How Long Does a 20 lb Propane Tank Last? A Runtime Calculator, and Why It Quits in the Cold

Your tank isn’t empty. It’s out of pressure.

Every page on this question does the same sum: pounds of propane times BTU per pound, divided by the burner rating. A 48,000 BTU patio heater on a full 20 lb tank gets you 9 hours. Then a cold night comes, the flame shrinks after three or four hours, and you walk the tank back to the store with half of it still inside.

The 9 is what the fuel holds. How much the cylinder can deliver is a different number, and it depends on the temperature and how full the tank is. This guide has both, along with a calculator that shows them side by side. The delivery figures come from a printed cylinder chart and Emerson’s engineering bulletin. Our hour estimates are a model built on those, and we have not timed a tank ourselves, so they are labelled as estimates wherever they appear.

Propane tank runtime calculator

The paper number

Propane holds about 21,500 BTU per pound, so a 20 lb tank holds about 431,000 BTU and a 15 lb exchange tank about 323,000. Divide by the rating:

Hours = pounds × 21,500 ÷ BTU per hour

A manual check confirms it: Cuisinart prints 1.77 lb an hour at 38,200 BTU on its COH-400. Blue Rhino says a gallon lasts about 3 hours at 30,000 BTU, which matches.

Printed figures vary. Mi-T-M’s heater spec prints 2.3 lb an hour and 8.5 hours at 50,000 BTU and marks it as an estimate, which is honest. Mr. Heater’s 48,000 BTU page says up to 10 hours, which is true only turned down: full flame gives 9.0. Bromic’s blog says about 9 to 10 hours for models that work out to 10.8 and 18.3, so neither matches. Some rival pages are simply wrong. One fire pit retailer prints “approximately 18-20 hours at a consumption rate of 2-4 gallons per hour”, but a 20 lb tank holds 4.7 gallons, so that rate would empty it in under 2.5 hours. Its 45,000 BTU pit burns about half a gallon an hour.

Across the pages we read, nobody prints a temperature beside an hours figure. That is the part that matters.

What a cylinder can actually deliver

A propane cylinder is a tank of boiling liquid. The gas your burner uses comes off the top, and turning liquid into gas takes heat. That heat comes from the air, through the steel, and only through the part of the steel that has liquid behind it. Emerson’s bulletin says what happens when you draw faster than that:

“A tank will meet load demands until gas is removed faster than boiling can replace it. When this occurs, the outside walls of the tank are chilled, inducing precipitation on the walls from the surrounding atmosphere.”

Then frost forms, and frost insulates. PERC: “Tank frosting acts as an insulator, reducing the vaporization rate.” The liquid chills, pressure falls, and Emerson says the regulator’s capacity falls with it.

So a cylinder has a speed limit, set by two things: how cold the air is and how much steel is wet. RegO’s manual says it plainly: “the worst conditions for vaporization rate are when the container has a small amount of liquid in it and the outside air temperature is low.”

Most handbooks publish this for 100 lb cylinders and up. We found one chart with a 20 lb column and stated assumptions, from US Carburetion, which makes propane fuel systems: vertical cylinder, 25 percent full, minimum 10 psi. Its 20 lb column is the bottom row below. We scaled the other rows by the wetted steel, and that scaling is our arithmetic. Emerson’s published formula reproduces the printed column to within 2 percent with a heat-transfer value of 1.9, and runs 6 to 7 percent high at its published average of 2, which is why we trust the method.

What a 20 lb cylinder can supply continuously, BTU per hour, holding 10 psi

Read it against your burner. A 48,000 BTU heater is inside the line only on a full cylinder at about 40°F or warmer. A typical 15 lb exchange tank falls short below the upper 50s. A half-full tank at 40°F can give 28,000. A 60,000 BTU fire table is close to the line on a full tank even at 60°F.

