How Many Pounds Are in a 100-Gallon Propane Tank?

A 100-gallon propane tank holds 80 gallons of liquid propane, which weighs about 339 pounds at 60°F. The 100 is the tank's water capacity, meaning the volume it would hold if you filled it with water, and NFPA 58, the Liquefied Petroleum Gas Code, caps the liquid at 80 percent of that number so the propane has somewhere to expand. At the industry reference density of 4.24 pounds per gallon, 80 gallons comes to 339 pounds of fuel; the NIST Chemistry WebBook's saturated-liquid value at 60°F, 31.631 lb/ft³, works out to 4.23 pounds per gallon and gives 338. The steel adds its own tare weight, roughly 260 pounds for a vessel that size, so a full tank on a scale reads close to 600 pounds.

What the 100 in "100-gallon" actually counts

Fourteen years of buying material for stage sets is fourteen years of watching a label fail to match a caliper. A nominal 2×4 measures 1½ by 3½ inches under PS 20, the American Softwood Lumber Standard. The offcuts from my kid's loft bed are still stacked by my desk, and every one says the same thing: the label is a category, the measurement is a contract.

Propane tanks carry labels the same way, and this is the one detail that changes the answer. Three different vessels get called a 100-gallon propane tank in ordinary conversation, and they hold 100 pounds, 339 pounds, or 407 pounds of fuel depending on which one is sitting in your yard.

| | 100-gallon water capacity ASME tank | 118-gallon ASME tank, sold as "120-gallon" or "420 lb" | 100 lb DOT cylinder | |---|---|---|---| | What the number rates | water capacity, in gallons | water capacity, in gallons | propane, in pounds | | Liquid propane held | 80 gal | 95.9 gal | 23.6 gal | | Weight of that propane at 60°F | ~339 lb | ~407 lb | 100 lb | | Tare (empty) weight | ~260 lb | 291 lb | 71 lb | | Full weight on a scale | ~600 lb | ~698 lb | 171 lb | | Stored energy | 7.3 million Btu | 8.8 million Btu | 2.2 million Btu |

Those tare figures come from Manchester Tank & Equipment's portable steel tanks catalog, number 49501. Manchester lists no 100-gallon ASME model at all; the line runs 28.6, 57, and 118 gallons. Their 57-gallon tank is stamped 188 pounds empty and the 118-gallon 291, so a true 100-gallon vessel lands near 260. That is an interpolation. The data plate on your tank carries the real figure.

The 420 lb name in that middle column comes from federal filling rules rather than from gallons. A 420 lb DOT cylinder has a 1,000-pound water capacity, and 49 CFR 173.304a permits a filling density of 42 percent for propane with a specific gravity between 0.504 and 0.510. Forty-two percent of 1,000 is 420. At 4.24 pounds per gallon that is 99 gallons of propane, which is why dealers started calling it a hundred-gallon tank.

Why the tank stops at 80 percent

Liquid propane expands hard with temperature. NIST's saturation data puts it at 32.620 lb/ft³ at 40°F and 29.425 lb/ft³ at 100°F, the same mass occupying 10.9 percent more volume. A tank filled to the brim on a January morning would have nowhere to put that growth by a July afternoon.

The 20 percent vapor space absorbs it, and it keeps the pressure relief valve doing the job it was designed for. A valve venting vapor is a nuisance. A valve submerged in liquid discharges liquid propane, which expands roughly 272 to 1 on the way out, per the properties table in the Canadian Standards Association's propane storage and handling code.

On an ASME tank, the 80 percent line is not the driver's judgment call. A fixed maximum liquid level gauge, a dip tube cut to end at that depth, runs off a #54 outage valve orifice in Manchester's spec. The driver cracks it while filling; vapor whistles out, invisible. The moment liquid reaches the tube, a dense white plume appears and the fill stops. That plume is the measurement, and the dial on the front of the tank is not.

Temperature is the other number in the calculation

Wind has been shoving at my windows since before daylight while the air in this room has not moved once, and that split is the right picture for a tank outdoors. Wind chill is defined for exposed skin. It does not drive steel below the surrounding air temperature; it only gets the steel there faster. What sets your pounds is the temperature of the liquid inside, which follows the air and arrives late.

