How many gallons does a 20 lb propane take?

A standard 20 lb propane tank holds approximately 4.7 gallons of propane when full. The exact amount can vary slightly based on temperature and altitude, but 4.7 gallons is a good average. Propane weighs about 4.2 lbs per gallon, so a 20 lb propane tank contains about 4.7 gallons (20 / 4.2 = 4.7).

What is the capacity of a 20 lb propane tank?

Most 20 lb propane tanks have a capacity of 18-19 gallons. However, they are not filled completely full with liquid propane. This is because propane expands significantly from liquid to gas form. Filling the tank 100% full with liquid propane would create unsafe pressure levels.

Instead, 20 lb propane tanks are filled to only about 80% capacity with liquid propane, which takes up around 4.7 gallons. The remaining capacity is left as head space for the propane to expand into gas form.

How is the capacity of a propane tank measured?

Propane tank capacity is measured by weight, not volume. A “20 lb” propane tank means the tank holds about 20 lbs of propane when full. Since propane weighs 4.2 lbs per gallon, a 20 lb tank holds about 20 / 4.2 = 4.7 gallons.

Measuring by weight instead of volume accounts for changes in propane density due to temperature. 4.7 gallons of propane weighs 20 lbs whether it’s liquid or gas. If measured by volume, the amount would vary significantly between liquid and gas states.

Do all 20 lb propane tanks hold 4.7 gallons?

Most standard 20 lb propane tanks will hold around 4.7 usable gallons of propane, but the exact amount can vary slightly depending on:

  • Tank size – Some 20 lb tanks may be a little larger or smaller in capacity.
  • Temperature – Propane density increases at lower temperatures.
  • Altitude – Propane density decreases at higher altitudes.
  • Tank pressure – Tanks may not always be filled to the maximum safe level.

So while 4.7 is a good standard estimate, the usable capacity may range from 4.5 to 5 gallons for a 20 lb propane tank.

Why are propane tanks only 80% full?

Propane tanks are only filled to about 80% capacity with liquid propane because propane expands greatly as it converts from liquid to gas form. Filling the tank all the way would create unsafe pressure levels as the liquid propane expands.

The remaining 20% capacity is left as head space where the propane can expand into gas form. The head space allows the propane gas to have room to expand and prevents pressure from building up inside the tank.

Some key facts about propane expansion:

  • Propane takes up about 270 times more space in gas form than liquid form.
  • Pouring 20 lbs of liquid propane into a hot tank would result in over 1000 psi pressure.
  • Typical tank pressure relief valves release at 375 psi.
  • Filling to only 80% capacity leaves room for propane to expand safely.

Does temperature affect how much propane will fit?

Yes, temperature has a significant effect on the density of propane. Colder temperatures allow more liquid propane to fit safely in the tank:

Temperature Gallons held by 20 lb tank
70°F 4.5 gallons
60°F 4.6 gallons
30°F 4.8 gallons

As shown, a 20 lb propane tank can hold about 4.8 gallons at cooler 30°F temperatures. But at hot 70°F temperatures, the capacity drops to 4.5 gallons.

The effect is reversed when going from liquid to gas. Cold temperatures decrease propane expansion, allowing more liquid to fit. Hot temperatures increase expansion rates, decreasing capacity.

Does altitude affect propane tank capacity?

Yes, altitude also affects the density and capacity of propane. At higher altitudes, lower atmospheric pressure causes propane to expand more. This means less liquid propane can fit safely in the tank:

Altitude Gallons held by 20 lb tank
Sea level 4.7 gallons
2000 ft 4.6 gallons
5000 ft 4.5 gallons

At 5000 ft altitude, the 20 lb propane tank only holds around 4.5 gallons. At sea level, capacity increases to 4.7 gallons. Lower atmospheric pressure at higher altitudes decreases propane density.

Does tank color affect capacity?

No, the color of a propane tank does not directly affect its capacity. Standard 20 lb propane tanks are available in several different colors like silver, black, or white. But they have the same internal volume and propane capacity regardless of color.

However, propane tank capacity can be indirectly affected by tank color. Dark-colored tanks will absorb more heat from sunlight. This warmer temperature will cause a slight decrease in propane density and capacity compared to cooler-colored tanks.


In summary, a standard 20 lb propane tank holds about 4.7 gallons when filled to 80% capacity. This number can vary slightly based on temperature, altitude, and other factors. Colder temperatures and lower altitudes allow for higher propane density and increased capacity. But regardless of conditions, a 20 lb propane tank will hold between 4.5-5 gallons for typical consumer use.

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