You are halfway through burgers when the flame softens and the tank runs dry. If you have wondered how long a 20 lb propane tank lasts on a grill, the answer is mostly math: energy in the cylinder divided by how hard your burners are working. The same math applies to a flat-top griddle, with one important difference: many griddle cooks run with the surface uncovered, so real-world consumption can sit at the high end of the range.
(Last modified date: November 9, 2026)
A full 20 lb cylinder holds approximately 430,000 BTUs of liquid propane, according to Ferrellgas. Blue Rhino cites about 91,000 BTUs per gallon as the energy content of propane. Divide the energy in the cylinder by your appliance's actual burn rate and you get a realistic hour range, not a single magic number.
How long a 20 lb propane tank lasts on a grill or griddle
A full 20 lb tank usually lasts about 8 to 20 hours. Heat setting and burner count matter more than tank size. On high with every burner lit, a large grill or island can drain a tank in one afternoon. A lid-off griddle cook often lands at the shorter end of that range.
U-Haul's propane runtime guide also stresses that BTU rating, burner count, and conditions change the result. Weber's consumer-care article gives a simpler planning figure of about 18 to 20 hours for a standard 20 lb tank on most of its grills, and about 1.5 to 2 hours for a small disposable cylinder. Treat those as planning tools, not a guarantee for every appliance.
BTU math: how to estimate burn time
BTU stands for British thermal unit. On a grill or griddle label it means fuel energy burned per hour at full output, not the temperature on your food. Once you know that number, runtime is a short equation.
Hours on high is approximately 430,000 divided by total BTU per hour of the burners you actually light. At medium, many cooks assume roughly half the rated input, which roughly doubles the hours. If your manual lists each burner separately, add only the burners you keep lit.
Example: a 40,000 BTU grill on high lasts about 10.8 hours on a full tank. The same grill near half input lasts about 21 hours. Always check the rating plate or owner's manual for your exact model rather than guessing from burner count alone.
Table: grills, griddles, and island loads
| Setup (typical or listed) | Assumed or listed BTU on high | Hours on high, full 20 lb tank | Hours on medium, about 50 percent |
|---|---|---|---|
| 2-burner portable or compact grill | About 20,000 to 30,000 | About 14 to 21 | About 28 to 40-plus |
| 4-burner family grill | About 36,000 to 48,000 | About 9 to 12 | About 18 to 24 |
| 6-burner or large cart | About 48,000 to 60,000-plus | About 7 to 9 | About 14 to 18 |
| Big Horn grill and griddle combo island, all six burners | 65,000 listed | About 6.6 | About 13 if run near half input |
| Big Horn 4+2 burner black island, all burners | 74,600 listed | About 5.8 | About 11.6 if run near half input |
The first three rows are continuous-burn estimates for common size classes, not lab tests of a particular model. The last two rows use the BTU totals published on Big Horn product pages and the 430,000 BTU tank figure. Real cookouts cycle burners on and off, so many households still see a full tank last through multiple weekends.
How a griddle changes the picture
A flat top is often cooked without a closed hood, so heat loss to wind is higher and you may leave more burners on to hold a wide, even surface. That is why owners also ask how long a propane tank lasts on a Blackstone-style griddle: the formula is the same, but lid-off, full-width cooking can use more fuel than a covered grill doing the same menu.
On a combo island, the griddle section is only part of the load. The 65,000 BTU grill and griddle combo uses six tubular burners for a grate section and a dedicated griddle. Lighting only the griddle-side burners stretches the tank; lighting every burner for a party cook shortens it toward the 6 to 7 hour mark on high. The 74,600 BTU 4+2 burner black island is a higher-draw example if every burner is open.
Those islands live in a larger plan described in how to lay out an outdoor kitchen with a griddle and in the built-in versus freestanding comparison.
How to tell what is left
Guessing from flame color is unreliable. Weigh the cylinder on a bathroom scale and subtract the tare weight stamped on the collar. The difference in pounds is roughly the propane left. A tank gauge on the valve or a clip-on gauge is faster for weekly checks. The warm-water method, pouring warm water down the side and feeling for a cool line, shows the liquid level in a pinch.
If the flame weakens mid-cook, shut the burner valves, then the tank valve, and swap tanks outdoors. Never try to shake more liquid into the regulator while burners are lit.
Habits that stretch a tank
Preheat just until the surface is ready, then cook. Light only the burners under the food, and use the rest of the surface as a warm zone with burners off. Keep a grill lid down when you can. On a griddle, a basting dome or a wind guard can reduce the urge to turn every knob to high. Clean grease that smothers burners, and park the appliance out of strong wind.
Store spare cylinders upright outdoors, never indoors or in a hot trunk, and inspect hoses for cracks before the season starts. NFPA grilling safety, Ready.gov home fire guidance, and propane consumer guidance are the right references for siting and storage.
When a second tank is the smarter buy
A spare tank is cheaper than a ruined roast. Buy a second cylinder when you regularly cook longer than two hours, host large groups, or grill far from a refill station. Keep one tank on the appliance and one full in outdoor storage. Swap at the first sign of fading heat instead of mid-sear.
If the whole kitchen needs to travel, the packing approach in how to build a portable outdoor kitchen is a better match than a single oversized cylinder you cannot easily exchange.
Island grills and natural gas as the no-refill option
Built-in and island kitchens burn more fuel when many burners and side appliances run together. If you cook several nights a week and dislike mid-party refills, ask whether a natural-gas hookup is available at your patio and whether your specific model supports conversion through the manufacturer. The American Gas Association explains residential natural gas service at a high level. Conversion kits, orifice sizes, and regulator requirements are model-specific. Do not improvise.
For the wider cooking-surface map, see the outdoor griddle guide.
FAQ
How many hours should a 20 lb propane tank last?
On a typical backyard grill, expect roughly 8 to 20 hours of cooking from a full 20 lb tank, depending on burner count and heat setting. A compact 2-burner on medium can stretch past a weekend of cookouts, while a large 6-burner or a high-draw island on high can empty the tank in a single long party. Use your appliance's total BTU rating for a tighter estimate.
How long does a propane tank last on a flat-top griddle?
Use the same formula: about 430,000 BTUs in a full 20 lb cylinder, divided by the burners you actually light. Because many griddle cooks run uncovered, real-world time often sits toward the shorter end of the grill range, especially in wind. Check the rating plate rather than assuming a single hour number for every brand and size.
How do I know if my 20 lb propane tank is empty?
Weigh the tank and subtract the tare weight stamped on the collar; the difference is the propane left in pounds. You can also pour warm water down the side and feel for a cool band at the liquid level, or use a tank gauge. A sputtering flame or sudden drop in heat is a late warning, not an early one.
Does propane in a grill tank go bad?
Propane itself does not spoil like food. What fails over time is the cylinder: rust, damaged valves, outdated requalification dates, or leaks. Have tanks inspected or exchanged when the date stamp expires, store them upright outdoors, and never use a tank that smells strongly of gas or shows heavy corrosion.
What's the average cost to fill a 20 lb propane tank?
Refill prices vary by region, season, and whether you exchange the cylinder or refill your own. Exchange cages often cost more per gallon than a metered refill station. Check local propane retailers and hardware stores for current pricing rather than relying on a national average, and weigh cost per usable pound, not just the sticker price.
Blackstone is a trademark of its respective owner. Big Horn Outdoors is not affiliated with or endorsed by Blackstone. Weber is a trademark of its respective owner. Big Horn Outdoors is not affiliated with or endorsed by Weber.
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