What size garage heater do I need? 18 heaters from 5 makers
Garage heater size in BTU and watts from your garage's size and winter low, with 18 current electric and gas garage heaters from Dr. Heater, Fahrenheat, King Electric, Modine, Mr. Heater, each maker's rating, wiring and coverage claim.
Ratings, wiring and coverage claims read by hand on each maker's product page or manual on 21 October 2026. We have not installed or measured any of these heaters. How we research | How links work
The short answer
A typical two-car garage, 24 x 24 ft with an 8 ft ceiling, needs about 24,500 BTU/hr of heat output to stay 40°F warmer than outside, by the common rule of cubic feet x 0.133 x temperature rise. That is a 7,500 W electric heater on a 240 V circuit or a gas unit heater with about 25,000 BTU/hr of output, which is 30,000 BTU/hr of input on Modine's Hot Dawg. Makers disagree: King Electric's own chart asks for about half that wattage in a well insulated garage, so match the rule to your walls and door.
Size a heater for your garage
Enter the inside size of your garage, how cold it gets outside, and how warm you want it. The rise is the difference between the two.
About 24,515 BTU/hr
24 x 24 x 8 ft at a 40°F rise: 4,608 cubic feet x 0.133 x 40.
Every heater: electric by watts, then gas by input
Electric output is watts x 3.413, since all of an electric heater's power becomes heat. Gas output is the maker's own figure where it gives one. "Covers by the rule" is the floor area each heater holds at a 40°F rise with an 8 ft ceiling, by the 0.133 rule. Amps marked "ours" are watts / 240. Empty cells mean the maker does not give the figure on the page we read.
| Heater | Fuel | Watts | Gas input (BTU/hr) | Output (BTU/hr) | Amps at 240 V | Wiring or circuit as stated | Maker's coverage claim | Covers by the rule (sq ft) | Note | Source |
|---|---|---|---|---|---|---|---|---|---|---|
| Fahrenheat FUH54C | Electric, 240 V | 5,000 | 17,065 | 20.9 | 30 A fuse (maker table); 208 V gives 3,750 W | 401 | Maker states 17,065 BTU/hr at 240 V | Fahrenheat | ||
| King Electric GH2405TB | Electric, 240 V | 5,000 | 17,065 | 20.8 | 240/208 V; 13% fewer amps at 208 V | 401 | Maker states 17 kBTUH | King Electric | ||
| Dr. Heater DR-988 | Electric, 240 V | 5,600 | 19,113 | 23.3 (ours) | Plug-in: NEMA 6-30P on a dedicated 30 A 250 V outlet | 449 | The only plug-in model here; the rest are hardwired | Dr. Heater | ||
| Dr. Heater DR-966 | Electric, 240 V | 6,000 (3,000 W low setting) | 20,478 | 25.0 (ours) | Hardwired; 8 AWG copper | Heating coverage: 600 sq ft | 481 | Dr. Heater | ||
| Dr. Heater DR-975 | Electric, 240 V | 7,500 (4,700 W low setting) | 25,598 | 31.3 (ours) | Hardwired; 8 AWG copper | 601 | Dr. Heater | |||
| Fahrenheat FUH724A | Electric, 240 V | 7,500 | 25,598 | 31.3 | 45 A fuse (maker table); 208 V gives 5,625 W | 601 | Maker states 25,598 BTU/hr at 240 V | Fahrenheat | ||
| King Electric GH2407TB | Electric, 240 V | 7,500 | 25,598 | 31.2 | 240/208 V | 601 | Maker states 25.6 kBTUH | King Electric | ||
| Dr. Heater DR-910F | Electric, 240 V | 10,000 | 34,130 | 41.7 (ours) | Hardwired; 6 AWG copper | 802 | Dr. Heater | |||
| Dr. Heater DR-910M | Electric, 240 V | 10,000 | 34,130 | 41.7 (ours) | Hardwired; 6 AWG copper; on a cart | 802 | Dr. Heater | |||
