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. The shop heater calculator uses this same model with shop-sized presets.
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.
Running cost model
The running cost calculator treats the sizing result as delivered heat output. For electric resistance heat, it divides output BTU/hr by 3,412 to find kWh, then multiplies by the electric price and duty cycle. For natural gas and propane, it divides delivered output by 80% efficiency, then divides by 100,000 BTU per therm or 91,452 BTU per gallon before multiplying by the fuel price. Monthly cost multiplies that hourly result by hours per day, days per week, and 4.345 weeks per month.
Duty cycle accounts for thermostat cycling. A heater normally runs for only part of each hour after the garage is warm, so multiplying rated power by every scheduled hour would overstate the bill. The defaults are editable: electricity is $0.19/kWh, rounded from the EIA Electric Power Monthly April 2026 residential average of 18.83 cents/kWh; propane is $2.67/gal from the EIA weekly residential heating fuels survey in late March 2026; and natural gas is $1.68/therm, rounded from the BLS Utility (piped) gas per therm, U.S. city average of $1.676 for April 2026, published in FRED series APU000072620.
Installation cost model
The installation cost calculator adds low, typical, and high component bands for equipment, labor, and the utility work a visitor selects. It totals each column independently and rounds the displayed subtotal to the nearest $50. Unknown work is never assigned a hidden midpoint. Permit fees, sales tax, possible panel upgrades, and new utility gas service remain outside the known-work subtotal.
Equipment values are representative U.S. retail snapshots reviewed in August 2026 from retailers and manufacturers including Tractor Supply, SupplyHouse, and Mr. Heater. Labor and general installed-cost context use HomeGuide. HomeAdvisor and Angi are treated as one overlapping consumer-cost system rather than independent sources.
The dedicated-circuit and possible panel bands use ENERGY STAR electrical-readiness guidance and supporting contractor cost references. Gas-line bands use HomeGuide gas-line data plus independent contractor and cost references. Propane setup uses HomeGuide and This Old House. Manufacturer documentation from Modine and Mr. Heater supports the distinction between power venting and separated-combustion configurations. The City and County of Denver quick-permit page is one official example showing unit heaters, gas piping, and gas vents within local mechanical-permit scope; requirements vary by jurisdiction.
Imported delivered BTU/hr supplies shopping context only. V1 does not apply one price-per-BTU multiplier because capacity, brand, construction, controls, and vent configuration do not form a stable nationwide price curve. It also omits a regional labor multiplier because employee wage data does not directly measure contractor billing. The model is a budgeting aid, not a quotation, permit decision, load calculation, pipe-sizing result, or vent approval.
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.