Key takeaways
- 49 CFR 173.6 allows Class 3 liquids such as gasoline in containers up to 30 L (8 gal) each, 440 lb of hazmat per vehicle, with no outer packaging when the can is secured in a box or compartment.
- The rule covers carrying for a business. An individual driving a private vehicle for non-commercial purposes is outside the federal hazmat rules altogether.
- UWS crossover box instructions carry a DANGER label against storing or transporting containers of gasoline, solvents and propane tanks.
- Gasoline's flash point is -45 F and its lower explosive limit 1.4 percent. About 26 mL of liquid, under 1 fl oz, holds enough vapor to reach that limit in an 11 cu ft box.
- A closed vehicle gains about 40 F in an hour in sun, which takes a 72 F day past gasoline's 102 F boiling point.
Two answers circulate, and both are quoted correctly. The hazmat rule for work trucks names boxes and compartments as acceptable places for a fuel can. The tool box's own manual says, under a DANGER heading, never to do it. The regulation sets what a business may lawfully transport. The box maker says what its box is safe to hold. Below are both layers, then the arithmetic that decides between them.
Layer one: the federal materials-of-trade rule
Gasoline on the road falls under the Hazardous Materials Regulations (HMR) in 49 CFR. Full compliance means shipping papers, placards and trained drivers, so the HMR carve out materials of trade: small amounts a business carries in support of work that is not itself transportation.
The definition in 49 CFR 171.8 covers three purposes: the health and safety of the operator or passengers, operating or maintaining the vehicle and its auxiliary equipment, and carriage by a private motor carrier "in direct support of a principal business that is other than transportation by motor vehicle." A contractor's generator fuel sits in the third.
What 49 CFR 173.6 then allows for gasoline:
| Requirement | What 173.6 says | What it means for a gas can |
|---|---|---|
| Quantity per container | Class 3, Packing Group II and III: 30 kg (66 lb) or 30 L (8 gal) | A 5 gal can qualifies. A 10 gal transfer tank does not. |
| Packing Group I | 0.5 kg (1 lb) or 0.5 L (1 pint) | Does not apply to gasoline (see below) |
| Container | "For gasoline, a packaging must be made of metal or plastic" and meet the HMR or OSHA 29 CFR 1910.106(d)(2) or 1926.152(a)(1) | An approved safety can or a DOT-compliant can, not a milk jug or a drinks bottle |
| Condition | Leak tight for liquids | Cap and spout closed and sealing |
| Outer packaging | Not required for receptacles "secured against shifting in cages, carts, bins, boxes, or compartments or by other means" | The can may ride bare if it cannot slide or tip |
| Marking | Marked with "a common name or proper shipping name" | "Gasoline" on the can |
| Vehicle total | Aggregate gross weight of all materials of trade "may not exceed 200 kg (440 pounds)" | Every hazmat on the truck, containers included |
| Driver | Must be informed of the hazmat present and of the requirements of the section | Whoever drives the truck has to know the can is there |
Why gasoline is Packing Group II. 49 CFR 173.121 assigns Class 3 liquids by flash point and initial boiling point. Packing Group II is a flash point under 73 F (23 C) with an initial boiling point over 95 F (35 C). The NIOSH Pocket Guide gives gasoline a flash point of -45 F (-43 C) and a boiling point of 102 F (39 C), so on those figures it lands in Packing Group II and gets the 8 gal per container limit, not the 1 pint limit.
The 440 lb is generous for fuel. At about 6.1 lb per gallon, the figure used in truck box weight, payload and fuel cost, a full 5 gal can is 30.5 lb of liquid and a full 8 gal can 48.8 lb, before the can's own weight. The 440 lb binds when fuel shares the truck with cylinders, paint and solvent, not when fuel rides alone. Every pound also comes out of payload.
Read the outer-packaging row again. The federal rule names boxes and compartments as acceptable ways to secure a receptacle. That sentence is where the "it's legal in the tool box" answer comes from, and on the law it is correct.
Who the federal rule does not cover
The materials-of-trade exception is a carve-out for businesses. The opposite case sits outside the HMR entirely. 49 CFR 171.1(d)(6) lists, among functions the HMR do not apply to, "transportation of a hazardous material by an individual for non-commercial purposes in a private motor vehicle, including a leased or rented motor vehicle."
| Who is carrying the fuel | Federal hazmat status | What governs the box |
|---|---|---|
| Homeowner fueling a lawn mower, personal truck | Outside the HMR under 171.1(d)(6) | The box manufacturer's instructions |
| Self-employed contractor, fuel for jobsite equipment | Material of trade under 173.6, if within limits | 173.6, plus the box manufacturer's instructions |
| Employee in a company truck | Material of trade under 173.6, if within limits | 173.6, the employer's policy, and the box manufacturer's instructions |
| Over 8 gal per container or 440 lb per vehicle | No longer materials of trade | Full HMR: this page does not cover it |
That table is the federal layer only, and an employer can be stricter than the regulation.
An employer policy that is stricter than the rule
The USDA Forest Service published a question and answer sheet telling its drivers how to carry fuel under the materials-of-trade exception. It is an older document (it cites the 2000 Emergency Response Guidebook), but it shows how a large fleet applied 173.6 in practice.
| Point | 49 CFR 173.6 | Forest Service driver guidance |
|---|---|---|
| Gasoline container size | Up to 8 gal (30 L) | One answer allows "a gross capacity not over 8 gallons", a summary table limits gasoline to "a container that is no larger than 5 (five) gallons" |
| Fill level | Not specified | "No container will be more than 90% full at any time" |
| Approved containers | Metal or plastic, meeting HMR or OSHA rules | Approved by DOT (UN), UL, FM or OSHA. Older military jerry cans whose only marking is the obsolete DOT 51 or DOT 53 cannot be used |
| Securing | Secured against shifting, boxes and compartments named | Secured "in 4 directions of movement or placed in a secure box or compartment attached to the vehicle" |
| Cab | Not addressed | "Flammables cannot be carried inside the passenger compartment of any vehicle" |
| Aggregate | 440 lb | 440 lb, "including load on a pulled trailer" |
The sheet disagrees with itself on can size, so pick the 5 gal can: both answers allow it. The 90 percent fill leaves room for the liquid to expand as it warms.
