Can you mail fuel canisters to a trail resupply?

Yes, inside the United States, by USPS Ground Advantage only. Isobutane canisters are a flammable gas, so they may never fly. They ride by truck as a "Limited Quantity" with a marked diamond on the box. The limit is 1 litre per box, and it counts the size of the cans, not the weight of the fuel. You cannot mail them to or from another country.

What the rule actually says#

The rule is in USPS Publication 52, the Postal Service's book on hazardous mail: section 342 for gases, and Packaging Instruction 2A for flammable gases. Three terms from those pages decide everything else.

Division 2.1 flammable gas. This is the hazard class your canister fuel falls in. Pub 52 defines it as a material that is a gas at 20 degrees C (68 degrees F) at normal air pressure, and burns in air. Isobutane, butane and propane all qualify. Pub 52 names butane and propane by name as mailable "via surface transportation as Limited Quantity materials".

Limited Quantity surface. A Limited Quantity is a small amount of a hazardous material, in a small package, that is let off the full hazmat paperwork. "Surface" means it travels by ground or by boat, never by plane. Pub 52 says these materials "must not, under any circumstances, be transported by air". It also says they "generally must also qualify as a Consumer Commodity". That is a product packaged for retail sale and for personal or household use. A shop-bought canister is that kind of product.

Capacity. This is how much water the empty can would hold if you filled it to the brim. It is not the weight of fuel printed on the label. A "230 g" can holds 230 g of fuel, but the steel shell around it has a capacity of at least 465 mL, as worked out below.

With those three terms, section 342.3 reads simply. Metal cans are allowed "with a water capacity up to 33.8 fluid ounces", which is 1 litre. Flammable gases are limited to 1 litre "in a metal primary receptacle per mailpiece". The fuel must not fill the can completely at 130 degrees F (54.4 degrees C). The valve must be protected so it cannot go off by accident. Instruction 2A adds that several cans may share one strong box, "provided the total volume of flammable gas does not exceed 33.8 fluid ounces (1 liter) per mailpiece".

Does 1 litre mean the can or the fuel?#

The can. Two parts of Pub 52 say so in so many words. Section 342.3 states the limit as "water capacity". The hazardous materials table in Appendix A lists canisters as "Receptacles, small, containing gas (gas cartridges)... not exceeding 1 L capacity". Both measure the container.

Instruction 2A's box limit is worded as "total volume of flammable gas". A gas has no volume of its own: it fills whatever holds it. So the only volume it can mean is the volume of the cans, added up. That is also the only reading that agrees with 342.3, which puts the 1 litre limit "in a metal primary receptacle per mailpiece".

Counting only the liquid fuel would give a looser answer, and nothing in Pub 52 supports it. This page uses the capacity reading throughout.

Two pressure rules also apply, and canister fuel clears both. Pub 52 bans any can above 180 psig (pounds per square inch above air pressure) at 130 degrees F. It requires a DOT 2P or 2Q can from 140 psig up. We model an 80/20 isobutane and propane blend, MSR's stated mix, by Raoult's law (which treats the blend as an ideal mix), from NIST vapour pressures. It reaches about 136 psig at 130 degrees F. Pure propane reaches about 258, which is why the propane cassette question has a different answer.

Why "Retail Ground" and "three canisters" keep circulating#

Neither piece of advice matches the current rule.

Retail Ground. This was the ground service sold at the post office counter until 2023. USPS's final rule, effective 9 July 2023, has a heading that reads "USPS Retail Ground Discontinued". Its replacement is USPS Ground Advantage. Pub 52's definitions page now says surface rules apply to hazardous mail "sent at the Parcel Select Lightweight or USPS Ground Advantage prices". So the advice survives, renamed. Ask for Ground Advantage.

The reverse mistake costs more. Priority Mail and Priority Mail Express are air services in Pub 52's terms. Section 327 says material banned from air "may not be sent as Priority Mail Express or Priority Mail". Section 336 adds that you may not put surface labels on Priority Mail to avoid air transport. A Flat Rate box is Priority Mail, so fuel cannot go in one.

Three canisters max. No count appears anywhere in Pub 52's gas rules. The limit is litres. Three small cans may fit or may not. Three standard 230 g cans cannot, as the next section shows. A clerk accepting a box does not make it legal.

How many canisters fit in 1 litre?#

No canister maker we found publishes the water capacity of its cans. Two numbers can still be worked out, and between them they settle most boxes.

