Quick answer: One US gallon of liquid propane weighs 4.24 pounds at 60°F (15.6°C) — the figure US suppliers, AmeriGas, and industry property tables quote. At a warmer 77°F (25°C) the same gallon weighs 4.11 pounds, because liquid propane expands as it warms. For everyday math, use 4.24 lb per gallon; for precise work, match the temperature.
The Short Answer: 4.24 Pounds per Gallon
If you only need one number, it is 4.24 lb per gallon. That is the standard US trade figure for liquid propane at 60°F, and it is the number behind nearly every tank label, refill quote, and delivery ticket in the United States.
But if you have ever searched this question before, you have probably also seen 4.11 pounds — and both numbers are correct. They simply describe propane at different temperatures. Propane is stored as a liquid under pressure, and like most liquids it gets less dense as it warms up. A gallon of cold propane weighs slightly more than a gallon of warm propane, because the warm gallon has expanded and contains fewer molecules.
That is why honest answers always come with a temperature attached. A number without a temperature is only half an answer.
Why You See Two Different Numbers: 4.24 vs 4.11
The two figures come from two different reference temperatures, and both trace back to published density data:
| Figure | Temperature | Source of the number |
|---|---|---|
| 4.24 lb/gal | 60°F (15.6°C) | US trade standard; quoted by AmeriGas and ELGAS LPG property tables |
| 4.11 lb/gal | 77°F (25°C) | Density of 0.493 kg/L, the published reference value at 25°C |
The difference is only about 3%, but it explains the confusion. A supplier quoting 4.24 lb/gal is using the 60°F trade figure. A calculator or science reference quoting 4.11 lb/gal is using the warmer 25°C laboratory density of 0.493 kg/L. Neither is wrong — they are measuring the same liquid at different temperatures.
The practical rule: use 4.24 lb/gal for buying, refilling, and tank math in the US, because that is the figure your supplier’s paperwork uses. Use 4.11 lb/gal only when you are working from the 25°C density figure, for example in a science or engineering calculation.
Propane Weight by Temperature
Because density changes with temperature, the weight of a gallon is not a single fixed number. These are the published anchor points:
| Temperature | Weight of 1 US gallon | Density |
|---|---|---|
| 32°F (0°C) | 4.37 lb | Coldest common reference — densest propane |
| 60°F (15.6°C) | 4.24 lb | US trade standard |
| 70°F (21°C) | 4.19 lb | Typical mild-day figure |
| 77°F (25°C) | 4.11 lb | Laboratory reference (0.493 kg/L) |
As a rule of thumb, a gallon of propane loses roughly 0.005 lb for every degree Fahrenheit it warms in the normal outdoor range. So on a hot 90°F afternoon, a gallon weighs closer to 4.05 lb; on a freezing 20°F morning, it weighs closer to 4.43 lb. The swing is small — a few percent — but it is real, and it is the reason suppliers fill tanks to only 80% of capacity: the liquid needs room to expand on a hot day without over-pressurising the tank.
Gallons to Pounds: Conversion Chart
Multiply gallons by 4.24 to get pounds at the 60°F trade figure:
| Gallons of propane | Weight (lb) at 60°F | Weight (kg) |
|---|---|---|
| 1 gallon | 4.24 lb | 1.92 kg |
| 2 gallons | 8.48 lb | 3.85 kg |
| 5 gallons | 21.2 lb | 9.62 kg |
| 10 gallons | 42.4 lb | 19.23 kg |
| 20 gallons | 84.8 lb | 38.46 kg |
| 25 gallons | 106 lb | 48.08 kg |
| 50 gallons | 212 lb | 96.16 kg |
| 100 gallons | 424 lb | 192.32 kg |
| 120 gallons | 508.8 lb | 230.79 kg |
| 250 gallons | 1,060 lb | 480.81 kg |
| 500 gallons | 2,120 lb | 961.62 kg |
| 1,000 gallons | 4,240 lb | 1,923.23 kg |
The formula is simple: weight (lb) = gallons × 4.24. In metric, weight (kg) = litres × 0.493 at 25°C, and one litre of liquid propane weighs about 1.09 lb.
Pounds to Gallons: The Reverse Conversion
Tanks are sold by weight, so the reverse question comes up just as often. Divide pounds by 4.24 to get gallons:
| Pounds of propane | Gallons (US) |
|---|---|
| 5 lb | 1.18 gal |
| 10 lb | 2.36 gal |
| 20 lb | 4.72 gal |
| 30 lb | 7.08 gal |
| 40 lb | 9.43 gal |
| 100 lb | 23.58 gal |
The formula: gallons = pounds ÷ 4.24. At the warmer 77°F density (4.11 lb/gal), the shortcut is gallons = pounds × 0.243.
