How long does an 8 oz butane canister last?

An 8 oz (227 g) butane canister holds about 10,650 Btu, so on full power it lasts that figure divided by the stove's rating: about 53 minutes on a 12,000 Btu burner, 64 minutes on 10,000 and 84 minutes on 7,650. An 8.8 oz (250 g) can holds about 11,700 Btu and runs roughly a tenth longer. The "1.5 to 3 hours" figure that circulates is only possible with the flame turned well down.

How much energy is in an 8 oz butane canister?#

The method is the one used for the 1 lb propane cylinder: heat content per gram, times grams in the can, divided by the stove's rated input.

Heat per gram. NIST gives the enthalpy of combustion of n-butane gas as 2,877.5 kilojoules per mole. At 58.12 g per mole that is 49.5 megajoules per kilogram, or 46.9 Btu per gram. The EIA agrees by another route: 4.353 million Btu per barrel of normal butane is 103,600 Btu per gallon, and NIST puts saturated liquid n-butane at about 584 kilograms per cubic metre at 60 degrees F, so a gallon weighs 2.21 kg and carries 46.9 Btu per gram. Isobutane comes out at 46.8 by both routes. The blend in the can makes no practical difference.

Grams per can. Most "8 oz" butane cans in the US are 227 g net; Iwatani lists its BU-6 as "8oz.(227g)", and Gas One's GS-3900PB manual specifies a "227g (8 Oz.)" canister. Coleman's is 8.8 oz, and its manual names the CP250 250 g cartridge. Iwatani's Japanese cassettes are also 250 g.

Energy per can. 227 times 46.9 is 10,650 Btu (11.2 megajoules). 250 times 46.9 is 11,700 Btu (12.4 megajoules). Both are the gross figure, the same basis as the 22,200 Btu quoted for a 16.4 oz propane cylinder, so the two pages compare directly: an 8 oz butane can holds just under half the energy of a 1 lb propane cylinder.

Runtime by rated Btu per hour#

The rating on the box or plate is the stove's input at full output: fuel burned per hour, expressed as heat.

For a stove not in the table: 10,650 divided by the rating is hours for an 8 oz can, 11,700 for an 8.8 oz can. For grams per hour, divide the rating by 46.9.

Read the "1.5 to 3 hours" figure against the table. Ninety minutes from 227 g needs an average of 7,100 Btu per hour, below the full rating of every stove in the claims table further down; three hours needs 3,550, a low simmer. Both happen in real cooking, which boils briefly and simmers long. On high, the answer is about an hour.

What manufacturers claim, checked against the can#

At the rated input, an 8 oz can cannot supply more than 10,650 Btu. A stated runtime above that ceiling means the stove is not burning at its rating for the whole time.

Coleman's claim is conservative. 1.25 hours at 7,650 Btu uses 82 percent of a 250 g can.

Iwatani's ZA-3HP and Eco-Premium figures sit on the ceiling. Within a few percent, and inside the rounding of an "approx." figure.

The VA-30 and 35FW figures do not fit a steady burn. Seventy minutes at 12,000 Btu and 55 minutes at 15,000 both need about 30 percent more energy than 227 g of butane carries. Iwatani's Japanese data, next, show how a stove gets there.

Why a real stove on high runs longer than the arithmetic#

Iwatani's Japanese specification pages publish two separate numbers for each cassette stove. Gas consumption is measured over the first 30 minutes at 20 to 25 degrees C and scaled to an hour. Continuous burn time is measured on high until one 250 g cassette is used up, at the same temperature. Divide 250 g by the burn time and you get the average rate over the whole can, which can be compared with the rated rate.

Every stove ran 5 to 32 percent longer than its rated consumption allows: averaged over the can, they burned 76 to 95 percent of their rated rate.

Iwatani does not say why. The physics gives a strong candidate: every gram that leaves the can has to boil first. NIST puts n-butane's enthalpy of vaporization at 22.4 kilojoules per mole, about 385 kilojoules per kilogram, so a 12,000 Btu burner drawing 256 g an hour pulls about 27 watts of heat out of the liquid. The can cools below room temperature, its vapour pressure falls, less gas reaches the jet, and the flame shrinks while the runtime stretches. The ZA-3HP's manual describes the opposite fix built into that stove: "Through the heat panel, the heat of burner is transferred to the canister to increase the vaporization inside the canister maximizing energy output."

So two numbers are useful:

• Energy divided by rating is the floor, if the stove truly holds its rating. Use it to count cans for cooking that needs real heat.
• The measured figure is a stove left on high at room temperature: about an hour per can, with a weaker flame towards the end.

If you would rather plan in litres than minutes: at the 18 g per litre derived in stove fuel planning for a pot with a lid and a windscreen, 227 g boils about 12 litres from cold. The stove fuel calculator runs it from a menu.

Why the "12,000 to 13,000 Btu" figure is too high#

Gas One's own blog says "Butane burns at roughly 26,000 BTU per pound of fuel" and that a 227 g cartridge therefore holds approximately 12,000 to 13,000 Btu. The NIST and EIA figures above give 46.9 Btu per gram, which is 21,300 Btu per pound. The 26,000 figure is about 22 percent high, and every runtime built on it inherits the error: the post's 72 minutes on a 10,000 Btu stove is 64 by the arithmetic, and its 48 minutes at 15,000 is 43.

The method is right; the constant is wrong.

