PROPANE GUIDE
How long does propane last? The math you need.
One number makes propane planning simple: 91,502. That is how many BTUs are in a single gallon of propane. Everything else is division.
THE FORMULA
One number, one formula, every answer.
Propane has a fixed energy density. A gallon of propane always contains approximately 91,502 BTUs, regardless of brand, supplier, or where you bought it. That constant makes fuel planning straightforward arithmetic.
The runtime formula
Runtime (hours) = Usable gallons × 91,502 ÷ Appliance BTU/hr
Usable gallons is the tank's total water capacity in gallons multiplied by 0.80. Propane tanks are filled to 80% of their water capacity, never 100%. The remaining 20% is headspace for the liquid to expand safely as temperature changes.
Appliance BTU/hr is the input rating stamped on the appliance's data plate or listed in the owner's manual. This is the rate the appliance consumes fuel, not its heat output (though for propane appliances, the two are closely related).
For appliances with variable settings (a heater with low/medium/high), use the BTU rating for the setting you actually run. Running a 9,000 BTU heater on low (4,000 BTU) doubles your runtime compared to running it on high.
Worked examples
20-LB CYLINDER + CAMP STOVE BURNER (10,000 BTU/HR)
Usable propane: 4.7 gallons. Energy: 4.7 × 91,502 = 430,059 BTU. Runtime: 430,059 ÷ 10,000 = about 43 hours of cooking.
20-LB CYLINDER + PORTABLE HEATER (9,000 BTU/HR ON HIGH)
Same 430,059 BTU. Runtime: 430,059 ÷ 9,000 = about 48 hours of heat. On low (4,000 BTU/hr), that stretches to about 107 hours.
100-LB CYLINDER + DUAL-FUEL GENERATOR (FULL LOAD, ~75,000 BTU/HR)
Usable propane: 23.6 gallons. Energy: 23.6 × 91,502 = 2,159,447 BTU. Runtime: 2,159,447 ÷ 75,000 = about 29 hours at full load. At half load, closer to 58 hours.
500-GAL ASME TANK + PROPANE FURNACE (60,000 BTU/HR, 50% DUTY CYCLE)
Usable propane: 400 gallons. Energy: 400 × 91,502 = 36,600,800 BTU. Effective draw at 50% duty cycle: 30,000 BTU/hr. Runtime: 36,600,800 ÷ 30,000 = about 1,220 hours, or roughly 50 days of continuous heating.
APPLIANCE RATINGS
What propane appliances actually draw. Check the data plate.
Every propane appliance has a BTU/hr input rating, either stamped on a data plate on the appliance itself or listed in the owner's manual. The ranges below cover common household and preparedness appliances. Your specific appliance may differ; always use the actual rating from the data plate for your calculations.
COOKING
BTU/hr input rating. Cooking uses surprisingly little propane. Two meals a day on a camp stove uses under a half-gallon per week.
HEATING
Heating is the largest propane consumer in most households. Furnace duty cycle matters: a 60,000 BTU furnace running 50% of the time draws an effective 30,000 BTU/hr.
POWER GENERATION
Generator BTU draw depends heavily on electrical load. Half load roughly halves propane consumption. A well-managed load strategy doubles your generator runtime.
HOT WATER & OTHER
Tankless water heaters have a high BTU rating but only draw propane when hot water is flowing. Actual daily consumption is typically modest.
RUNTIME TABLES
Tank meets appliance. Hours at a glance.
These tables show approximate runtime in hours for common tank-and-appliance pairings. All figures assume 80% fill, a single appliance running continuously at the stated BTU rate, and moderate temperatures. Real-world runtime is often longer because most appliances cycle on and off rather than running continuously.
DOT cylinder runtimes (hours)
| Appliance | BTU/hr | 20 lb | 30 lb | 40 lb | 100 lb |
|---|---|---|---|---|---|
| Camp stove (1 burner) | 10,000 | 43 | 65 | 86 | 216 |
| Buddy heater (high, 9,000) | 9,000 | 48 | 72 | 96 | 240 |
| Buddy heater (low, 4,000) | 4,000 | 108 | 161 | 215 | 540 |
| Big Buddy (high, 18,000) | 18,000 | 24 | 36 | 48 | 120 |
| Generator, 3kW (full load) | 40,000 | 11 | 16 | 22 | 54 |
| Generator, 7.5kW (full load) | 75,000 | 6 | 9 | 11 | 29 |
Usable BTUs per cylinder: 20 lb = ~430,000 | 30 lb = ~646,000 | 40 lb = ~860,000 | 100 lb = ~2,159,000. Rounded to nearest whole hour.