The floor. Pub 52 says the fuel must not fill the can at 130 degrees F (54.4 degrees C). Liquid expands as it warms. So the can must hold at least the volume of its fuel at that temperature. NIST gives isobutane 511 kg per cubic metre and propane 440 at 54.4 degrees C. An 80/20 blend then takes up about 2.02 mL per gram. That gives each can a smallest possible capacity.

The ceiling. A can fits inside a straight cylinder of its own width and height. Its domed ends and rim only make it smaller. So the outside cylinder is the largest the capacity can be. The makers publish those dimensions.

Add the cans in a box and compare against 1,000 mL. If the ceilings add up to less, the box is within the rule. If the floors add up to more, it is over. If 1,000 falls between them, the box cannot be shown to pass.

The 450 g can sits on that line by itself. Its floor is 909 mL, so it uses at least nine tenths of a box. The European canister standard, EN 417, covers cartridges of 50 to 1,000 mL, so an EN 417 can should not be over 1 litre. MSR does not say its can is made to EN 417, and the outside cylinder is 1.5 litres. Without the maker's capacity figure, one 450 g can is not a box you can show is within the rule.

Which boxes are allowed#

Each row is the sum of the table above, set against the 1,000 mL limit in 342.3 and Instruction 2A.

"Not provable" is not a polite yes. The floors show the box might be legal. Nothing published shows that it is. If a counter clerk asks, you cannot show it.

The fix is simple. The limit is per box, not per customer. Two boxes, each inside 1 litre, are two legal mailpieces.

A worked case: two hikers, eight days, one post office#

Take two hikers who plan an eight-day stretch to a trail town. They cook on a canister stove with a lid and windscreen. Our stove fuel planning page puts that kind of trip at about 20 g of gas per person per day.

1. Fuel needed. 20 g times two people times eight days is 320 g, before any spare.
2. The obvious box. One 450 g can covers it. But its capacity is at least 909 mL, and no figure shows it is under 1,000. That box is not provable.
3. The next obvious box. Two 230 g cans give 460 g. Their floors add to 930 mL and their ceilings to as much as 1,900. Also not provable.
4. What does work. Box one holds one 230 g can, at no more than 950 mL. Box two holds two 110 g cans, at no more than 860 mL. That is 450 g of fuel in two boxes, and each box is provably within the rule.
5. The send. Both boxes go by Ground Advantage, each with the Limited Quantity diamond on the address side, caps on, cans packed so they cannot move.

Walked through, the answer is two boxes rather than one. It also shows the margin that small cans give. Two 110 g cans carry less fuel than one 230 g can, but they leave more room under the limit. Use the fuel calculator to set the grams before you pick the cans.

How to pack and mark the box#

Instruction 2A and section 342.4 set out the steps.

1. Leave the cans as sold. A retail can in its original form matches Pub 52's description of a "Consumer Commodity", which the surface rules expect. Do not refill or decant.
2. Protect the valve. Pub 52 requires "screw-thread caps" or similar to stop an accidental release. Keep the plastic cap on every can.
3. Cushion each can. Instruction 2A asks for enough cushioning "to absorb shock and prevent damage". Wrap them so they do not touch the box walls or each other.
4. Use a strong box. It must hold the cans "firmly and securely". A resupply box packed with food is fine if the cans cannot shift.
5. Mark the address side. Add the Limited Quantity surface mark: a diamond, black top and bottom, white centre. Each side must be at least 100 mm (3.9 inches), with a 2 mm border. A 50 mm mark is allowed only where the box is too small. No shipping name, ID number or shipping paper is needed for surface mail.
6. Write full addresses. Instruction 2A requires "a complete return and delivery address".
7. Ask for Ground Advantage. Tell the clerk the box holds a flammable gas. Never send it Priority Mail.

Common mistakes#

Mailing a used can home in the bounce box. Pub 52 says empty used gas containers are "mailable under the same restrictions as when filled". A part-used can counts as a full one. New, never-filled containers are the only exemption. To judge what is left before you decide, see how much gas is left in a canister.

Hiding the fuel in a box of food. An unmarked box with flammable gas inside breaks the marking rule, whatever else is in it. It also takes away the one sign that tells handlers to keep it off a plane.

Leaving the box in a hot car. Pub 52 sets its can limits at 130 degrees F (54.4 degrees C). A car parked in the sun can get that hot inside, as can you leave propane in a hot car shows with the measured cabin figures.

