How Long Does a Propane Tank Last on a Heater?

When a winter storm knocks out grid power for three days, running out of heating fuel at 2:00 AM is more than an inconvenience—it is a household safety emergency. Most people who buy an indoor-rated radiant propane heater grab a two-pack of green 1-pound camping canisters at the hardware store without doing the math, only to discover that two 1 lb canisters burn out in less than five hours on High.

Before buying fuel or lighting a burner, start with household medical safety and the exact BTU-per-gallon runtime formula. Knowing how to convert cylinder weight into usable British Thermal Units (BTU)—and why fire codes forbid bringing a 20 lb barbecue cylinder indoors—lets you plan a 72-hour winter heating reserve without guesswork.

Start with household safety: when combustion heat is not the right plan

Before calculating tank hours, evaluate who is inside your home. Unvented indoor-rated propane heaters (ANSI Z21.11.2) consume room oxygen and release water vapor and trace combustion gases directly into the living space:

• Households with infants, pregnant women, older adults (65+), or anyone with asthma, COPD, or cardiovascular disease face heightened risks from indoor combustion byproducts and moisture buildup.
• High-rise renters and condo residents should check local fire ordinances and building rules first; many multi-family buildings prohibit storing or operating liquid propane heaters indoors.
• Never run an unvented propane heater while sleeping or unattended. Use the heater during waking hours to warm your consolidated room (target 65°F–68°F), shut the gas valve completely OFF before bed, and transition to high-R-value sleeping bags, insulated sleeping pads, or a low-wattage (50–70 W) electric heated mattress pad powered by a portable battery station overnight.

The propane heater runtime formula, step by step

To calculate how long any propane cylinder will run any heater, you only need two physical constants from the U.S. Energy Information Administration (EIA):

1. Weight of liquid propane: ~4.20 to 4.24 pounds per gallon at 60°F.
2. Energy content of liquid propane: ~91,500 BTU per gallon (which equals ~21,500 to 21,600 BTU per pound of liquid propane).

Step 1: Convert cylinder size to usable gallons and total BTUs

Propane cylinders are never filled to 100% liquid capacity because liquid propane expands rapidly as ambient temperature rises. Every modern cylinder uses an Overfill Prevention Device (OPD) that stops filling at 80% liquid volume, leaving 20% headspace for vapor expansion:

• 1 lb Disposable Camping Canister (16 oz / 0.236 gal liquid): 1.0 lb × 21,500 BTU/lb = ~21,500 BTU total energy
• 20 lb Grill Cylinder (holds ~20 lbs or 4.7 gallons of liquid propane when filled at a weigh-scale refill station): 4.7 gallons × 91,500 BTU/gal = ~430,050 BTU total energy (Warning: Supermarket “tank exchange” cages often under-fill 20 lb cylinders to 15 lbs (3.5 gallons = 320,250 BTU) to offset transport costs—reducing your outage runtime by 25%! Always check the net weight printed on the exchange collar or get your tank filled by the gallon at an U-Haul, tractor supply, or propane dealer.)

Step 2: Divide total cylinder BTUs by the heater’s BTU/hr setting

Runtime (Hours) = Total Cylinder BTUs / Heater Input Rating (BTU/hr)

Worked example for a standard 20 lb cylinder (430,050 BTU full fill):

• At 4,000 BTU/hr (Low setting on a single-burner Buddy heater): 430,050 BTU / 4,000 BTU/hr = 107.5 hours (~4.5 days of continuous burn, or 8.9 days if run 12 waking hours per day)
• At 9,000 BTU/hr (High setting on a Portable Buddy): 430,050 BTU / 9,000 BTU/hr = 47.8 hours (~2.0 days continuous, or 4.0 days at 12 hours/day)
• At 18,000 BTU/hr (High setting on a dual-tile Big Buddy): 430,050 BTU / 18,000 BTU/hr = 23.9 hours (~1.0 day continuous, or 2.0 days at 12 hours/day)

Complete propane cylinder runtime reference table

Use the table below to plan your 72-hour winter storm fuel inventory across all common propane cylinder sizes:

Matching heater BTU output to room square footage

How many BTUs do you actually need to heat your consolidated emergency room? A widely used residential planning rule of thumb is roughly 20 to 25 BTU per hour for every 1 square foot of floor space in a moderately insulated room with 8-foot ceilings during cold weather:

