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Pour warm water down the side of the tank and feel for the cold line. Where the metal turns from warm to cool marks the top of the liquid propane inside — cold all the way down means nearly full, a cold line near the bottom means nearly empty, and no cool spot at all means empty. It works because liquid propane draws heat off the tank wall while the empty vapor space above stays warm. The most exact alternative is weight: a full 20-pound grill tank weighs about 37 pounds, and subtracting the empty weight tells you the pounds of gas left.
How does the warm water method work?
This is the no-tool answer, and it is more accurate than people expect. Liquid propane conducts heat away from the steel it touches; the vapor above it does not. So pour a cup or two of warm — not boiling — water down one side of the tank, then run your palm down that wet stripe. Above the fuel line the metal feels warm; at and below the line it turns distinctly cool, because the liquid pulled the heat out. That temperature boundary is the propane level, usually readable to within an inch. It costs nothing and takes ten seconds.
How do you use weight to check the level?
Weighing is the precise method. Every cylinder has two numbers stamped on its collar: TW, the tare (empty) weight, and WC, the water capacity. A standard 20-pound tank has a tare weight of about 17 pounds and holds 20 pounds of propane when full, for roughly 37 pounds total. Set it on a bathroom scale, subtract the tare weight, and the remainder is pounds of propane. Since propane weighs about 4.2 pounds per gallon, you can convert to gallons too.
| Scale reading (20 lb tank) | Propane remaining | Roughly |
|---|---|---|
| ~37 lb | ~20 lb | Full |
| ~32 lb | ~15 lb | Three-quarters |
| ~27 lb | ~10 lb | Half |
| ~22 lb | ~5 lb | Quarter — refill soon |
| ~17 lb | ~0 lb | Empty |
How much cooking is left in what remains?
Once you know the pounds left, you can estimate cooking time. A full 20-pound tank runs a typical mid-size gas grill for about 18 to 20 hours on medium, or roughly 10 to 12 hours wide open on a large grill — so half a tank is still many weekends. Cold weather is the wrinkle: vapor output falls in deep cold, so the flame can weaken before the tank is truly empty, which is covered in our note on leaving a propane tank outside in winter. The habit that never fails you is keeping one filled spare so an empty tank never ends a cook.
What about clip-on and inline gauges?
Magnetic strip gauges that stick to the tank and change color with the liquid level use the same warm-cold principle as the water trick and are handy, though slow to read in cold weather. Inline gauges that thread between the tank and regulator mostly measure pressure, and propane pressure stays high until the tank is almost empty — so they tend to warn you late rather than early. For a truly accurate reading, weight is the gold standard; for a quick check with nothing on hand, the warm-water method wins. Either way, knowing your level is part of getting the most life from the grill, which ties back to how long a gas grill lasts.
We do not buy, test or physically handle any product on this site. Methods here reflect propane industry guidance and cylinder markings — see how we rank.
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