How cold is too cold for a butane canister?#

n-butane's normal boiling point is about minus 0.5 degrees C (Iwatani's safety data sheet rounds it to minus 1). Below that, a can of pure n-butane has no pressure above the outside air to push gas to the burner. Iwatani's BU-6 is not pure: its safety data sheet lists butane at 50 to 70 percent, isobutane at 25 to 35 and propane at 0 to 5. Isobutane boils at about minus 12 degrees C, so it adds a little margin.

At 0 degrees C the blend has about a sixth of its 20 degree pressure, and the liquid is colder than the air once the stove has been burning for a while. This is why Iwatani's US manuals for the ZA-3HP, VA-30 and 35FW all say the stove is "not for use at temprature below freezing (32°F / 0°C)", and the 35FW manual warns that "it may be difficult to ignite the stove if the canister is cold". Iwatani's Japanese range has a separate cassette, Power Gold, described as having a higher isobutane ratio for low temperatures.

Coleman's manual says to store and operate its butane stove where the temperature will not fall below minus 18 degrees C. Read that as a limit for the appliance, not a promise about the fuel: at minus 18 degrees C neither n-butane nor isobutane has any gauge pressure at all.

If any propane is in the can, it boils off first, so a part-used can is weaker in the cold than a new one. Canister stoves in the cold works through that fractionation arithmetic and the safe ways to keep a can warm. For trips where nights fall below freezing, the 1 lb propane cylinder still has 1.5 bar of gauge pressure at minus 20 degrees C, and camping stove types compared covers the other options.

Measure your own stove's burn rate#

A kitchen scale turns the rating into a figure for your stove and your setting.

1. Weigh a new can to the gram and write the gross weight on it.
2. Run the stove for exactly 15 minutes at the setting you care about, outdoors or under a hood, in air above 15 degrees C so cooling does not throttle it.
3. Weigh again. Multiply the loss by 4 for grams per hour.
4. Convert to Btu per hour by multiplying grams per hour by 46.9. A 55 g loss is 220 g an hour, about 10,300 Btu per hour.
5. Compute your runtime: 227 divided by grams per hour is hours per 8 oz can. At 220 g an hour, about 62 minutes.
6. Repeat on your simmer setting. Most meals spend more time there than on high, and that number decides how many cans a trip needs.

To read a part-used can, weigh it and subtract the empty can. Iwatani's Japanese 250 g cassette weighs about 360 g, so its can is about 110 g; for other brands, weigh one empty can of the type once. How much gas is left in a canister explains the method and lists empty weights for screw-on backpacking canisters.

Common mistakes#

Quoting runtime without a rating. "A can lasts two hours" describes a stove averaging about 5,300 Btu per hour. On a 12,000 Btu burner held on high it is wrong by more than half.

Trusting a 26,000 Btu per pound figure. Butane carries about 21,300 Btu per pound. A runtime built on the higher number is about a fifth too long.

Using a cassette stove below freezing. At 0 degrees C a typical butane blend has about 0.2 bar of pressure to work with, and Iwatani rules it out. The flame fades or fails, and the can is not empty.

Blaming an empty can when it is a cold one. A frosted can with a weak flame is usually cold. Weigh it before you throw it away.

Running a pot that covers the can. Iwatani's ZA-3HP manual forbids cookware that extends over the canister cover: the can overheats. A heat panel designed in by the maker is not a licence for your own.

Leaving cans in a hot car. Iwatani says to store cans under 40 degrees C, Coleman under 49 degrees C, and Gas One's manual not above 110 degrees F. A car parked in sun beats all three, as set out in can you leave fuel canisters in a hot car.

Frequently asked questions#

How long does an 8 oz butane canister last on high?#

About an hour. An 8 oz (227 g) can holds about 10,650 Btu, which is 53 minutes on a 12,000 Btu burner held at its rating and 64 minutes on 10,000. Iwatani's measured times on high, using a 250 g can at 20 to 25 degrees C, run from 55 to 78 minutes, because the flame weakens a little as the can cools.

How many Btu are in an 8 oz butane canister?#

About 10,650 Btu. Butane carries about 46.9 Btu per gram (49.5 megajoules per kilogram, from NIST's enthalpy of combustion, confirmed by the EIA's conversion factor), and an 8 oz can holds 227 g. An 8.8 oz (250 g) can holds about 11,700 Btu, and a 1 lb propane cylinder about 22,200.

How long does an Iwatani butane canister last?#

Iwatani states approx. 55 minutes on the ZA-3HP (12,000 Btu per hour) and the 35FW (15,000), approx. 70 on the VA-30 (12,000) and approx. 65 on the Eco-Premium (10,000), each at maximum with a new can at 20 to 25 degrees C. Its US BU-6 can is 227 g. At lower settings it lasts longer: about 3.5 hours at a steady 3,000 Btu per hour.

Can I get 3 hours from one butane canister?#

Only at a low flame. Three hours from 227 g of butane means an average of about 3,550 Btu per hour, under half the full rating of every stove in the claims table above. That fits a meal that is mostly simmering. It does not fit boiling water or frying, which run near full output.

Does a butane stove work below freezing?#

Not reliably. n-butane boils at about minus 0.5 degrees C, and a typical blend with about a third isobutane has only around 0.2 bar of pressure at 0 degrees C. Iwatani says its US stoves are not for use below 0 degrees C. In cold weather, a propane cylinder or an isobutane-propane canister on a remote stove keeps working.

How do I know how much butane is left in a can?#

Weigh it and subtract the weight of an empty can of the same type. A Japanese Iwatani 250 g cassette is about 360 g full, so about 110 g empty. Divide the grams left by your stove's burn rate, about 256 g an hour for a 12,000 Btu burner on full, for the time remaining.