Handbook numbers can look much higher, and that is worth knowing if you find them. RegO’s 100 lb table reads 113,000 BTU at 0°F, while Emerson’s handbook reads 71,000 for the same cylinder, and both then tell installers to design to 50,000 at 0°F. RegO’s table fits Emerson’s formula with the liquid at propane’s 0 psig boiling point, at which nothing flows, so it reads as a ceiling. Also, the row labelled “20” in that RegO table means 20 lb left in a 100 lb cylinder, not a 20 lb tank.

Why you get more than the line for a while

A full cylinder can beat that limit for a few hours, because the liquid itself is a store of heat. It cools as it runs, and only when it reaches the pressure floor does the flame start to fade. That is the headline number in the calculator.

Estimated hours at full flame on a steady load. These are estimates from a heat balance using Emerson’s formula, with the tank starting at air temperature. They have not been checked against a timed burn. Wind or sun can move them about 30 percent one way, and a shroud or frost about 30 percent the other, so read them as “about”.

Three things follow.

The tank is not empty when it fades. At 48,000 BTU and 40°F, the run ends with about 12 pounds still in it. Weigh it and you will see. It comes back when the tank warms, without you doing anything to it.

An exchange tank loses more than a quarter. It starts with 25 percent less fuel and about 21 percent less wet steel, so 3.7 hours becomes about 2.5. That is a drop of about a third.

Griddles and grills have an easier life. Those estimates are for a steady load on high. A griddle or grill cook is usually short, and you turn it down or off between batches. Emerson says that under intermittent loading, capacity “might be three to four times greater.” A griddle fades in real cold on a long cook or a low tank. A patio heater or fire pit on high all evening is the true test of a cylinder. That is why the griddle cooks fine in the same weather that kills the heater.

Fuel use at a fixed knob does not rise in the cold, whatever some pages say: the burner flow is set by the valve. Cold lengthens the cook, and it shortens how long the tank can keep up.

Is it the cold, or the regulator?

These look alike and have different fixes. Check when it happens.

Low from the moment you light it, on a warm tank: that is probably the flow limiting device. Blackstone explains: US regulators contain one, and it can “accidentally be tripped by opening a propane tank too quickly, especially with a full tank, or opening the propane tank when a burner knob is not in the off position.” Weber’s symptom is a grill that only reaches 250 to 300°F on high.

To reset it, per Blackstone’s manual: burner knobs off, close the tank, disconnect the regulator, wait five minutes, reconnect, and open the valve slowly, about half a turn. Blackstone suggests taking about 30 seconds for the first quarter turn. Weber’s editions differ on the exact steps, so follow your own manual.

Fine for a while, then fading, on a cold or low tank, maybe with frost: that is vaporization. No reset will fix it. Turn the burner down or swap the tank.

Uline’s patio heater manual is the plainest we found: “Burner flame is low”, cause “Outdoor temperature is less than 40°F and tank is less than 1/4 full”, fix “Use a full cylinder.” Also “Burner won’t light”, cause “Propane cylinder is frosted over”, fix wait until it warms up. On the table above, a quarter tank at 40°F gives about 18,500 BTU, or 40 percent of what a 46,000 BTU heater wants, so the manual’s line is if anything generous.

A heater often goes out, and does not just fade, because a shrinking flame may no longer heat its flame sensor, which closes the valve. That last step is likely but we only have secondary support for it.

What frost tells you

Frost on the outside of a tank means you are drawing past the line. It is also a free gauge: the top of the frost is the liquid level, because that is where the cold stops. Blue Rhino explains the frost line as high demand on the cylinder, and UK caterers’ trade body NCASS calls a wall of frost “a telltale sign that a cylinder is undersized.”

Frost on the regulator or valve is a slightly different case, and Blackstone’s manual attributes a frosty regulator to overfill.

Does propane freeze? No. NIST puts its melting point near -306°F, and it boils at about -44°F. Below that, a tank gives you no pressure, but the liquid is still liquid. What freezes is water on the outside.