How Much Propane Common Tanks Actually Hold
Portable cylinders are named by the weight of propane they hold, while home tanks are named by their total water capacity — and neither is filled to 100%. The 80% fill rule means a tank’s usable propane is less than its nameplate capacity:
| Tank | Propane it holds | Propane weight | Notes |
|---|---|---|---|
| 20 lb grill cylinder | 4.7 gallons | 20 lb | The standard backyard grill tank; about 37 lb full including the ~17 lb steel tank |
| 30 lb RV cylinder | 7.1 gallons | 30 lb | Common on travel trailers |
| 40 lb cylinder | 9.4 gallons | 40 lb | Forklifts and larger RVs |
| 100 lb cylinder | 23.6 gallons | 100 lb | Portable home-heating and construction use |
| 120-gallon home tank | ~96 gallons usable | ~407 lb | 80% fill of a 120-gal tank |
| 250-gallon home tank | ~200 gallons usable | ~848 lb | 80% fill of a 250-gal tank |
| 500-gallon home tank | ~400 gallons usable | ~1,696 lb | 80% fill of a 500-gal tank |
| 1,000-gallon home tank | ~800 gallons usable | ~3,392 lb | 80% fill of a 1,000-gal tank |
The 20 lb grill tank is the one most people meet. When it is “full”, it holds about 4.7 gallons of propane — not 5, and not the full water capacity of the cylinder. The 20 lb figure is the weight of the fuel alone; the steel cylinder itself (the tare weight, stamped “TW” on the collar) adds roughly 17 pounds, so a full grill tank weighs about 37 pounds in total.
How to Weigh Your Tank to See How Much Is Left
Propane tank gauges are notoriously vague, and many grill tanks have no gauge at all. Weighing is the accurate method, and it uses the same 4.24 figure:
- Find the tare weight (TW). It is stamped on the metal collar around the valve — for a standard 20 lb tank it is usually 17 to 18 lb.
- Weigh the tank. Stand it on a bathroom scale and note the total weight.
- Subtract the tare weight. Total weight minus TW equals the weight of the propane inside.
- Divide by 4.24. That gives you the gallons remaining.
Worked example: Your tank weighs 27 lb on the scale and the collar says TW 17 lb. That leaves 10 lb of propane. 10 ÷ 4.24 = 2.36 gallons left — a little over half a tank.
Second example: A tank weighs 32 lb with TW 17 lb. That is 15 lb of propane, or 15 ÷ 4.24 = 3.54 gallons — 75% full, good for roughly 15–18 hours of medium-heat grilling.
A quicker rough check is the hot-water method: pour warm water down the side of the tank and feel for the cold line where the liquid propane sits inside. It tells you roughly where the level is, but it cannot tell you the weight — only the scale can do that. How the methods compare:
| Method | Accuracy | Cost | Best for |
|---|---|---|---|
| Digital luggage scale | 95–99% | $15–25 | Precise tracking over time |
| Bathroom scale | 90–95% | $0 (already owned) | Quick home checks |
| Hot-water method | 70–80% | $0 | Rough check in a pinch |
| Magnetic level indicator | 80–85% | $15–20 | Visual monitoring between weighings |
| Pressure gauge | 50–60% | $10–30 | General indication only |
Pressure gauges deserve a warning: they measure vapour pressure, not fuel level, so they can read “fine” right up until the tank runs dry. A scale never has that problem.
Propane vs Water: Why Propane Feels So Light
One reason the 4.24 lb figure surprises people is that water weighs almost twice as much per gallon:
| Liquid (1 US gallon, ~60°F) | Weight |
|---|---|
| Water | 8.34 lb |
| Liquid propane | 4.24 lb |
| Gasoline | ~6.1 lb |
| Diesel | ~7.1 lb |
Propane is roughly half the weight of water, gallon for gallon. That is why a 20 lb tank feels manageable to carry while a 5-gallon jug of water (over 41 lb) feels like a workout. It is also why volume and weight get confused so easily with propane: a tank that looks small can still hold a useful amount of energy, because the energy is in the chemistry, not the weight.
How Much Energy Is in a Gallon of Propane?
Weight is only half the story — what most people really want to know is how long a gallon lasts. According to the US Energy Information Administration, one gallon of propane contains 91,452 BTU of energy.
What that means in practice:
- A typical gas grill burner running at 30,000 BTU per hour burns about one gallon every 3 hours.
- A 40,000 BTU furnace uses about one gallon every 2.3 hours at full output.
- A standard 20 lb grill tank (4.7 gallons, ~430,000 BTU) gives roughly 14 hours of grilling at 30,000 BTU per hour.
These are full-output figures; real appliances cycle on and off, so a gallon usually stretches further in normal use.
How Long Does a Gallon of Propane Last?
