn it over.
The relationship is not linear and it is not intuitive, which is exactly why it is worth calculating rather than eyeballing. A house at half the width in height is a true semicircle: maximum volume for the pipe, straight-up shoulders that a person can stand near, and the roundest profile for shedding. Below that you get a flatter, cheaper, lower-volume tunnel with more headroom lost at the edges. Above it the arch starts to lean gothic and the pipe cost climbs quickly.
Hoop Spacing, End Bays and the Off-Cut Nobody Plans For
The calculator places a hoop at both ends of the run and divides the rest evenly, which is how tunnels are actually framed — you never want an end wall floating between bows. If your length is not a whole multiple of the spacing, something has to give. This page rounds the bay count up so the last bay is short rather than long, because a short bay is stronger than a stretched one and a bay wider than you designed is where covers tear and pipe deflects. The summary line tells you the actual on-centre spacing that results.
Pipe waste is the second surprise. If a bow needs 27.67 ft and your supplier sells 21 ft sticks, one stick will not do it and two leave a 14 ft off-cut. Many builders splice two shorter pieces per bow with a swage or a coupler at the peak, which changes the arithmetic completely. The stick count printed here is a straight length-divided-by-length figure, so treat it as the floor rather than the order. For angles cut into posts or braces, use the angle cut calculator.
What This Page Deliberately Does Not Do
It publishes no snow load table, no wind speed table and no pipe schedule, because none of those can be typed from memory honestly. Ground snow load is site-specific and comes from ASCE 7 or your building department. Wind exposure depends on terrain and surrounding structures. Whether a 1⅜ in top rail at 4 ft centres is adequate for your county is a structural question with a real answer, and that answer is not on this page. If you need the roof load figure itself, our snow load calculator applies the published ASCE method to a ground snow load you supply, and explains why it will not hand you the ground figure.
Many jurisdictions treat a hoop house or high tunnel as an agricultural structure with lighter permitting than a building, and many do not. Some require a permit above a floor area threshold, some require an engineer's letter for anything with a permanent foundation, and some regulate it as a greenhouse under a separate code chapter. Check before you order pipe, not after.
Cover Sizing, Sheet Widths and the Real Order
Greenhouse poly is sold in fixed roll widths and cut to length, so the sheet size this page returns almost never matches a stock size exactly. You round up to the next available width. The important part is knowing the true arc plus tuck, because ordering to the base width instead of the arc length is the classic and expensive mistake — a 20 ft wide house needs a sheet nearer 31 ft wide, not 20 ft.
End walls are quoted separately and are computed from the segment area printed in the summary. Most builders frame end walls in timber with a door and a vent, so the poly needed is the segment area minus the openings. If you are sheathing them in plywood or polycarbonate instead, the plywood sheet calculator converts an area into a sheet count with a waste factor.
Siting, Ground Posts and the Things Geometry Cannot Tell You
Ground posts are driven, not calculated. Depth depends on soil, frost line and the uplift you expect, and a tunnel that fails almost always fails at the anchor rather than in the middle of a bow. Orientation matters too: a house running north to south lights more evenly through the day in most latitudes, while an east to west run collects more winter light at low sun angles. Neither is a number this calculator can give you, and both are covered well by the free extension material linked below.
Drainage is the other input that never appears in a spreadsheet. Water shed off both sides of a tunnel has to go somewhere, and on a slope it goes inside. A perimeter trench sized with our french drain calculator is cheap insurance compared with re-siting a finished house, and the elevation grade calculator tells you how much fall you actually have to work with.
Reliable, non-commercial guidance on high tunnel construction and management is published by the university extension services. Penn State Extension's high tunnel production resources and the Cornell Small Farms Program both cover siting, ventilation, crop scheduling and the practical build details that geometry alone will not settle.
Arb Digital builds websites and search visibility for growers, contractors and suppliers who sell locally. If people are searching for what you produce and finding somebody else, that is a fixable problem.
Web Design Services SEO ServicesCommon Mistakes to Avoid
- Ordering poly to the base width instead of the arc length — the curve is always longer than the span, and on a tall house it is half again as long.
- Measuring peak height to the underside of the pipe on one build and the top on the next — pick one convention and use it for the whole frame, or your hoops will not match.
- Assuming the last bay will just work out — a run that is not a whole multiple of the spacing leaves an odd bay, and it should be the short one.
- Treating the arc length as the cut length after bending — a hoop bender consumes a small amount of material and springback changes the finished rise, so bend one test hoop and measure before cutting twelve more.
- Building to a spacing copied from a kit of a different width — hoop spacing is a load decision tied to the specific pipe, span and site, and it does not transfer between designs.
Related Free Tools From Arb Digital
Work the roof geometry of a framed structure with the roof pitch calculator or the rafter length calculator, get the load side of the question from the snow load calculator, size a gravel pad with the gravel calculator, and estimate the timber for end walls with the wall framing calculator. For a smaller structure entirely, the chicken coop size calculator works the same footprint logic. Browse the full free online tools hub, or contact us if something you need is missing.
Frequently Asked Questions
It is the arc length of the curve, not the width of the house. For a 20 ft wide hoop house with an 8 ft peak the arc is about 27.7 ft, so a 20 ft span needs nearly 28 ft of pipe per bow before any bender allowance.
That is a design choice driven by the crop, the equipment you drive inside and the snow and wind at your site. A common starting point is somewhere between one third and one half of the width, but this page calculates the consequences of your choice rather than recommending one.
Hoop spacing is a structural decision that depends on pipe size and gauge, the span, and the snow and wind loads at your site. This calculator takes spacing as an input and will not suggest one, because a spacing copied from a different design can be badly wrong.
The sheet has to cover the arc plus enough on each side to bury, clamp or wrap, and the length plus an allowance at each end. The calculator returns that finished size; you then round up to the nearest stock roll width your supplier carries.
It depends entirely on your jurisdiction. Some treat high tunnels as agricultural structures with light or no permitting, others regulate them as greenhouses or as buildings once they exceed a floor area or gain a permanent foundation. Ask your building department before ordering materials.
No. It computes geometry only. It publishes no snow, wind or pipe capacity figures, and it makes no judgement about whether a frame is adequate. Load capacity is an engineering question for a licensed engineer or your kit manufacturer.
Because the sine form breaks down once the peak height exceeds half the width and the arc passes beyond a semicircle. The cosine form stays correct across the whole range, including tall gothic-leaning profiles.
This tool computes geometry for planning and material estimating only. It is not a structural design, it is not stamped, and it publishes no load, span or pipe capacity figures. Whether a frame, spacing, anchor or covering is adequate for the snow, wind and site conditions where you build is a question for a licensed engineer, the kit manufacturer's engineer, and the authority having jurisdiction.