Layer two: what the tool box maker says
The installation manual for UWS crossover truck tool boxes (UWS is part of CURT Group) carries this under a DANGER heading:
Danger of explosion. Do not use this product for storing or transporting flammables, explosives, hazardous materials or hazardous waste, such as containers of gasoline, solvents, gun powder, dynamite, propane tanks, acetylene tanks and cutting torches.
That is a statement about the product, not a regulation, and it reaches further than 173.6 in two ways. It covers transporting as well as storing, so "only for the drive to the site" is not an exception. And it names propane and acetylene tanks, which 173.6 lets trades carry daily, but not in this box.
When a regulation permits something and the equipment's maker forbids it, follow the stricter instruction. The regulation never assessed that particular box.
Why the maker is right: vapor and heat
A truck box is built to keep water and dust out, with a lid gasket and latches that pull it down tight, in every one of the crossover, side mount, chest and low profile formats. That seal is what turns a fuel can into a hazard.
The vapor arithmetic
Gasoline vapor will burn when it makes up between 1.4 and 7.6 percent of the air, the lower and upper explosive limits in the NIOSH Pocket Guide. Its flash point of -45 F (-43 C) means it gives off enough vapor to ignite at any temperature above -45 F, which is nearly every day a truck works. There is no season when an open film of gasoline is safe in a closed space.
Now the volume. A full-size crossover box holds roughly 11 cu ft (0.31 m3), about 311 L of air.
Under an ounce: the drip down the outside of a can after filling, or a spout not quite closed. It takes no leaking can, only a handled one.
The box does not have to be airtight for this to matter, and a gasketed lid is not. It only has to exchange air more slowly than the vapor arrives, and a gasket is there to make that exchange slow.
Even a closed, compliant can is not perfectly tight. The EPA limit for portable fuel containers, 40 CFR 59.611, is 0.3 g of hydrocarbons per gallon per day, so a legal 5 gal can may emit up to 1.5 g a day. In a box that breathes, that alone is not a fast route to 19.6 g. It does mean "the cap is on" is not "no vapor."
The heat arithmetic
The same rule lets a can carry a manually operated pressure relief device, provided it closes automatically. A modern can is built to stay shut, so as it warms, the pressure inside rises. The vapor comes out all at once, when someone opens the cap or presses the relief.
No published measurement covers the air inside a truck box. The closest measured case is a closed vehicle. A Stanford study published in Pediatrics in July 2005 found a parked car's interior gained an average of 40 F in an hour on sunny days with highs of 72 to 96 F, with 80 percent of the rise in the first half hour.
| Day's high | Closed vehicle after one hour | Against gasoline's 102 F (39 C) boiling point |
|---|---|---|
| 72 F (22 C) | about 112 F (44 C) | 10 F above |
| 80 F (27 C) | about 120 F (49 C) | 18 F above |
| 96 F (36 C) | about 136 F (58 C) | 34 F above |
A closed metal box with its lid facing the sun is the same kind of space, and a dark lid runs hotter, as the cool roof figures in whether you can leave tool batteries in a truck box show. So on an ordinary sunny day, a can in the box spends the afternoon above the temperature at which gasoline starts to boil. The pressure is highest exactly when the box is shut and the truck is parked.
Put the two together and the box maker's word "explosion" is precise. A box full of vapor needs only a spark, and a tool box is a steel or aluminum shell full of steel tools, often with a lid light wired in.
Where the gas can should ride
The aim is to meet 173.6 without using the box: secured against shifting, in open air, away from the cab.
- Use the right can. Metal or plastic, approved under the HMR or OSHA rules, marked "Gasoline," 5 gal.
- Fill it to 90 percent. Leave the expansion space the Forest Service sheet asks for, because the liquid grows as it warms.
- Wipe it dry. The ounce in the vapor box above is the drip on the outside of the can.
- Ride it in the open bed, upright. Strap it to a tie-down or block it in a bin or rack so it cannot slide or tip. That is "secured against shifting by other means," which 173.6 accepts.
- Never in the cab. The Forest Service bars flammables from the passenger compartment outright.
- Shade it if you can, and do not open it hot. Let a hot can cool before you release the cap, and open it pointed away from you.
- Take it out at the end of the day. The truck is not storage. The tools that stay overnight are what the box is for, protected by the layers in jobsite box security, and the fuel can is not one of them.
Where this answer stops applying
- Over the limits. Above 8 gal in one container or 440 lb of materials of trade on one vehicle, the exception ends and the full HMR apply. Bed-mounted transfer tanks are a separate subject.
- State rules. This page covers the federal layer. A state can regulate transport within its borders on its own terms.
- Other compartments. The UWS warning is for a general-purpose crossover tool box. If the maker of a service body or compartment states in writing that it is designed to carry fuel, follow that maker's instructions. If they are silent, treat it as a tool box.
- Diesel and other fuels. The vapor arithmetic above uses gasoline's figures and does not transfer to other fuels. The UWS warning names flammables generally, so the box answer stays the same.
- Propane and acetylene. Covered by the same box warning. Cylinder securing rules are not covered here.
What is in the box instead should be the tools, and the box itself should be chosen on the trade-offs in aluminum vs steel vs polymer truck boxes.