Is there a difference in propane quality?

For home heating, very little. US propane is made to published industry specifications that cap what else can be in the tank besides propane, so one dealer's gallon burns much like another's. Where quality does differ, it is in the grade a particular use calls for, in contamination picked up along the way and in the service wrapped around the fuel, not in some premium molecule one supplier sells and another does not.

The specifications behind every gallon

Two industry standards define propane quality in the US: ASTM D1835, the standard specification for liquefied petroleum gases, and GPA Standard 2140 from the Gas Processors Association. ASTM's covers propane, propylene (also called propene), butane and mixtures of them, for home, commercial and industrial heating and for engine fuel. Both set limits on composition and on physical properties such as vapor pressure and residue.

California's rule for propane sold as motor fuel shows what those limits look like in practice. It caps vapor pressure at 208 psig at 100°F, limits the oily residue left when a sample evaporates, caps sulfur at 80 parts per million by weight, and requires the fuel to pass a moisture test and a copper-strip corrosion test. It also requires an odor strong enough to detect at no more than one-fifth of the concentration at which propane can ignite. Propane has that smell because an odorant, ethyl mercaptan, is added for leak detection, per DOE's Alternative Fuels Data Center.

HD-5, the grade most people mean

The best-known grade is HD-5. Under the Gas Processors Association's HD-5 specification, as DOE's Alternative Fuels Data Center summarizes it, the fuel must be at least 90 percent propane, no more than 5 percent propylene and 5 percent other gases, mainly butane and butylene. It is the tighter grade, and it is the one specified for propane used in vehicles. The same standards also define a commercial propane grade with looser limits.

California writes its own motor-fuel limits, a mix often called HD-10 in the trade: at least 85.0 percent propane and up to 10.0 percent propene, with butanes capped at 5.0 percent, butenes at 2.0 percent and pentenes and heavier at 0.5 percent. The biggest difference between the two is the propylene allowance: 5 percent against 10.

Propane and propylene travel together

Propylene is a close chemical relative of propane that moves through the same supply chain, and EIA's weekly inventory report counts the two as one number.

The US stocks chart above is propane and propylene in primary storage, so a reader comparing it with any propane-only figure is not comparing like with like. The grades above are how the industry keeps the propylene share of a delivered gallon inside known limits.

Worked example: where the price gap really comes from

The grade of a gallon is set upstream, long before a dealer prices it, and the site's own numbers show where the price of a delivered gallon actually builds up: between the hub and the tank.

In the last week of EIA's heating-season survey, ending March 30, 2026, the US residential price was $2.674 a gallon and the gap over the Mont Belvieu spot price was $1.839, so the hub price that week was about 83.5 cents. That $1.839 pays for pipelines and terminals, small-load truck delivery, the tank, service calls, carrying inventory into winter and the dealer's margin, the layers traced in how propane is priced. The hub itself trades year-round; Mont Belvieu propane is $0.919/gal at its latest daily reading.

Where quality problems actually start

A dealer does not make propane. It buys fuel produced at gas processing plants and refineries and moved through pipelines, storage and terminals before it ever reaches a delivery truck. The specification tests screen for water, heavy residue, sulfur and corrosiveness, the contaminants a gallon can carry whoever delivers it. A household comparing suppliers is mostly comparing price, delivery reliability, tank arrangements and contract terms, all of which vary more from dealer to dealer than fuel made to the same specification can.

The Fill Line is a free weekly read of the public propane numbers: the state price survey from October through March, and stocks, the Mont Belvieu spot and the weekly supply balance year round. One email a week on what moved.