| King Electric GH2410TB | Electric, 240 V | 10,000 | 34,130 | 41.6 | 240/208 V | 802 | Maker states 34.1 kBTUH | King Electric | ||
| Modine Hot Dawg HD30 | Natural gas or propane, vented | 30,000 | 24,900 | 1 to 1-1/2 car garage | 585 | Modine and FAQ | ||||
| Modine Hot Dawg HD45 | Natural gas or propane, vented | 45,000 | 37,350 | 2 to 2-1/2 car garage | 878 | Modine and FAQ | ||||
| Mr. Heater Big Maxx MHU50 | Natural gas or propane, vented | 50,000 | Heats up to 1,667 sq. ft. | under 1,175 | Output not stated on the product page; natural gas, propane kit included | Mr. Heater | ||||
| Modine Hot Dawg HD60 | Natural gas or propane, vented | 60,000 | 49,800 | 3 to 3-1/2 car garage | 1,170 | Modine and FAQ | ||||
| Modine Hot Dawg HD75 | Natural gas or propane, vented | 75,000 | 62,250 | Larger garages and higher altitudes | 1,463 | Modine and FAQ | ||||
| Mr. Heater Big Maxx MHU80 | Natural gas or propane, vented | 80,000 | Heats up to 2,667 sq. ft. | under 1,880 | Output not stated on the product page | Mr. Heater | ||||
| Modine Hot Dawg HD100 | Natural gas or propane, vented | 100,000 | 82,000 | Larger garages, out-buildings and higher altitudes | 1,927 | Modine and FAQ | ||||
| Modine Hot Dawg HD125 | Natural gas or propane, vented | 125,000 | 102,500 | Larger garages, out-buildings and higher altitudes | 2,408 | Modine and FAQ |
For the example garage, 12 of the 18 heaters have a stated or computed output of at least 24,515 BTU/hr. The smallest electric ones are the 7,500 W models: Dr. Heater DR-975, Fahrenheat FUH724A, King Electric GH2407TB. Mr. Heater gives input but not output on its Big Maxx pages, so its "covers" figure is an upper limit.
Two coverage claims are generous by the rule. Dr. Heater states 600 sq ft for the 6,000 W DR-966; by the rule that holds a rise of about 32°F at an 8 ft ceiling, not 40°F. Mr. Heater states 1,667 sq ft for the 50,000 BTU/hr MHU50; even if every BTU of input reached the garage, the rule gives a rise of about 28°F. Modine's claims are closer: the HD45's 37,350 BTU/hr output holds a 700 sq ft two-car garage at about a 50°F rise.
Makers' sizing rules side by side
King Electric's chart is for a 60°F rise in a garage "adequately insulated for the local climate". The 0.133 rule is the one Ferrellgas publishes and the top sizing calculators use; it makes no insulation allowance. Same garage sizes, both in watts:
| Ceiling | 1-car, 450 sq ft: King chart (W) | 2-car, 700 sq ft: King chart (W) | 3-car, 900 sq ft: King chart (W) | 4-car, 1100 sq ft: King chart (W) | 1-car: 0.133 rule (W) | 2-car: 0.133 rule (W) | 3-car: 0.133 rule (W) | 4-car: 0.133 rule (W) | Source |
|---|---|---|---|---|---|---|---|---|---|
| 8 ft | 4,400 | 6,850 | 9,100 | 11,750 | 8,417 | 13,093 | 16,834 | 20,575 | King, rule |
| 10 ft | 4,850 | 7,500 | 10,900 | 12,950 | 10,522 | 16,367 | 21,043 | 25,719 | King, rule |
| 12 ft | 5,300 | 8,200 | 12,600 | 14,100 | 12,626 | 19,640 | 25,252 | 30,863 | King, rule |
The rule asks for 1.9 times King's wattage for a two-car garage at an 8 ft ceiling. Neither is wrong for its own assumption. If your garage door is bare steel and the walls are open studs, the rule is the safer number. If the walls, ceiling and door are insulated, King's chart is closer. Modine and Mr. Heater state coverage by car count or floor area without a rise or insulation level, so they cannot be checked the same way.
