How the runtime is worked out
A heater's BTU rating is how much heat it burns in an hour. A tank holds a fixed amount of heat. So the runtime is one divided by the other:
BTU in the tank ÷ heater BTU per hour = hours the tank lasts
The heat in the tank depends on how it is sold:
- Cylinders are named by the pounds of propane they hold. Each pound holds about 21,548 BTU (Ferrellgas), so pounds × 21,548 = BTU. Ferrellgas puts a 20 lb cylinder at about 430,000 BTU, which is 20 × 21,548.
- Stationary tanks are named by their size in gallons but are only filled to 80%, which leaves room for the propane to expand (AmeriGas tank sizes: a 500 gallon tank "holds 400 gallons of propane when full"). Each gallon holds 91,452 BTU (EIA), so tank size × 0.8 × 91,452 = BTU. Ferrellgas quotes 91,502 BTU a gallon. The two differ by 50 BTU, about 0.05%; the tool uses the EIA's figure.
The cost comes from the gallons burned: BTU per hour ÷ 91,452 = gallons per hour, and gallons per hour × price per gallon = cost per hour. Per day is that times your hours, per month is 30 days, and refills a month is the month's BTU divided by the BTU in one tank. The "share of that time" slider covers thermostats: a burner that is on 60% of the time burns 60% of the fuel, so the tank lasts longer and every cost shrinks by the same share.
Propane prices are the EIA's weekly residential propane price by state. Where the EIA's survey does not cover a state, the calculator uses the region or US average and says so beside the price. That is a price for delivered propane; if you buy by the cylinder, type your own price per gallon.
The electric comparison uses the EIA's conversion of 1 kWh = 3,412 BTU, so a 1,500 W plug-in heater gives 1,500 × 3.412 = 5,118 BTU an hour, at your state's average residential electricity price. Comparing the cost of 10,000 BTU from each fuel shows which is cheaper per unit of heat. A vented propane heater sends some of its heat up the flue, so the propane figure is the best case for a vented model.
A word on safety. An unvented propane heater puts its exhaust into the room, so use one only in the spaces its maker rates it for, and read the manual for the room size and the fresh air it needs. A patio heater is built for outdoors. Our guide to using a propane heater indoors covers the safety side: oxygen depletion sensors, carbon monoxide alarms and ventilation.
Worked example
- 20 lb × 21,548 = 430,960 BTU in the cylinder.
- 430,960 ÷ 20,000 = 21.5 hours on full. Mr. Heater's own spec for its 20,000 BTU vent-free heater says 21.59 hours.
- 20,000 ÷ 91,452 = 0.219 gallons an hour.
- 0.219 × $2.638 = $0.58 an hour; × 6 hours = $3.46 a day; × 30 days = $103.85 a month.
- 6 hours × 30 days = 180 hours a month; 180 ÷ 21.5 = 8.4 cylinders a month.
The same heat from plug-in electric heaters: 10,000 BTU of propane costs 10,000 ÷ 91,452 × $2.638 = $0.29, and 10,000 BTU of electricity costs 10,000 ÷ 3,412 × $0.1831 = $0.54 at the US average rates.
How much heat is in each tank?
Cylinders use 21,548 BTU a pound (Ferrellgas). Stationary tanks use 80% of their size (AmeriGas) at 91,452 BTU a gallon (EIA).
| Tank | Propane in it | BTU | Where the figure comes from |
|---|---|---|---|
| 1 lb cylinder | 1 lb | 21,548 | The size Mr. Heater's Buddy heaters take; BTU per pound from Ferrellgas |
| 20 lb cylinder, exchange fill | 15 lb | 323,220 | AmeriGas: its grill tanks "contain a net weight of 15lbs"; about 3.5 gallons by the two factors above |
| 20 lb cylinder, full | 20 lb | 430,960 | Ferrellgas puts it at about 430,000 BTU |
| 30 lb cylinder | 30 lb | 646,440 | Pounds × 21,548 |
| 40 lb cylinder | 40 lb | 861,920 | Pounds × 21,548 |
| 100 lb cylinder | 100 lb | 2,154,800 | Pounds × 21,548 |
| 120 gallon tank | 96 gal | 8,779,392 | AmeriGas: a 120 gallon tank holds 96 gallons of propane |
| 250 gallon tank | 200 gal | 18,290,400 | AmeriGas: a 250 gallon tank holds 200 gallons when full |
| 500 gallon tank | 400 gal | 36,580,800 | AmeriGas: a 500 gallon tank holds 400 gallons when full |
A stationary tank rarely runs a space heater alone. If it also feeds a furnace, a water heater or a range, those burn from the same tank and it empties sooner.