How to keep it burning in the cold

Most of these cost nothing.

1. Start with a full tank. Wet steel is capacity. A refilled 20 lb tank beats a 15 lb exchange twice over.
2. Turn it down. At half flame, a 48,000 BTU heater asks for 24,000, which a full tank can supply down to about 6°F.
3. Swap to a second tank. Keep a spare outside, away from the appliance. Swap when it fades, and let the first one recover on its own.
4. Go bigger. US Carburetion’s printed ratios to a 20 lb cylinder are 1.33 times for a 30 lb, 1.67 times for a 40 lb, and 3.3 times for a 100 lb, at any temperature. A tall heater will not take a larger tank in its base, so that suits a fire table on a hose or a griddle.
5. Or skip the cylinder. A permanent gas line has no such limit, and electric patio heaters do not care how cold it is.

What not to do. No flame on a cylinder, ever: Flame Engineering says “DO NOT apply flame to cylinders to increase pressure.” Heat raises pressure until the relief valve vents gas. No bringing it inside to warm up, which owners do suggest on forums: the PERC guide printed by U-Haul says “NEVER store or place a propane cylinder indoors or in an enclosed area such as a basement, garage, shed, or tent.”

Tank warmers. Powerblanket sells a listed 120-watt cylinder warmer held at 90°F. NCASS, the UK trade body, says not to use “concentrated heat sources e.g. cylinder blankets”. Those positions conflict, we found no US code sentence settling it, and we do not recommend one. A second tank is cheaper and has no such question.

Reading the level

Weigh it. That is the only method that measures contents. The empty weight is stamped on the collar after TW, usually 17 to 19 pounds. Gross weight minus TW is propane left. NIST gives a new cylinder’s tare tolerance as minus 3 or plus 1 percent, so this is good to about half a pound.

Warm water poured down the side leaves a line you can feel where the liquid starts. Ferrellgas says it will not give a precise reading. Disconnect the cylinder first.

Inline pressure gauges cannot read level. Cylinder pressure depends on temperature until the liquid is nearly gone: about 102 psig at 60°F, full or nearly empty, by PERC’s table. That point is our reading from the table and not a maker’s sentence. A low reading on a cold tank is the temperature talking.

Built-in scales drift. Weber’s own troubleshooting notes a scale showing gas in an empty tank.

Exchange or refill

NIST: “Though it will safely hold 20 lb, most 20 lb cylinders are filled and labeled to a net weight of 15 lb.” So an exchange buys you less fuel and, as above, less capacity in the cold. Our patio heater showdown prices that out: a typical exchange costs more per BTU than a 120 V electric heater. Check the printed net weight, because brands differ.

The rules a buyer actually hits

Requalification. The common line says 12 years, then 5, 7 or 12. The regulation reads differently: 49 CFR 180.209 lets a DOT 4BA, 4BW or similar cylinder in LP service go 12 years from manufacture, then 12 more by volumetric expansion test, 10 by proof pressure, or 5 by external visual inspection, which is the one most counters do. The 7 belongs to fire extinguishers. An out-of-date cylinder “may remain in service until it is emptied”, so you can burn it off, but it cannot be refilled. Blackstone’s manual says within five years, which is stricter than the rule.

OPD. A cylinder needs an overfill prevention device, the triangular handwheel with OPD on it, to be refilled in many states.

Storage. Outside, upright, in a cool dry place. Not indoors, not in a garage, not under or near a grill, and not anywhere over 120°F, per the PERC guide. Minnesota’s fire marshal says the same of garages. For how far a tank sits from an open fire, see how far a fire pit must be from a propane tank.

Transport. Upright and secure, straight to your destination, and never left in a hot vehicle or a closed trunk.

Where to next

Picking the appliance: patio heaters, propane fire pits, fire pit tables, flat-top griddles and gas grills. Under a roof? What each maker’s manual says. Then read how far a fire pit must be from the house and why the garage is not the backup plan.