Methodology
How the Garage Heater Calculator Works
Every number this site shows comes from one transparent formula. This page documents the formula, the factors, the rounding rules, and the limits of the estimate, so you can judge the result instead of trusting a black box.
The formula
Required BTU/hr = garage volume in cubic feet x insulation factor x temperature rise. Volume is length x width x ceiling height. Temperature rise is the difference between your target indoor temperature and the coldest outdoor temperature you realistically expect. This is a widely used industry rule of thumb for sizing garage and workshop heaters, not something we invented.
Insulation factors
The published rule of thumb uses roughly 0.133 for an average garage and up to about 0.2 for a poorly insulated one. We extend the same scale downward for better construction.
| Level | Factor | What it looks like |
|---|---|---|
| Poor | 0.20 | Exposed framing, an uninsulated metal door, visible gaps or drafts. |
| Average | 0.133 | Some wall insulation, a standard garage door, normal air leakage. |
| Good | 0.10 | Insulated walls and door, weather sealing in reasonable shape. |
| Excellent | 0.08 | Finished, tightly sealed, insulated on all sides including the door. |
A worked example
A 24 x 24 ft two-car garage with an 8 ft ceiling holds 4,608 cubic feet. With average insulation and a 40 F rise (20 F outside, 60 F target), the estimate is 4,608 x 0.133 x 40, which is about 24,500 BTU/hr. That converts to roughly 7,200 watts of electric heat, and the next standard unit size is 25,000 BTU/hr.
Rounding and unit-size rules
Required output is rounded to the nearest 500 BTU/hr, and electric equivalents to the nearest 100 watts (1 watt is 3.412 BTU/hr). The suggested purchase size always rounds UP to the next capacity heaters are commonly sold in: 2,500 BTU/hr steps below 15,000 and 5,000 BTU/hr steps above. We never round a purchase suggestion down, because a slightly larger unit works while a slightly smaller one fails on the coldest day. Gas and propane guidance converts required output to listed INPUT capacity assuming roughly 80% combustion efficiency.
Sources
The cubic-feet formula and factor range are documented across the heating industry, for example in sizing guides from The Furnace Outlet, LearnMetrics, and manufacturer guidance such as King Electric. Where sources disagree, we use the more conservative published value.
What this estimate cannot see
This is a planning estimate, not an ACCA Manual J heat-loss calculation. It cannot see your window area, slab temperature, wind exposure, air-change rate, or how fast you want the space to warm up from cold. If you are installing permanent equipment, sizing a heat pump, or working with unusual construction, have a professional run a real heat-loss calculation. Use this tool to narrow the shortlist and sanity-check quotes.