• 150 to 200 sq ft bedroom or office: 180 sq ft × 20 BTU/sq ft = 3,600 BTU/hr → Matches the 4,000 BTU/hr (Low) setting on a compact indoor-rated propane heater.
• 300 to 400 sq ft living room or family room: 350 sq ft × 20 BTU/sq ft = 7,000 BTU/hr → Matches a 9,000 BTU/hr setting cycled intermittently.
• 450 to 600 sq ft open basement or great room: 500 sq ft × 25 BTU/sq ft = 12,500 BTU/hr → Requires up to 18,000 BTU/hr during initial warm-up, then throttling back to 4,000–9,000 BTU/hr.

Treat 20 BTU/sq ft as a starting assumption; for a more precise figure based on your home’s actual insulation R-value and outdoor temperature drop (ΔT), run the equation in our winter blackout home heat-loss calculation guide.

72-hour propane heater planning checklist

Verify every item below before relying on a portable propane heater for a multi-day winter storm:

• Calculate waking-hour fuel demand (not 24/7 burn): Because you must shut off unvented heaters while sleeping (~8 hours/night), size your fuel for 14 to 16 waking hours per day (e.g., 15 hrs/day × 4,000 BTU/hr = 60,000 BTU/day → requires 3× 1 lb canisters per day or 9× 1 lb canisters for a 72-hour outage).
• Get 20 lb tanks filled by the gallon at a refill station: Avoid 15 lb pre-filled exchange cages so you receive the full 4.7 gallons (430,050 BTU) of capacity.
• Use the correct hose + fuel filter pairing:
  • Standard high-pressure rubber hoses (1"-20 male throwaway cylinder thread) leach oily plasticizers into the heater valve unless you screw a fuel filter (F273699 style) directly into the heater inlet and replace the filter annually.
  • Regulated quick-connect hoses (dedicated Big Buddy quick-coupler or green composite zero-leach hoses) do not require the inline filter—check your exact hose manual.
• Enforce the 1 sq in per 1,000 BTU/hr window ventilation rule: Crack a window slightly (~1/4 to 5/16 inch on a 30-inch sash) and keep two working battery-powered carbon monoxide (CO) alarms in the room and hallway.

For a deep dive into how the heater’s pilot light and thermocouple shutoff work—and why they do not detect carbon monoxide—read our indoor propane heater ODS sensor limits guide.

Frequently asked questions

How long does a 20-pound propane tank last on a heater?

A full 20-pound propane cylinder holds roughly 4.7 gallons of liquid propane, which contains about 430,000 BTU of heat energy (4.7 gallons × 91,500 BTU/gallon). Divide 430,000 BTU by your heater’s hourly BTU setting: at 4,000 BTU/hr (Low) it runs for about 107 hours, at 9,000 BTU/hr (Medium/High) it runs for about 48 hours, and at 18,000 BTU/hr it runs for about 24 hours. Note that supermarket tank-exchange cages typically fill cylinders to only 15 pounds (~3.5 gallons or 320,000 BTU), which reduces runtime by 25%.

Is it safe to use a propane heater indoors?

Only portable propane heaters explicitly certified to ANSI Z21.11.2 for indoor use (equipped with an Oxygen Depletion Sensor and mechanical tip-over shutoff) may be used indoors, and even those carry carbon monoxide and fire risks. You must provide fresh outdoor air ventilation (at least 1 square inch of window opening per 1,000 BTU/hr), maintain a 3-foot clearance from bedding and curtains, install working battery-powered carbon monoxide alarms, keep 20 lb bulk tanks outdoors, and never run the heater while sleeping.

How many BTU do you need to heat a room?

A common residential planning estimate is roughly 20 BTU per hour for each square foot of a moderately insulated room in cold weather. For a 200 sq ft bedroom, 200 × 20 = 4,000 BTU/hr (Low setting). For a 300 to 400 sq ft living room, 300 × 20 = 6,000 to 8,000 BTU/hr. Poorly insulated rooms with single-pane windows or sub-zero outdoor wind chills require more—use our heat-loss equation or insulate windows with plastic film and blankets to reduce BTU demand.