How Long a Propane Tank Actually Lasts With a Space Heater

A 20-pound propane tank weighs about 17 pounds when full of gas and holds roughly 430,000 BTU. A typical portable propane space heater burns between 4,000 and 9,000 BTU per hour on its low and high settings. Divide one by the other and you get a hard number: somewhere between 48 and 108 hours of burn time, depending on how high you run the dial. That range is wide enough to be useless on its own, which is why most people run out of fuel mid-weekend. The real answer depends on heater size, tank size, thermostat setting, outdoor temperature, and how often the unit cycles on and off.

If you are working out the running cost side of this rather than the runtime, the electricity cost calculator covers the plug-in comparison for electric heaters, which is the fair thing to weigh a propane unit against.

How Much Propane a Tank Actually Holds

Propane tanks are never filled to 100 percent. The industry standard is an 80 percent fill limit, which leaves headroom for the liquid to expand as it warms. That means a “20 lb” tank contains about 4.7 gallons of usable propane, not the 5 gallons the tank could physically hold. At 91,500 BTU per gallon, usable capacity works out to roughly 430,000 BTU per full tank.

Larger tanks scale the same way. A 100-pound tank holds about 23.6 gallons, or roughly 2.16 million BTU. A 500-gallon whole-house tank holds about 400 gallons usable, or roughly 36.6 million BTU. Whatever the size, the arithmetic is identical — take usable gallons, multiply by 91,500, and you have the total energy budget you are spending down.

Matching BTU Output to Burn Hours

The runtime calculation is simple division, but the input that trips people up is the BTU rating. Manufacturers quote a range because heaters have multiple settings. A “30,000 BTU” heater typically runs 10,000 BTU on low, 20,000 on medium, and 30,000 on high.

Tank size	Usable BTU	@ 5,000 BTU/hr	@ 10,000 BTU/hr	@ 20,000 BTU/hr
20 lb (4.7 gal)	~430,000	86 hours	43 hours	21.5 hours
40 lb (9.4 gal)	~860,000	172 hours	86 hours	43 hours
100 lb (23.6 gal)	~2,160,000	432 hours	216 hours	108 hours

Notice that the same 20-pound tank that lasts 86 hours on a low setting runs dry in under a day at high output. Choosing a setting is effectively choosing your refill schedule.

Why Real-World Runtime Is Shorter Than the Math

Three factors eat into the theoretical number. First, cold weather reduces tank pressure, so the last 10 to 15 percent of a tank can burn weakly or not at all in sub-freezing conditions — plan on about 85 percent of the calculated runtime in winter. Second, most heaters cycle rather than run flat-out, which helps, but the thermostat is measuring the air near the unit, not the room, so it overshoots. Third, if the heater is undersized for the space, it runs continuously and delivers the worst-case number every time.

A useful rule of thumb: a propane heater needs roughly 20 to 30 BTU per square foot for a well-insulated room and 40 to 50 BTU per square foot for a drafty one. A 400-square-foot drafty garage therefore needs 16,000 to 20,000 BTU, which puts you firmly in the 21-hour-per-tank column on a 20-pound cylinder.

Working Through a Real Example

Consider a 300-square-foot insulated workshop with a 20,000 BTU heater used four hours a night, five nights a week. Running on medium at 10,000 BTU per hour, each evening consumes 40,000 BTU. A full 20-pound tank gives 430,000 BTU, or about 10.75 evenings — call it two weeks of weekday use. Drop the setting to low at 5,000 BTU per hour and the same tank stretches to over four weeks. That is the entire economic case for a lower setting in one calculation.

Propane Versus Electric Running Cost

Propane at roughly $3.50 per gallon works out to about $0.0000383 per BTU. An electric space heater drawing 1,500 watts delivers about 5,120 BTU per hour and, at a national average of $0.17 per kWh, costs about $0.255 per hour — or roughly $0.0000498 per BTU. Propane is cheaper per delivered BTU at those prices, but only by about 23 percent, and that gap closes fast in regions with expensive propane or cheap electricity.

The trade-off is portability and ventilation. Propane heaters require combustion air and produce carbon monoxide, which is why indoor-rated units carry oxygen depletion sensors and why you should never use an outdoor-only heater indoors. Electric heaters need neither. If you are sizing the electrical side or converting a heater’s rating, the horsepower to watt calculator handles the unit conversion cleanly.

Practical Refill and Storage Advice

Weigh the tank, do not shake it. The tare weight is stamped on the collar — usually around 17 pounds for a 20 lb cylinder. Subtract it from the current weight and multiply by 91,500 to get remaining BTU.
Keep a spare. Because a 20-pound tank at high output lasts under 24 hours, a second cylinder is not optional for anything beyond occasional use.
Store upright and outdoors. Never store propane indoors, in a basement, or in a vehicle. The cylinders vent if pressure rises, and a venting cylinder in an enclosed space is an explosion risk.
Watch for icing. At high draw rates the tank can frost over and lose pressure. A tank that suddenly burns weakly is usually cold, not empty.

Frequently Asked Questions

How long does a 20 lb propane tank last with a heater? Between 21 and 86 hours depending on the heater’s output setting, based on 430,000 usable BTU and outputs from 5,000 to 20,000 BTU per hour. High settings burn through it in a day; low settings last over a week.

Can I use a propane heater indoors? Only an indoor-rated unit with an oxygen depletion sensor, and only in a ventilated space. Outdoor-rated heaters, including most patio and construction models, must never be used indoors because of carbon monoxide.

Does a full propane tank weigh 20 pounds? No. The “20 lb” figure is the gas capacity. The empty cylinder weighs about 17 pounds, so a full one weighs roughly 37 pounds. This is why weighing the tank requires subtracting the tare weight.

Is propane cheaper than electric heat? Per delivered BTU, usually yes — about 23 percent cheaper at typical US prices. But the advantage is small and reverses in areas with high propane costs or low electricity rates, so run the numbers for your own prices before assuming.

For readers doing the cooking side of propane math — timing a roast, converting oven temperatures, or scaling a recipe — the cooking time calculator covers the same kind of practical conversion work.