Next step
Measure your garage inside the walls, decide how warm you need it, and check your panel has room for a 240 V breaker of the size on the heater's label. For gas, confirm the venting and gas line with the maker's installation manual before you buy.
Common questions
What size heater do I need for a 2-car garage?
About 24,500 BTU/hr of heat output for a 24 x 24 ft garage with an 8 ft ceiling and a 40°F rise, by the common rule of cubic feet x 0.133 x temperature rise. That is about a 7,200 W load, so a 7,500 W electric heater such as the Fahrenheat FUH724A (25,598 BTU/hr), or a gas unit with at least that output, such as the Modine Hot Dawg HD30 (24,900 BTU/hr output). A colder target or poor insulation needs more.
Why do makers' own sizing charts disagree?
They assume different garages. King Electric's chart gives 6,850 W for a 700 sq ft two-car garage with an 8 ft ceiling at a 60°F rise, for a garage "adequately insulated for the local climate". The 0.133 rule gives 13,093 W for the same garage, about 1.9 times as much. Use the chart that matches your insulation, and size up if your door and walls are bare.
Is a 30,000 BTU gas heater enough for a 2-car garage?
Check the output, not the input. Modine rates its Hot Dawg HD30 at 30,000 BTU/hr input and 24,900 output and lists it for a 1 to 1-1/2 car garage. It lists the HD45 (37,350 output) for 2 to 2-1/2 cars.
What circuit does a 240 V garage heater need?
It depends on the watts. Fahrenheat's manual lists a 30 A fuse for the 5,000 W FUH54C and 45 A for the 7,500 W FUH724A. Dr. Heater specifies 8 AWG copper for its 6,000 and 7,500 W models and 6 AWG for 10,000 W, and its plug-in DR-988 needs a dedicated 30 A NEMA 6-30 outlet. Have an electrician size the circuit to the label and local code.
Method
- Models. Garage heaters from Dr. Heater, Fahrenheat, King Electric, Modine, Mr. Heater that sit on a 240 V circuit or a gas line, read one page or manual at a time on 21 October 2026. Plug-in 120 V heaters and portable propane heaters are not included.
- Load. Cubic feet x 0.133 x temperature rise, in BTU/hr. Example: 24 x 24 x 8 = 4,608 cubic feet; x 0.133 x 40 = 24,515 BTU/hr. Divide by 3.413 for watts.
- Output. Electric: watts x 3.413. Gas: the maker's stated output (Modine). Where a gas maker gives only input (Mr. Heater), we compare the input and say so.
- Claims. Each maker's coverage claim is copied as written. The rise each claim implies is the output divided by (floor area x 8 ft x 0.133).
- Not covered. Heat loss from your actual walls, door and air leaks, which a heat-loss calculation by an HVAC contractor would measure. Venting, gas line sizing and electrical code. We did not test any heater.
Sources
- Dr. Heater USA product pages: DR-966, DR-975, DR-988, DR-910F, DR-910M (watts, wiring, coverage), read 2026-10-21
- Marley Engineered Products, Fahrenheat FUH Series installation manual (Table 2: BTU/hr, watts, fuse size, amps), read 2026-10-21
- King Electric GH compact heavy duty garage heater, ordering information (kW, BTUH, amps), read 2026-10-21
- King Electric garage heaters brochure 200022, dated 2 December 2019, still hosted by King (sizing chart by car count and ceiling height, PDF), read 2026-10-21
- Modine Hot Dawg garage unit heater sheet 6-114.13 (input and output BTU/hr, PDF), read 2026-10-21
- Modine HVAC homeowners FAQs: what size Hot Dawg is needed to heat a garage, read 2026-10-21
- Mr. Heater Big Maxx MHU50 and MHU80 product pages (input BTU, heating area), read 2026-10-21
- Ferrellgas, How many BTUs do you need: the 0.133 rule (temperature change x cubic feet x 0.133), read 2026-10-21
- Our garage heater dataset, CSV
Changes
- : Published. 18 garage heaters from Dr. Heater, Fahrenheat, King Electric, Modine, Mr. Heater, each read on the maker's page or manual on 21 October 2026, with King Electric's sizing chart and the 0.133 rule side by side.