Greenhouse Heating Calculator

About this calculator

Sizing a greenhouse heater starts with the heat that escapes through its glazing: multiplying the surface area by the temperature difference between inside and outside gives the base heat loss, and multiplying that by a standard glazing factor of about 1.1 (accounting for the extra losses typical of single-layer glass or plastic covering) gives the heater output needed, usually expressed in BTU/hr, to hold a target temperature.

This method is used by gardeners, greenhouse growers, and hobbyist horticulturists deciding what size heater to buy before winter — undersizing leaves plants exposed to frost during cold snaps, while oversizing wastes money on both the unit and its running cost. It's the same basic heat-loss logic professional greenhouse and grow-room designers use, simplified to the inputs a home grower actually has on hand.