Size a hobby or commercial greenhouse in 7 steps: plants, benches, capacity, ventilation, heater, glazing, and water — all computed in your browser.
These are industry rule-of-thumb calculations, not an engineering design. Spacing, ventilation, and heating rules vary by crop, climate, and local code. Use this tool to plan and budget, then confirm equipment choices with your greenhouse supplier or a qualified contractor.
Planning default 4 sq ft per plant (2-ft spacing).
Extra space for 36-in aisles and working room.
Planning default 3 ft (36 in) aisles. Standard bench working height is 30 in.
Share of floor used for plants after aisles (0–1).
Must be ≥ eave height.
Tip: compute it exactly in the Glazing Area tab.
Coldest expected night — size for the coldest night.
Planning estimate 0.75 gal (3 quarts) for mature tomatoes; adjust by crop and climate.
Every number below is a published rule of thumb or a disclosed estimator assumption — the calculator applies them transparently.
| Growing space | 4 sq ft per plant (planning default, 2-ft spacing; editable) |
| Aisle width | 36 in (3 ft) default between and beside benches |
| Bench height | 30 in standard working height (reference only) |
| Summer ventilation | 1 full air change per minute → CFM = greenhouse volume (UGA Extension B792) |
| Winter ventilation | Minimum 1 air change per 5 minutes → CFM = volume ÷ 5 (disclosed planning default) |
| Natural ventilation | Roof vent area ≥ 15% of floor area (ASABE recommendation) |
| Heat loss | HL = glazing area × U × ΔT (BTU/hr) |
| U-factors | Single 1.25 · Double poly 0.80 · Twin-wall 0.70 · Triple-wall 0.60 Btu/hr·sq ft·°F |
| Heater sizing | Required heater input = heat loss ÷ 0.80 (typical 80% efficient heater) |
| Water | 0.75 gal (3 quarts) per plant per day — mature-tomato planning estimate (MSU Extension) |
Sources: heat-loss equation and U-factors — Greenhouse Management, “Basic greenhouse engineering calculations”; 1-air-change-per-minute summer ventilation — UGA Cooperative Extension Bulletin 792; bench height ≈ 30 in — WSU Extension greenhouse construction chapter; 3 quarts/plant/day water planning — Mississippi State Extension (via Growing Produce); roof-vent 15–20% of floor area — ASABE standard cited by Greenhouse Management.
Plan on 4 sq ft per plant: 100 x 4 = 400 sq ft of growing area. Add a 30% aisle allowance and you need about 520 sq ft of floor - roughly a 22x24 ft footprint (528 sq ft).
An 8x10 greenhouse has 80 sq ft of floor. At 75% growable space and 4 sq ft per plant that holds about 15 plants (80 x 0.75 / 4 = 15).
Size fans for 1 full air change per minute: CFM = greenhouse volume. A 12x20 greenhouse with 8-ft eaves and an 11-ft ridge holds 2,280 cu ft of air, so you need 2,280 CFM of exhaust capacity for summer. For winter minimum ventilation (1 air change per 5 minutes) you only need 456 CFM - use staged or two-speed fans to cover both rates.
Heat loss = glazing area x U-factor x temperature difference. Example: 1,000 sq ft of double-layer poly (U 0.80) with a 40 F rise needs 1,000 x 0.80 x 40 = 32,000 BTU/hr. With a typical 80% efficient heater, buy 40,000 BTU/hr of input - then round up to the next standard size.
Plan on 3 quarts (0.75 gal) per mature plant per day. For 50 plants that is 37.5 gal/day (142.0 liters), about 262.5 gal per week. Size a storage tank for at least a 3-day reserve: 112.5 gal. Young plants need far less - size for peak maturity.
About 30 inches above the floor - the standard working height recommended in extension greenhouse guides. Benches 32-36 inches wide suit one-sided access; a 60-inch width works for a center bench with access from both sides.
Plan 36 inches (3 ft) for main working aisles - the default this calculator uses for bench layouts. Narrower maintenance aisles are possible with rolling bench systems, but 36 inches lets two people pass with a cart.
Add sidewalls (2 x length x eave) + end walls + roof. For a 20x40 greenhouse with 8-ft eaves and an 11-ft ridge: sidewalls 640 sq ft + ends 440 sq ft + roof 835.2 sq ft = 1,915.2 sq ft of glazing. Order about 10% extra for overlap and waste.
ASABE recommends roof vent area of at least 15% of the floor area. A 240 sq ft greenhouse needs 36 sq ft of open roof-vent area (more in warm climates; up to 20%).
With 3-ft aisles on both sides and between benches, ten 4x8 ft benches need a 10-ft-wide by 113-ft-long house = 1,130 sq ft of floor.
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