Expecting a retailer to ship to a PO box. Primus says it ships fuel to the lower 48 states only, in case packs, and "cannot ship fuel to PO boxes". MSR does not sell its fuel online, "due to hazardous materials shipping restrictions". A trail town shop is often the simpler source.

Check your own box before you mail it#

You need a ruler and the label on each can.

1. Measure each can. Write down its width and its height, in centimetres, valve included.
2. Work out the ceiling. Multiply 0.785 by the width squared and then by the height. The answer is in millilitres. A 10 cm wide, 9 cm tall can gives 0.785 times 100 times 9, or about 707 mL.
3. Work out the floor. Multiply the net grams on the label by 2.02. That gives the fuel's volume at 130 degrees F for an 80/20 blend. Blends with more n-butane are a little denser, so for those 2.02 gives a floor slightly too high. Treat a total just over 1,000 mL as not provable rather than over.
4. Add up the box. If the ceilings total under 1,000 mL, the box is within the rule. If the floors total over 1,000, it is not. Anything in between, split it into two boxes.

What this answer does not cover#

This page covers USPS domestic mail under Publication 52 as published on 3 November 2026, for screw-on isobutane and propane canisters. It does not cover UPS or FedEx. Their hazmat guides were not read for this page, and their rules are their own. It does not cover Hawaii, Alaska, US territories or military addresses. Section 327 lets only lithium battery devices use the air-only ZIP code route, so gas has no route to a place that mail reaches only by air. Liquid fuels such as white gas and alcohol fall under Pub 52's Class 3 rules, not this one. Butane cassettes and 1 lb propane cylinders are not covered either. By the filling arithmetic on our refill page, a 1 lb cylinder needs at least 1.1 litres of capacity, already over the limit. The capacity floors and ceilings are our arithmetic from NIST data and makers' published sizes. They are not measured capacities, and a maker's own figure overrides them. The final call on any mailpiece belongs to the Postal Service. It can rule on a specific item through its Pricing and Classification Service Center. Flying with a stove is a separate question, covered in can you take a camping stove on a plane.

Frequently asked questions#

Can you mail isobutane fuel canisters through USPS?#

Yes, within the United States, by USPS Ground Advantage only. Publication 52 treats canister fuel as a Division 2.1 flammable gas. It is mailable by ground as a Limited Quantity, with the Limited Quantity diamond on the address side. The cans must be metal, no more than 1 litre in capacity in total per box, with their valve caps on. Air mail, including Priority Mail, and international mail are prohibited.

How many fuel canisters can you mail in one box?#

There is no count in the rule. The limit is 1 litre of can capacity per box. One 227 or 230 g can fits, at no more than 950 mL. Two 100 or 110 g cans fit, at no more than 890 mL. Three 230 g cans need at least 1,377 mL, so they never fit. Two 230 g cans or one 450 g can cannot be shown to fit.

Can you send fuel canisters by Priority Mail or in a Flat Rate box?#

No. Publication 52 says material banned from air transport may not be sent as Priority Mail or Priority Mail Express. Flammable gases are always banned from air. Flat Rate boxes are Priority Mail, so they are out too. Putting a ground label on a Priority Mail box to get around this is itself prohibited.

Is USPS Retail Ground still the service for fuel?#

No, because Retail Ground no longer exists. USPS discontinued it on 9 July 2023 and folded it into USPS Ground Advantage. Publication 52 now says surface rules apply to hazardous mail sent at Ground Advantage or Parcel Select Lightweight prices. Ask for Ground Advantage at the counter, and say the box holds a flammable gas.

Does the 1 litre limit mean the fuel or the can?#

The can. Publication 52 states the limit as "water capacity", meaning how much water the empty can would hold. Its hazardous materials table lists gas cartridges "not exceeding 1 L capacity". A 230 g can holds at least 465 mL by capacity, because the fuel must not fill the can when heated to 130 degrees F.

Can you mail fuel canisters to Canada?#

No. Publication 52 prohibits all gases in international mail, and Instruction 2A repeats it for flammable gases. Inside Canada, the Non-mailable Matter Regulations bar dangerous goods unless Canada Post can carry them under transport law. Canada Post lists camping stove and BBQ cylinders as Class 2 dangerous goods. Plan to buy fuel in Canada.

Can you mail an empty fuel canister?#

Not as ordinary mail if it has ever held fuel. Publication 52 says empty used gas containers are mailable only "under the same restrictions as when filled", because some gas may remain. So a used can goes by Ground Advantage, marked and within the 1 litre limit, like a full one. Only new, never-filled containers are free of the rule.