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Forced-air heaters burn through propane fast compared to a radiant heater, simply because they run at much higher BTU output. Here is the real math, by BTU tier, so you can plan fuel instead of guessing.
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The Basic Conversion
One pound of propane holds about 21,600 BTU of energy. A heater running at its full rated output burns through fuel at a predictable rate based on that number. A standard 20 lb propane tank holds roughly 432,000 BTU worth of fuel when full, though most tanks are filled to about 80% capacity for safety, so plan on closer to 345,000 usable BTU in practice.
Runtime by BTU Tier on a 20 lb Tank
- 60,000 BTU — roughly 5.5 to 6 hours of runtime at full output.
- 100,000 BTU — roughly 3.5 hours of runtime at full output.
- 125,000 BTU — roughly 2.5 to 3 hours of runtime at full output.
- 150,000 BTU — a little over 2 hours of runtime at full output.
These numbers assume the heater runs continuously at its maximum rated output. In practice, most heaters cycle or run at a lower adjustable setting once a room reaches a comfortable temperature, which stretches actual runtime meaningfully beyond these full-burn figures.
Why a 20 lb Tank Is Not Enough for Regular Use
If you are heating a garage, workshop, or job site for a full work day, a 20 lb tank forces you to swap tanks multiple times. This gets expensive and inconvenient fast. A heater like the Dyna-Glo RMC-FA150DGD, run at its higher 150,000 BTU setting all day, would need multiple tank swaps just to get through a single 8-hour shift.
Moving to a 100 lb Tank
A 100 lb propane tank holds about 2,160,000 BTU worth of fuel when full, roughly five times a 20 lb tank. At 100,000 BTU output, that translates to somewhere around 17 hours of continuous runtime, enough to cover multiple full work days before needing a refill. Most people who run a forced-air heater regularly through a season eventually move to a 100 lb tank connected with a longer hose, specifically to avoid the constant swap-and-refill cycle a 20 lb tank demands.
Adjustable Output Saves Real Fuel
A heater with adjustable BTU output, rather than one fixed setting, lets you run lower on mild days and reserve full output for genuinely cold ones. Since fuel consumption scales directly with output, running an adjustable heater at 60% of its rated maximum instead of 100% meaningfully stretches a tank’s runtime. This is one of the most overlooked ways to cut fuel cost without buying a smaller heater.
Cost Per Hour to Run
Propane typically costs between $2.50 and $4 per gallon depending on region and whether you refill or exchange, and a gallon of propane holds about 91,500 BTU. At $3 per gallon, a 100,000 BTU heater running at full output costs roughly $3.30 per hour to run. A 60,000 BTU heater at the same price costs closer to $2 per hour. These numbers shift with local propane prices, so use them as a starting estimate rather than an exact figure for your area.
Refilling vs. Exchanging Tanks
Refilling a tank at a propane retailer is usually cheaper per gallon than exchanging it at a kiosk outside a hardware store. Exchange is faster and more convenient, but you pay for that convenience. If you run a forced-air heater regularly through a full season, the savings from refilling instead of exchanging add up fast. This matters even more at 100 lb tank sizes, where the price difference per fill is larger in real dollars.
Tracking Fuel Cost Across a Job
For a contractor billing heating costs back into a job, keeping a simple log of tank swaps and refill costs makes the actual fuel expense easy to justify later. Multiply the number of tank fills by the price per fill for a rough total, then compare it against the hours the heater actually ran. This is a more accurate number than guessing after the fact, and it helps size the next job’s heater budget correctly instead of estimating from memory.
Cold Weather and Fuel Efficiency
Propane vaporizes more slowly in very cold temperatures. This can slightly reduce a tank’s effective delivery rate on the coldest days. It rarely causes a heater to underperform in a way you would notice. Keeping a spare tank somewhere warmer than the ground, such as in a vehicle, can help maintain steady output during extended cold-weather use.
How Altitude Affects Fuel Burn
At higher elevations, thinner air means less oxygen is available for the heater to burn fuel completely. This can slightly reduce a forced-air heater’s real-world output compared to its rated BTU figure at sea level, an effect known as altitude derating. It matters most above roughly 2,000 to 3,000 feet. If you are working at meaningful elevation, check the specific heater’s manual for any altitude note, and consider sizing up slightly to compensate rather than assuming the rated BTU figure holds exactly.
Comparing Propane Cost to Electric Heating
Electric space heaters do not burn fuel, so there is no propane cost to calculate, but they draw a lot of amperage at comparable heat output and often trip a circuit if a job site’s power is limited. Propane’s real advantage on a job site is BTU output per dollar at scale. A single 150,000 BTU propane heater would need an enormous, often impractical electrical draw to match with an electric equivalent. For raw heating power on a site with limited electrical service, propane usually wins on cost even after factoring in fuel spend.
Why Two Heaters of the Same BTU Rating Can Use Different Amounts of Fuel
Two heaters with the same BTU rating do not always burn propane at exactly the same rate in practice. Burner efficiency, regulator calibration, and how well a specific unit is maintained all affect real-world fuel use slightly. A heater with a dirty burner or a fuel line that has not been checked in years can run less efficiently than its rated output suggests, using more fuel to produce the same amount of heat. Regular cleaning and maintenance, covered in our forced-air setup guide, keeps a heater running closer to its rated efficiency.
Estimating Fuel Needs Before a Multi-Day Job
For a job expected to run several days with a forced-air heater running most of each day, estimate total fuel need before the first day rather than restocking as you go. Multiply your heater’s approximate hourly burn rate by expected daily runtime, then by the number of days. Add a buffer of at least one extra tank, since colder-than-expected weather or a longer workday than planned both push actual use above the estimate.
Related Reading
For sizing a forced-air heater to your specific garage, see our forced-air garage setup guide. For general tank and hose guidance, see our tanks and accessories guide.