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Greenhouse Sizing Calculator
Size a greenhouse or high tunnel: grow area, heating load and the ventilation CFM the glazing demands.
Greenhouse Sizing Calculator
Chlorova · updated for 2026
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Click “Calculate” to see your results.
The heating figure is a rough rule-of-thumb load (footprint times a glazing U-factor times the design temperature rise), not a Manual J heat-loss calculation, and it ignores wind, foundation and shade loss. Ventilation uses a common twelve air-exchanges-per-minute heuristic rather than a certified fan curve, and usable grow area assumes about thirty-five percent is lost to walls, benches and paths. Treat every number here as a budgeting starting point and confirm equipment sizes against your climate, local code and the manufacturer rating before you buy a heater or fan.
A greenhouse is easy to sketch and easy to under-equip, because the frame is only the start of the decision. This greenhouse sizing calculator turns a footprint into the numbers that actually cost money: usable grow area, the heating load your cover creates, and the ventilation airflow it demands. Choose your glazing, enter the length and width and a design temperature rise, and it scales all three so you can budget a heater and fans before the first panel goes up.
Sizing a House in a Few Steps
- Pick the glazing that covers the structure you are planning to build.
- Enter the interior length and width of the footprint in feet.
- Set the temperature rise you want over the coldest outdoor low.
- Read both the gross footprint and the smaller usable grow area figure.
- Note the heating load in BTU per hour for your equipment search.
- Compare that to a rated heater sized at or just above the figure.
- Use the ventilation CFM to size exhaust fans and fresh-air inlets together.
How Each Number Is Derived
Footprint is simply length times width, and the usable grow area takes sixty-five percent of it, since walls, benches, paths and the door eat the rest. The heating load multiplies that footprint by the glazing U-factor and the design temperature rise, a rule-of-thumb that says how many BTU per hour leak out on the coldest night you plan to heat against.
The U-factor comes from the covering, not the frame, which is why double-inflated poly at roughly zero point six-five heats far cheaper than single glass near one point oh-four for the same box. Ventilation uses a common twelve air-exchanges-per-minute estimate tied to floor area so a summer afternoon does not cook the crop. These are sizing heuristics for budgeting gear, not certified load or fan curves, so confirm the final equipment against your real climate and the manufacturer's rated output at your test temperature.
Worked Example: A 20 × 40 Poly Tunnel
- You are framing a four-season high tunnel forty feet long by twenty wide.
- A double-inflated poly cover gives a low insulating U-factor of about 0.65.
- You are designing to hold about 25 °F above your coldest winter low.
- The overall footprint works out to a roomy 800 square feet of floor space.
- Usable grow area lands near 520 square feet after the beds and paths.
- Heating load lands near 800 × 0.65 × 25 = 13,000 BTU per hour.
- The ventilation demand is about 800 × 12, so roughly 9,600 CFM.
- So budget a 15,000 BTU heater plus fans that move all that air.
Approximate U-factors by Covering
Insulating value of common greenhouse coverings, shown as the U-factor the calculator uses for each glazing choice. Lower is warmer per square foot, so the same house and temperature rise cost less to heat as this number drops.
| Covering | U-factor (Btu/hr·ft²·°F) | Heat loss vs double poly |
|---|---|---|
| Double-inflated poly | 0.65 | baseline |
| Twin-wall polycarb | 0.82 | about 26% more |
| Single glass | 1.04 | about 60% more |
| Single poly film | 1.13 | about 74% more |
Key Terms
- U-factor
- How fast a covering leaks heat; higher numbers mean a colder, costlier-to-heat house.
- Design temperature rise
- The gap you hold indoors above the coldest outdoor design temperature for your region.
- BTU per hour
- The heating output a heater must sustain to replace what the glazing leaks on a cold night.
- CFM
- Cubic feet per minute of air the fans must move to fully refresh the house every minute.
- High tunnel
- A poly-covered hoop house, usually without sidewall vents, sized to a tractor or tiller width.
The Decisions That Move the Bill
- Glazing sets your running cost more than the frame does, so a jump from single film to double poly can cut the heating load by nearly half for the same footprint.
- Size heat to the design temperature rise, not an average winter day, because the heater earns its keep on the single coldest night, not the typical one.
- Ventilation and heating pull opposite ways, since a tighter, more insulating house still needs the same summer airflow, so budget for both rather than trading one for the other.
- Usable area shrinks fast on narrow houses, as a four-wide tunnel loses a far larger share of floor to walls than a twenty-wide one.
- Fan ratings use free air, not ducted or louvered air, so real airflow drops once you add shutters, and you should size with that loss built in.
Common Mistakes & Fixes
- Heating for a mild average January instead of the design coldest night, then freezing on the one night that matters.
- Reading gross floor area as grow area, then running out of bench space once paths and walls are subtracted.
- Under-fanning because the CFM looked huge, and cooking a summer crop that needed every air exchange.
- Choosing single film for the lowest up-front cost in a cold zone and paying it back monthly in fuel.
- Sizing a heater by house length alone while ignoring the temperature rise, so the BTU figure is a lucky guess.
When to Use This (and When Not To)
Use it early to budget a greenhouse or high tunnel, when you know the footprint and covering and want the heating and ventilation cost that implies. It sizes systems, not crops, so plan the plants and spacing with the garden planner, and work out how much bench soil you need with the potting soil calculator. For a passive cold frame you never heat, or a certified commercial build needing a Manual J load and engineered fan curve, this rule-of-thumb is the wrong tool.
Greenhouse Sizing Calculator: Frequently Asked Questions
How many BTUs do I need to heat a greenhouse?
Multiply the floor area by your glazing U-factor and the temperature rise you want to hold. A 200 square foot glass house kept 30 °F above outside needs roughly 200 × 1.04 × 30, or about 6,200 BTU per hour, before you add any wind or foundation losses.
Is double poly really worth it over single film?
In all but the mildest climates, usually yes, because the trapped air layer nearly halves heat loss for a small extra cost. The savings show up every winter month, and the film is cheap enough that the payback on a heated house is fast.
How much ventilation does a greenhouse need?
A common target is to exchange all the air about once a minute on a hot day, which this tool estimates as twelve CFM per square foot of floor. Size exhaust fans to that number, then let passive inlets make up the difference so the fans do not starve for makeup air.
Why is usable area less than the footprint?
Because walls, benches, aisles, the door and the curved edges of a tunnel all eat floor you cannot plant. Assuming about sixty-five percent is actually growable keeps a bed plan realistic instead of overstocking the house.
Does this tell me how many plants will fit?
No, it only reports the square footage you can grow in, not the plant count. Take that usable area into a spacing or garden-planning calculator to turn square feet into how many rows and crops actually fit.
What temperature rise should I design for?
Use the gap between your target growing temperature and your region's coldest design night, not an average winter low. That worst-case rise is what the heater must hold, so sizing to it prevents the single night when the crop is actually at risk.
Sources & Further Reading
- Sizing Greenhouse Heating Systems — Penn State Extension. Basis for the footprint × U-factor × temperature-rise heating rule used here.
- Naturally Ventilated Greenhouses — University of Missouri Extension. Underpins the air-exchange CFM ventilation estimate on this page.
- Greenhouse Covering Materials and Energy Use — North Carolina State University Extension. Source of the approximate glazing U-factor values in this calculator.
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated