propane
How Long Does a Propane Tank Last? The BTU Math, Worked Out
A 20 lb tank holds about 4.7 gallons, or roughly 431,600 BTU. Divide by the burner's BTU/h to get hours. Worked examples for grills, heaters, generators and furnaces.
Online answers to “how long does a propane tank last” range from “about 10 hours” to “a couple of weekends”, and none of them show the working. The answer is one division: the energy in the tank (gallons × about 91,500 BTU) divided by how fast the appliance burns it (its BTU/h rating). Everything else is working out those two numbers honestly, meaning how much propane is really in the tank and how much of the time the burner is actually lit. Below we go through each step with numbers from our propane usage calculator, then use the same BTU unit to look at the other half of the question: how big a heater or air conditioner a room needs.
Step 1: how much energy is in a gallon
The US Energy Information Administration’s British thermal units page lists propane at 91,452 Btu per gallon. The propane industry’s own teaching material from PERC (Lesson 5, “Measurements and Calculations with Propane”) rounds it to 91,000. You’ll see figures between those two and 92,000 quoted elsewhere. Our calculator uses 91,500, the usual industry rounding. The gap between these values is under 1 %, which is far smaller than the uncertainty in how you actually use the grill.
For reference, a BTU is the heat needed to raise one pound of water by 1 °F, and the same EIA page gives 1 kWh = 3,412 Btu. That second figure connects propane to electricity, and we’ll use it later.
Step 2: how much propane is actually in the tank
This is where most estimates go wrong, because tanks are sized two different ways.
Grill cylinders are sold by propane weight. A “20 lb” cylinder holds 20 pounds of propane, not 20 pounds of anything else. Propane weighs about 4.24 lb per gallon at 60 °F, so:
20 lb ÷ 4.24 lb/gal = 4.72 gal
4.72 gal × 91,500 BTU/gal ≈ 431,600 BTUManchester Tank’s DOT cylinder catalog gives the same answer: its 20 lb cylinder is listed at 4.7 gallons of LPG, with a water capacity of 47.6 lb, “nominal at 60 °F”. The cylinder is much bigger than the propane in it. Cylinders from 5 to 40 lb have an overfill prevention device (OPD), which stops filling before the liquid leaves too little room to expand. PERC’s lesson on storing propane states the rule: tanks “should only be filled to 80 percent of the tank’s capacity” so the liquid has room to expand on hot days. For a 20 lb cylinder, that headroom is already included in the rated 20 lb.
Home tanks are sold by water capacity. A “500-gallon” tank holds 500 gallons of water, but it is filled to 80 %, so it holds 400 gallons of propane when full. Its float gauge shows a percentage of the whole tank, so a full tank reads 80 and 60 % means 300 gallons. Our calculator applies the 80 % rule to the 120, 250, 500 and 1,000-gallon presets and converts gauge readings the same way.
| Tank | How it’s rated | Propane when full | Energy |
|---|---|---|---|
| 20 lb cylinder | 20 lb propane | 4.72 gal | ≈ 431,600 BTU |
| 100 lb cylinder | 100 lb propane | 23.6 gal | ≈ 2.16 million BTU |
| 500 gal tank | 500 gal water | 400 gal (80 %) | 36.6 million BTU |
How full is a cylinder? Weigh it
Most grill cylinders have no gauge, but a bathroom scale works fine. The collar of every DOT cylinder is stamped TW, the tare (empty) weight. PERC’s Lesson 5 puts it simply: TW “stands for the number of pounds that the empty bottle weighs,” and a filler sets the scale to TW + 20 lb to fill a 20 lb bottle. Working backwards:
propane left (lb) = scale reading − TW
gallons left = propane left ÷ 4.24Manchester’s 20 lb cylinder is stamped 18 lb. If yours weighs 27 lb on the scale, you have 9 lb of propane, which is 2.12 gallons, or 45 % of a full fill. Tare weights differ between manufacturers. Our calculator’s 20 lb preset assumes 17 lb, so type in the number stamped on your own collar. The “Weigh it” mode does this calculation for you.
Step 3: divide by the burn rate
An appliance’s BTU/h rating is its gas input at full flame. Hours of runtime are simply tank BTU ÷ appliance BTU/h. From our calculator, using a full 20 lb cylinder and an example price of $3.00 per gallon (enter your own local price):
| Appliance (full flame) | BTU/h | Burn rate | Runtime | Cost/hour |
|---|---|---|---|---|
| Portable / tabletop grill | 12,000 | 0.131 gal/h | 36.0 h | $0.39 |
| Tank-top “buddy” heater | 18,000 | 0.197 gal/h | 24.0 h | $0.59 |
| Mid-size grill | 30,000 | 0.328 gal/h | 14.4 h | $0.98 |
| 3-burner grill | 36,000 | 0.393 gal/h | 12.0 h | $1.18 |
| Patio heater | 40,000 | 0.437 gal/h | 10.8 h | $1.31 |
Two adjustments make this realistic:
- Add up every burner you run. A grill’s rating covers all burners on high. If you run one of three burners, use a third of the rating.
- Account for on-time. A grill at medium, or anything controlled by a thermostat, isn’t burning at full rate all the time. Our calculator has a “burner on-time” field. The same 36,000 BTU grill at 50 % on-time lasts 24.0 hours and costs $0.59 an hour. That’s why real grilling often gets far more than 12 hours from a tank.
If the grill is doing a holiday turkey, plan the other end too: the USDA thaw and temperature math works back from dinner to the day the bird has to go in the fridge.
Cold weather works against small cylinders. Propane boils at −44 °F (PERC Lesson 9), and a cylinder can only supply vapor as fast as the liquid inside can boil. Boiling pulls heat out of the tank, so on a cold day a high-demand burner can weaken before the tank is empty. Our calculator doesn’t model this. It shows the energy in the tank, and in winter you may not be able to use all of it at full flame.
Furnaces: input is not output
A furnace data plate lists both input and output BTU/h. Propane use depends on the input. The output is lower because of efficiency, measured as AFUE. The Department of Energy’s furnace purchasing guidance defines AFUE as the “ratio of useful energy output to energy input, expressed as a percentage … over the entire heating season,” and says ENERGY STAR requires 95 % AFUE in the northern region and 90 % in the southern region. A 95 % AFUE furnace with an 80,000 BTU/h input delivers about 76,000 BTU/h to the house. It still burns 80,000 BTU/h of propane while running.
A furnace also cycles. With a full 500-gallon tank (400 gal), an 80,000 BTU/h input and the burner running 40 % of the time, our calculator gives 0.35 gal/h averaged over the day, about 1,144 hours or 47.7 days of continuous heating, at roughly $1.05 per hour at $3/gal. On-time is the number you’re least sure of, so try a few values. A mild week and a cold snap can differ by a factor of two or more.
Generators: go by load, and trust the spec sheet
A generator’s propane use depends on how much power it is actually producing, not its nameplate rating. Our calculator’s “Generator by load” mode uses a rule of thumb of 20,000 BTU of propane per kWh of electrical output (about 17 % fuel-to-electric efficiency, since 3,412 ÷ 20,000 ≈ 17 %), or about 0.22 gal/h per kW of load. That constant is calibrated to the half-load figures on Generac’s spec sheets, below.
Compare that with a real spec sheet. Generac’s 22 kW home standby unit (model 7260) lists LP consumption of 2.45 gal/h at 50 % load (its full-load figure is given only as 135 ft³/h). The Wi-Fi model 7042 lists 2.53 gal/h at 50 % load and 3.71 gal/h at full load.
| 22 kW standby, 500 gal tank (400 gal) | Burn rate | Runtime |
|---|---|---|
| Our rule of thumb, 50 % load | 2.40 gal/h | 166 h ≈ 6.9 days |
| Generac spec (7260), 50 % load | 2.45 gal/h | 163 h ≈ 6.8 days |
| Generac spec (7042), 50 % load | 2.53 gal/h | 158 h ≈ 6.6 days |
Back-calculating from Generac’s figures, this engine uses about 20,400 to 21,000 BTU per kWh at half load and about 15,400 at full load. Generators are less efficient when lightly loaded, and a single flat rule can’t capture that. Our 20,000 BTU/kWh rule lands within 2–5 % of Generac’s half-load figures, but reads about 30 % high at full load (it predicts 4.8 gal/h against the listed 3.71). If you have the spec sheet, choose “Gallons per hour” and type in the manufacturer’s figure. Use the rule only when you don’t have one. For a smaller portable, the rule gives a 7 kW unit at half load 0.77 gal/h, or about 6.2 hours on a 20 lb cylinder and 31 hours on a 100 lb one. Small portables are often less efficient than a 22 kW standby engine, so treat those as rough estimates.