Weight and runtime are linked by how fast your appliance burns fuel. Typical burn rates for common equipment:
| Appliance | Typical burn rate | Hours from 1 gallon (4.24 lb) |
|---|---|---|
| Gas grill, medium heat | 1–2 lb per hour | ~2–4 hours |
| Gas grill, high heat | 2–3 lb per hour | ~1.5–2 hours |
| RV furnace | 1–2 lb per hour | ~2–4 hours |
| Outdoor patio heater | 2–3 lb per hour | ~1.5–2 hours |
A full 20 lb grill tank (4.7 gallons) therefore gives roughly 10–18 hours of medium-heat grilling. If you grill for two hours every weekend, one tank lasts about two months in summer.
Exchange vs Refill: The 15-Pound Trap
Here is a weight fact that costs people money. Exchange tanks from retailers — the swap-a-tank cages outside hardware and grocery stores — typically contain only 15 pounds of propane, not 20. That is 15 ÷ 4.24 = 3.54 gallons, a full gallon less than a properly refilled 20 lb tank, in a cylinder labelled “20 lb”.
A refill station, by contrast, fills your own tank to the full 20 lb (4.7 gallons). Typical US prices run about $20–25 for a 15 lb exchange versus $15–20 for a full 20 lb refill — so the exchange costs more per pound ($1.33–1.67/lb) than the refill ($0.75–1.00/lb). Over a year of regular grilling, refilling instead of exchanging can save $50–100. The only advantage of exchange is convenience: no waiting, and no worries about your tank’s recertification date.
What Else Changes the Weight of a Gallon?
Temperature is the main factor, but three smaller ones are worth knowing:
- Propane grade. Vehicle-grade HD-5 propane must be at least 90% propane with no more than 5% propylene. Its density differs from pure propane by only a few percent, so the 4.24 lb/gal figure stays a close estimate for HD-5 and commercial grades too.
- Liquid vs vapour. Everything in this article is about liquid propane — the form stored in the tank. Released as vapour, one gallon of liquid expands to roughly 270 times its volume as gas. Never feed vapour figures into a liquid conversion, or the numbers will be wildly wrong.
- Elevation and pressure. Liquid propane is stored at roughly 100–200 psi, and its liquid density barely changes with elevation. For home, grilling, and RV math, elevation is not worth correcting for — temperature is the only correction that matters, and commercial custody-transfer measurements are temperature-corrected to a reference point for exactly that reason.
US Gallons vs Imperial Gallons
If you are reading a UK or Canadian source, watch out: an imperial gallon is bigger than a US gallon (4.546 litres vs 3.785 litres). One imperial gallon of liquid propane weighs about 5.09 lb at 60°F. Whenever a figure looks about 20% too heavy, check which gallon the source is using — the US gallon is the smaller one, and the 4.24 lb figure belongs to it.
Metric equivalents for reference: one US gallon of propane is 1.92 kg at 60°F (1.87 kg at 77°F), and one litre is 0.493 kg at 25°C.
Frequently Asked Questions
How much does 1 gallon of propane weigh in pounds?
4.24 pounds at 60°F, the US trade figure. At 77°F it weighs 4.11 pounds. Use 4.24 for supplier quotes, tank math, and refill estimates.
How many gallons are in a 20 lb propane tank?
About 4.7 gallons when full (20 ÷ 4.24 = 4.72). A full 20 lb tank weighs about 37 lb in total, including the ~17 lb steel cylinder.
How much does a gallon of propane weigh in kg?
1.92 kg at 60°F and 1.87 kg at 77°F. One litre of liquid propane weighs 0.493 kg at 25°C.
Does propane weigh more when it is cold?
Yes. Cold propane is denser, so a gallon weighs more: about 4.37 lb at 32°F versus 4.11 lb at 77°F. The weight falls roughly 0.005 lb per degree Fahrenheit of warming.
How much does a full 20 lb propane tank weigh?
About 37 pounds — 20 lb of propane plus roughly 17 lb for the empty steel cylinder (the tare weight stamped on the collar).
How do I calculate the weight of propane in my tank?
Weigh the tank, subtract the tare weight (TW) stamped on the collar, and you have the propane weight in pounds. Divide that by 4.24 to get gallons remaining.
How many BTU are in a gallon of propane?
91,452 BTU per US gallon, according to the US Energy Information Administration. A 20 lb grill tank holds roughly 430,000 BTU in total.
Is propane heavier or lighter than water?
Much lighter. A gallon of water weighs 8.34 lb; a gallon of liquid propane weighs 4.24 lb — about half as much.
Why does my exchanged 20 lb tank feel lighter than a refilled one?
Exchange tanks typically contain only 15 lb of propane (3.54 gallons), not the full 20 lb (4.7 gallons) a refill station puts in. Same cylinder, a gallon less fuel.
Bottom Line
One gallon of liquid propane weighs 4.24 pounds at 60°F — the number to use for tank math, refills, and delivery quotes. Remember the temperature: the same gallon weighs 4.11 lb on a warm 77°F day and 4.37 lb on a freezing one. Divide pounds by 4.24 to turn a tank’s weight into gallons, weigh your tank against its stamped tare weight to see what is left, and never fill past 80% — the liquid needs that space to expand when the weather warms up.