ASME tank runtimes (days)
| Appliance | BTU/hr | 120 gal | 250 gal | 500 gal |
|---|---|---|---|---|
| Furnace, 60k BTU @ 50% duty | 30,000* | 12 | 25 | 51 |
| Furnace, 80k BTU @ 50% duty | 40,000* | 9 | 19 | 38 |
| Water heater (40-gal tank) | 40,000 | 9 | 19 | 38 |
| Standby generator, 20kW (full) | 200,000 | 2 | 4 | 8 |
| Cooking only (range, 2 hrs/day) | ~1,250** | 290 | 610 | 1,220 |
Usable gallons at 80%: 120 gal = 96 | 250 gal = 200 | 500 gal = 400. *50% duty cycle: the furnace runs about half the time during a heating hour. **Cooking average: ~15,000 BTU/hr burner for 2 hours/day = 30,000 BTU/day = 1,250 BTU/hr averaged over 24 hours.
WHAT REDUCES YOUR RUNTIME
The 80% rule. And what cold does to propane.
Why tanks fill to 80%
Propane is stored as a pressurized liquid. Like all liquids, it expands as temperature rises. A tank filled to 100% on a cool morning has no room for the liquid to expand as the day warms. The result would be dangerous internal pressure with no safety margin.
The 20% headspace gives the liquid room to expand safely. This is not a guideline. It is a physical safety requirement enforced by the hardware on every propane container.
On DOT cylinders (4 to 40 pounds), the OPD valve mechanically stops the fill when the liquid reaches 80%. The float inside the valve physically blocks the filling process. No human judgment is required.
On ASME tanks, the delivery driver opens the bleeder valve (fixed liquid level gauge) during filling. When liquid propane appears at the bleeder, the tank has reached the 80% mark and the driver stops.
Cold weather effects
Cold does not change the amount of energy stored in your propane. 91,502 BTUs per gallon at any temperature. But cold affects how the propane gets out of the tank.
Propane must vaporize (turn from liquid to gas) before an appliance can burn it. Vaporization happens inside the tank, driven by the liquid's vapor pressure. As temperature drops, vapor pressure drops. Below about 0°F, a small DOT cylinder may not deliver gas fast enough to keep a high-BTU appliance running. The tank is not empty; it is too cold to vaporize at the needed rate.
At -44°F (propane's boiling point), the liquid stops vaporizing entirely. The tank is effectively unusable until it warms up.
Larger tanks are less affected because they have more liquid surface area for vaporization. A 500-gallon ASME tank delivers reliably in temperatures that would stall a 20-pound cylinder.
Cold also increases heating demand, so your fuel gets used faster. Plan your winter propane reserve for higher consumption, not just the same daily rate you see in October.
FUEL PLANNING
Planning your propane reserve. Start with what you would actually run.
Step 1: List your outage appliances
Start by listing the propane appliances you would actually run during a power outage or supply disruption. Most households plan around three needs: cooking, heating one room, and intermittent generator power for refrigeration and charging.
Write down each appliance and its BTU/hr rating at the setting you would realistically use. Be specific. "The camp stove at 10,000 BTU/hr for 1 hour per meal" is useful. "Cooking" is not.
Step 2: Calculate daily BTU draw
For each appliance, multiply its BTU/hr rating by the hours per day you would run it. Add them up for your total daily draw.
EXAMPLE: 14-DAY WINTER OUTAGE PLAN
Step 3: Convert to gallons and pick your tanks
Multiply daily BTU draw by the number of days you want coverage. Divide by 91,502 to get gallons. Then choose the tank configuration that covers it.
CONTINUING THE EXAMPLE
135,000 BTU/day × 14 days = 1,890,000 BTU total
1,890,000 ÷ 91,502 = 20.7 gallons of propane
That is roughly five 20-lb cylinders (4.7 gal each = 23.5 gal), or one 100-lb cylinder (23.6 gal), or a 30-gallon margin inside a 120-gallon ASME tank kept at a minimum 50% fill.
Cooking alone uses very little
If your only propane need is cooking during an outage, the fuel math is reassuring. Two meals a day on a single camp stove burner at 10,000 BTU/hr for a total of 1.5 hours per day uses about 15,000 BTU per day. Over 14 days, that is 210,000 BTU, or about 2.3 gallons. A single 20-pound cylinder covers two weeks of cooking with fuel to spare.
Heating is where the fuel goes. That is the appliance to plan around.
Propane guides
Continue exploring
Propane Hub
The complete guide to propane for household energy. Start here for the overview.
Tank Types and Sizes
DOT cylinders vs. ASME tanks, OPD valves, and choosing the right size.
Storage and Safety
NFPA 58 distances, leak detection, CO prevention, and fire safety.
Getting Propane
Refill vs. exchange vs. bulk delivery. The real cost math.
Regulators and Connections
Regulator types, changeover valves, and natural gas conversion.
Propane Appliances
Every appliance propane can power, from cooking to generators to lighting.