Here is what that costs, using NIST saturated-liquid densities converted to pounds per gallon, on the same 80-gallon fill:

| Liquid temperature | Density (lb/gal) | 80 gallons weighs | |---|---|---| | 30°F | 4.42 | 354 lb | | 40°F | 4.36 | 349 lb | | 60°F | 4.23 | 338 lb | | 90°F | 4.01 | 321 lb | | 100°F | 3.93 | 315 lb |

A tank topped to the same 80 percent mark on a 40°F morning carries 349 pounds. The identical tank filled on a 90°F afternoon carries 321. Twenty-eight pounds of fuel, close to nine percent, decided by nothing but when the truck showed up.

Your supplier knows this, which is why the ticket has a net column. Custody transfer in the propane trade is corrected back to a 60°F base, so you are billed for temperature-corrected gallons rather than raw meter volume. Your dial has no such correction. It reports what the liquid is doing right now.

100-gallon tank versus 100-pound cylinder

These two get mixed up constantly, and the difference between them is a factor of about 3.4 in fuel.

A 100 lb cylinder is a DOT 4BW240 portable vessel, 15.1 inches in diameter and 46.6 inches tall in Manchester's catalog, with a 238-pound water capacity and a 71-pound tare weight. Apply the 42 percent filling density from 49 CFR 173.304a and you get 99.96 pounds of propane, which is where the name comes from. At 4.24 pounds per gallon that is 23.6 gallons. Full, it weighs 171 pounds, and one person is not carrying it.

Manchester's catalog is worth a second look. It lists that cylinder's LPG capacity as 22.9 gallons rather than 23.6. The internal volume is 28.6 gallons and 22.9 is exactly 80 percent of it, so the volume rule and the weight rule disagree by about three pounds here. Both are legitimate. Filling stations use the weight rule, because a 100 lb cylinder ships with no sight gauge; they set it on a scale and fill to the stamped TW plus 100.

A 100-gallon stationary tank is an ASME vessel, permanently sited, filled from a bulk truck through a fixed liquid level gauge, and it holds more than three times the fuel. If a delivery company quotes you gallons and a hardware store quotes you pounds, they are describing different objects.

Where gallons times 4.24 goes wrong

The arithmetic is fine. The inputs are where people lose money and, occasionally, a deck.

Multiplying 100 by 4.24 to get 424 pounds overstates the fuel by 25 percent, because it assumes a tank filled to 100 percent and no tank is. Ignoring temperature adds another 39 pounds of spread between a 30°F fill and a 100°F one. Dropping tare weight leaves a number useless for anything involving a scale, a hoist, or a trailer axle, since the steel is about 43 percent of a loaded 100-gallon tank.

The density constant is looser than it looks. The trade uses 4.24 pounds per gallon, the Canadian Standards Association's table lists 4.20 for commercial propane, and NIST's figure at 60°F is 4.23, a spread of about three pounds on a full tank.

Where I stop is where I stop on a cut list. If your number decides whether a pad or a trailer can carry the load, that is a structural calculation and it belongs to your installer or an engineer holding the drawing. I can tell you what the tank weighs. I cannot tell you what your slab will do about it.

Your gauge reads low right after a delivery

This is the most common complaint I hear, and it is usually four things at once rather than a short fill.

Float gauges on residential tanks are planning instruments, with a working accuracy that industry guidance puts near plus or minus 5 to 10 percentage points. On a 100-gallon tank that is 5 to 10 gallons, or 21 to 42 pounds. The dial also reports current liquid volume, so a tank filled warm and read cold has genuinely shrunk, and the ticket was corrected to 60°F while the dial was not.

Work through it in this order before calling anyone:

1. Read the dial cold and early. Take the reading in the morning before sun reaches the tank, and take every future reading at that same hour.
2. Find the data plate. Confirm the water capacity and tare weight stamped on the ASME plate or the collar. Everything downstream depends on those being your tank's numbers.
3. Convert the dial to gallons. The dial is a percentage of water capacity. On a 100-gallon tank one point equals one gallon; on a 118-gallon tank, 1.18 gallons.
4. Compare against the ticket. Add the delivered net gallons to your pre-fill gallons. If the total lands above 80 percent of water capacity, the dial is what is wrong; the outage valve cannot let the tank go past that line.
5. Call with numbers. If the gap still exceeds the gauge's 5 to 10 point tolerance after you correct for temperature, ask the supplier to check the meter and confirm the fill stopped on the outage valve.

Tare weight, appliance demand, and the rules for moving a tank

Three situations change which number you should be working from.

Tare weight rules whenever the tank leaves the ground. Full, a 100-gallon ASME tank is around 600 pounds and a 118-gallon close to 700, which is forklift territory. A 100 lb cylinder at 171 pounds is the edge of what a strong pair can walk up a ramp.