What the heaters burn
The presets are the makers' ratings, or the models our guides recommend. Where the maker lists a runtime, it is in the last column so you can check the arithmetic against it.
| Heater | BTU per hour | Heats up to | Maker's runtime and source |
|---|---|---|---|
| Mr. Heater Little Buddy | 3,800 | 95 sq ft | 5.6 hours on a 1 lb cylinder (Mr. Heater); the arithmetic gives 5.7 |
| Mr. Heater Portable Buddy | 4,000 low, 9,000 high | 225 sq ft | 3 hours at max and 6 at min on a 1 lb cylinder (Mr. Heater); the arithmetic gives 2.4 and 5.4. The same page lists 0.044 gallons an hour at 4,000 BTU and 0.099 at 9,000, which matches BTU ÷ 91,452 |
| Mr. Heater Big Buddy | 4,000, 9,000 or 18,000 | 450 sq ft | "Up to 11 hours with dual tanks", two 1 lb cylinders (Mr. Heater); the arithmetic gives 10.8 hours on low |
| Dyna-Glo IR12 vent-free wall heater | 12,000 (6,000 low) | 300 sq ft | Our pick for small spaces in the ventless propane heater guide |
| Mr. Heater 20,000 BTU vent-free blue flame | 20,000 | 700 sq ft | 21.59 hours at max on a 20 lb cylinder, the smallest it takes (Mr. Heater); the arithmetic gives 21.5 |
| Mr. Heater 30,000 BTU blue flame | 30,000 | Larger rooms | A pick in our propane wall heater guide |
| Patio heater | about 40,000 | Outdoors | Ferrellgas: "the most common patio heaters produce roughly 40,000 BTUs per hour" |
| Forced-air torpedo heater | 60,000 | Garages, work spaces | The Avenger FBDFA60V in our garage heater guide |
Guides that go with this tool
- Can you use a propane heater indoors? 7 safety tipsHow-to
- 6 best ventless propane heatersBuying guide
- 4 best propane heaters for a garageBuying guide
- 7 best propane wall heaters for indoor useBuying guide
- Blue flame vs radiant heaters: which warms better?Comparison
- 7 best torpedo heatersBuying guide
FAQ
How long does a 20 lb propane tank last on a heater?
Divide 430,960 BTU, the heat in 20 pounds of propane at 21,548 BTU a pound (Ferrellgas), by the heater's BTU per hour. A 20,000 BTU heater on full runs about 21.5 hours; Mr. Heater lists 21.59 hours for its 20,000 BTU wall heater. A 9,000 BTU heater runs about 48 hours and a 60,000 BTU torpedo heater about 7. An exchange cylinder that holds 15 pounds gives three quarters of those times.
Is a propane heater cheaper to run than an electric space heater?
Per unit of heat, usually yes at US average prices. At $2.638 a gallon (EIA, week of Oct 12, 2026), 10,000 BTU of propane costs about 29 cents. At 18.31 cents per kWh (EIA, July 2026), 10,000 BTU from a plug-in electric heater costs about 54 cents. Prices vary a lot by state, so pick yours in the calculator. A vented heater sends some of its heat up the flue, so it gives you less than the full figure.
How long does a 1 lb propane cylinder last?
A 1 lb cylinder holds about 21,548 BTU. That is about 5.7 hours on a 3,800 BTU Little Buddy (Mr. Heater lists 5.6 hours), about 5.4 hours on a Portable Buddy on its 4,000 BTU low setting, and about 2.4 hours at 9,000 BTU on high, where Mr. Heater lists 3 hours.
Sources
- Propane prices: EIA, weekly residential propane prices by state, week of Oct 12, 2026.
- Heat content: EIA, British thermal units: propane 1 gallon = 91,452 Btu; electricity 1 kWh = 3,412 Btu.
- Heat per pound and per cylinder, and the patio heater figure: Ferrellgas, How long can a propane heater run?
- Tanks filled to 80%: AmeriGas, propane tank sizes. Exchange cylinders at 15 lb: AmeriGas, grill tanks support.
- Electricity prices: EIA Electric Power Monthly, table 5.6.A, July 2026.
- Heater ratings: the Mr. Heater product pages and the Home Caprice guides linked in the table above.
Prices last checked 20 October 2026. The calculator does not store anything you type.