The same arithmetic lets you compare propane with batteries. A full 20 lb cylinder (431,600 BTU) run through a generator at 15,400 to 20,400 BTU/kWh (roughly Generac’s full-load and half-load figures) produces roughly 21 to 28 kWh of electricity. If your backup load is small, such as a router, a fridge or a sump pump, the UPS runtime calculator and battery runtime calculator show how long a battery bank would carry the same watts. You can then compare that with a cylinder or two.
The same BTU, used to size heating and cooling
BTU/h also measures how much heat a room gains or loses, so it’s the unit used to size heaters and air conditioners. A few fixed conversions:
- 1 ton of cooling = 12,000 BTU/h. The name comes from ice. Melting a short ton (2,000 lb) of ice absorbs 2,000 × 144 BTU/lb = 288,000 BTU, and spreading that over 24 hours gives 12,000 BTU/h, as MEP Academy explains.
- 1 W = 3.412 BTU/h, from EIA’s 3,412 Btu per kWh. A 1,500 W plug-in heater produces 5,118 BTU/h.
Rules of thumb vs Manual J
Our BTU calculator and AC tonnage calculator use per-square-foot rules of thumb, and both say so on screen. For cooling at 8 ft ceilings, they use 30 BTU/sq ft in hot climates, 25 in warm, 20 in moderate and about 17 in cold. The hot, warm and moderate figures come from the traditional contractor rule of one ton per 400 to 600 sq ft. The cold-climate figure (700 sq ft per ton) is our own extension. The result is then adjusted for insulation, sun, ceiling height and occupants. A 2,000 sq ft house with average insulation comes out at:
| Climate | Cooling load | Exact tons | Rounded (nearest ½ ton) |
|---|---|---|---|
| Hot | 60,000 BTU/h | 5.0 | 5 |
| Warm | 50,000 BTU/h | 4.2 | 4 |
| Moderate | 40,000 BTU/h | 3.3 | 3.5 |
| Cold | 34,286 BTU/h | 2.9 | 3 |
The real method is different. ACCA describes Manual J (ANSI/ACCA 2) as “the national ANSI-recognized standard for producing HVAC equipment sizing loads” for homes, and says a Manual J load calculation “is required by national building codes and most state and local jurisdictions.” Manual J works room by room from actual insulation, windows and their orientation, air leakage and local design temperatures. Rules of thumb tend to oversize. Energy Vanguard’s comparison of rules of thumb with real load calculations found an average cooling load of about 1,200 sq ft per ton across 75 hot-climate homes, against the 400–600 of the rule. That is why both our tools tell you to ask the contractor for a Manual J before buying central equipment.
Window units: check the ENERGY STAR chart
For a single room, ENERGY STAR publishes a room air conditioner sizing chart for 8 ft ceilings: 5,000 BTU/h for 100–150 sq ft, 6,000 for 150–250, 7,000 for 250–300, 12,000 for 450–550, and so on. It adds 10 % for a very sunny room, subtracts 10 % for a heavily shaded one, adds 600 BTU for each regular occupant beyond two, and adds 4,000 BTU in a kitchen.
The chart doesn’t depend on climate, while our per-square-foot rule does, so in moderate and cold climates our BTU calculator’s cooling figure for a small room comes out below the ENERGY STAR row:
| Room (8 ft ceiling, average) | Our cooling BTU/h (moderate) | Our window-AC pick | ENERGY STAR row |
|---|---|---|---|
| 200 sq ft | 4,000 | 5,000 | 6,000 |
| 300 sq ft | 6,000 | 6,000 | 7,000 |
| 500 sq ft | 10,000 | 10,000 | 12,000 |
In a hot climate the two agree more closely, or ours comes out higher (300 sq ft gives 9,000 BTU/h, rounded up to a 10,000 unit). We don’t hide the difference. The calculator displays the full ENERGY STAR chart beneath the result and highlights the row for your room. When the two disagree for a window unit, the ENERGY STAR row is the safer pick, particularly for a sunny room or one used by several people.
Doing it for your own tank
- Get the propane on hand. A full cylinder is its rated pounds ÷ 4.24. A partly used cylinder is scale weight minus TW, ÷ 4.24. A home tank is gauge % × water capacity.
- Get the burn rate. Add the BTU/h of every burner you’ll run, or use gallons per hour from a spec sheet, which is the better choice for generators.
- Set on-time for anything that cycles, then divide.
Our propane usage calculator does all three steps and also gives cost per hour and the value of the fuel in the tank. It runs entirely in your browser. If the propane is heating a room, run the room through the BTU calculator first so you know the heater is the right size. For a spec sheet in kW, multiply by 3,412 to get BTU/h.