Appliance input rate rules when you are sizing for a cold snap rather than for a season. A 30,000 Btu per hour heater running flat out on a full 100-gallon tank has 7.3 million Btu available, using the U.S. Energy Information Administration's factor of 91,452 Btu per gallon. That is 244 hours of continuous fire, a little over ten days. The trap is that a full tank can still fail to keep up, because vaporization depends on wetted surface area and liquid temperature, so a large burner on a low tank at 5°F outruns the propane's ability to boil. Your supplier runs that calculation, and it is why they ask about peak load rather than annual usage.

Transport rules apply the moment you consider hauling anything yourself. For cylinders, 49 CFR 177.840 requires the pressure relief device to stay in communication with the vapor space, and a 100 lb cylinder's valve sits in the top head, so it rides upright and strapped. NFPA 58 Chapter 9 bars closed-bodied passenger vehicles from carrying more than 215 pounds of water capacity in total, and no more than 108 pounds per cylinder, absent a vaportight partition. A 100 lb cylinder is 238 pounds of water capacity by itself, over the limit alone, and belongs in an open bed.

For the stationary tank, 49 CFR 173.315(j)(1) lets consumer storage containers charged to 5 percent or less move by private motor carrier. Paragraph (j)(2) is the one a 100-gallon owner needs: containers not exceeding 500 gallons water capacity, charged above 5 percent, may be transported by private motor carrier one way only, from the consumer's premises to the container owner's nearest facility. Manchester's catalog puts the general case at 80 percent for DOT cylinders and 5 percent for ASME tanks. Butane-Propane News has reported that NFPA 58 does not override the federal transport rules, so ask your supplier to pump the tank down.

Tracking propane through a year

A 100-gallon water capacity tank has one property I wish more equipment shared. Each dial point is one gallon, which makes a usable log possible without buying a monitor.

Read the gauge on the first of each month, same hour, tank in shade. Subtract this month's percentage from last month's and you have gallons burned, roughly 4.2 pounds each. Write the reading beside the month's heating degree days, which your local National Weather Service office publishes. After one winter you will know your gallons per degree day, and that number decides whether 80 gallons is a full season for you or six weeks of one.

Order at 25 to 30 percent rather than at 10. A bulk truck can be a week out in February, and a tank that runs dry needs a leak test and a relight before it goes back into service. Judge consumption in gallons off the delivery ticket, not in dial points off the tank, for the temperature reason above.

Frequently asked questions

How much does a 100-pound propane tank weigh when empty?

Between 63 and 71 pounds, depending on the manufacturer. Manchester Tank's DOT 4BW240 model is stamped at 71 pounds. Your cylinder's exact figure is stamped on the collar as TW, and that stamp is the only number a filling station should use when it puts the tank on the scale.

How much does it cost to fill a 100-pound propane tank?

About $63 in propane, since the cylinder takes 23.6 gallons and the EIA recorded a U.S. residential average of $2.674 a gallon for the week ending March 30, 2026. Retail refills usually land between $70 and $100 once handling is added, and state averages ranged from roughly $1.64 to $4.15.

Can I transport a 100-pound propane tank lying down?

No. 49 CFR 177.840 requires the pressure relief device to stay in communication with the tank's vapor space, and a 100-pound cylinder's relief valve sits in the top head. Laid on its side, the valve would vent liquid instead of vapor. Haul it upright and strapped, in an open bed.

How long will a 100-pound propane tank last on a 30,000-BTU heater?

About 72 hours at continuous full fire. The cylinder holds 23.6 gallons, and at the EIA's figure of 91,452 Btu per gallon that is roughly 2.16 million Btu, divided by 30,000 Btu per hour. A thermostat that cycles the burner near half the time stretches it past six days.

What size is a 100-gallon propane tank?

There is no single standard. Most vessels sold under that name are the 118-gallon ASME vertical tank, 30 inches across and 53.8 inches tall with a 291-pound tare weight in Manchester Tank's catalog. A true 100-gallon water capacity tank runs smaller. The ASME data plate settles which one you have.

Why is a propane tank filled only to its stated maximum level?

Liquid propane expands with heat. NIST saturation data shows it swells 10.9 percent in volume going from 40°F to 100°F, so a tank filled liquid-full on a cold morning would have nowhere to put that growth by afternoon. The 20 percent vapor space absorbs it and keeps the relief valve venting gas.