Greenhouse and season extension, by the numbers

Most greenhouse mistakes are sizing mistakes.

A heater two sizes too small, a fan that moves half the air the house needs, shade cloth at the wrong percentage, seedlings under a fixture delivering a third of the light they want. This site does that math first, then tells you what to buy. For vegetables, herbs, fruit and flowers.

Run the numbers See the three builds

Quick heater sizing

Width ft

Length ft

Delta °F

BTU per hour 7,700

Assumes 4 mm twin-wall polycarbonate and an 8 ft ridge. Full calculator handles film, glass, wind exposure and every glazing combination.

Where most people land

Palram

Palram Canopia Mythos 6 x 6 Greenhouse

36 sq ft of twin-wall from a maker with a documented spare-parts supply, which is the difference that shows up in year three when a panel clip breaks.

Check price $622.42

Compare 6 polycarbonate houses in three tiers

Bio Green Palma 1500 W Greenhouse Heater with Digital Thermostat

Heat Bio Green Palma $161.19

VIVOSUN 10 x 20.75 in Seedling Heat Mat and Digital Thermostat Set

Propagate VIVOSUN mat and thermostat $24.78

Complete builds

Three greenhouses, every item priced and linked

Compare all three

Not the same list at three prices. These are three genuinely different structures: unheated season extension, a heated hard-glazed house with one automated vent, and a climate-controlled house running twelve months a year. Each page states what to skip and what to upgrade first.

Beginner

Cold frame and low tunnel starter

A first season of growing under cover, on a patio or in an existing bed, with no power run outside.

Footprint: 10 sq ft under glazing plus a 4-shelf indoor rack
Power: Indoor power only
Line items: 15

Build total $390

Mid-range

Heated polycarbonate greenhouse

A gardener who has outgrown a cold frame and wants a frost-free house they can stand up in and walk away from for a weekend.

Footprint: 36 sq ft, roughly 6 by 6 ft
Power: One dedicated 15 A outdoor circuit
Line items: 20

Build total $1,527

Expert

Climate-controlled production greenhouse

A grower running the house all twelve months, propagating for a whole garden, and treating the structure as equipment rather than furniture.

Footprint: 180 sq ft, roughly 10 by 18 ft
Power: A 240 V circuit for heat plus two 15 A 120 V circuits
Line items: 54

Build total $4,228

Ten calculators

The math nobody else does properly

All calculators

Most gardening sites hand-wave greenhouse sizing with a rule of thumb. Getting heater BTU or daily light integral wrong does not produce a slightly worse result, it produces dead plants, so every figure here shows the formula it came from.

Researched gear

Every category, in three tiers

All gear guides

Beginner, buy-once and expert, with a real price and a real reason at each step, and a plain statement of who should not buy the expert tier. Selections are researched from published specifications, listed certifications and verified owner reviews. We do not claim hands-on testing.

Reference charts

Dense tables, specific numbers

All charts

By crop

What each plant actually wants under cover

Temperature range, daily light integral, germination soil temperature, spacing, humidity and the specific failure each crop is prone to in a greenhouse. Twenty-six vegetables, herbs, fruits and ornamentals, each with the numbers rather than the adjectives.

All crop pages

Guides

A full year under glass

All guides