The greenhouse sizing calculators

Almost every expensive greenhouse mistake is a sizing mistake. A heater that cannot hold the temperature it was bought for. A fan that moves a third of the air the house needs. A bed filled to eight inches when the crop wanted sixteen. A grow light hung at a height that delivers half the photons the seedlings require. None of those are equipment failures, and none of them are visible until the season is underway.

Every calculator on this site exists because one of those decisions has a right answer that can be computed from figures you already know, and because the general advice available elsewhere substitutes adjectives for arithmetic. "A good sized heater" is not a specification. Four thousand nine hundred BTU per hour is.

All eleven share one library of greenhouse maths, so a figure produced by a calculator and a figure printed in a reference chart on this site cannot disagree with each other. Each one names the standard it follows, and each one shows its working in the writing below the tool rather than presenting a number and expecting trust.

The order to run them in

The calculators are not independent. Several take their input from the output of another, and running them in the wrong order produces a self-consistent answer to the wrong question. The sequence that works is size, envelope, heat, air, light, water.

Start with the size calculator. It works backwards from the crop you actually want to the floor area that holds it, once paths, benching and storage are added back. Most people start from a kit size and discover afterwards that the growing area is a fraction of the footprint. On a 6 by 8 foot house, a 24 inch bench takes 16 square feet, a 24 inch path takes another 16 and storage takes 6 to 8, leaving roughly 8 to 10 square feet of actual growing space out of 48.

Then the glazing comparison. Envelope choice sets the heating bill for the life of the structure, and it is the one decision on this list that cannot be revised cheaply later. Glazing R-values run from about 0.83 to 2.50, a factor of three, while light transmission over the same range falls only from about 88 percent to 62 percent. Insulation varies far more than light does, which is why moving up the scale is worthwhile at all and why the top of the scale is a poor buy for most hobby houses.

Then the heater. Heat loss is envelope area multiplied by the glazing U-factor multiplied by the temperature difference you intend to hold, with an allowance for air infiltration. The area in that expression is the glazed envelope, not the floor, and using floor area is the single most common error in greenhouse heater sizing. Read the answer against a hard limit rather than a budget: a 120 volt 15 amp circuit supplies 1,800 watts at full load, a continuous load should stay under 80 percent of that, and a 1500 watt heater at 12.5 amps is already at the ceiling.

Then ventilation. A greenhouse needs roughly one complete air change per minute, which is about 8 CFM per square foot of floor before corrections for house length, elevation, light load and the temperature rise you will accept. Passive vent area should total roughly one fifth of the floor, split between ridge and side vents, and almost no kit greenhouse ships with anywhere near that.

Then light, shade and water, which are crop decisions rather than structure decisions and can be revisited each season without touching the building.