SizingKitEIA heat content · NFPA 58
Propane Usage Calculator
This turns an appliance schedule into gallons and pounds — per hour, per day and per year — and says how long the container lasts at that duty. It also does the thing a consumption calculator normally leaves out: it works out the heat that has to enter the liquid through the tank wall to keep the vapor coming at your peak draw, which is the constraint that strands people in January with a gauge still reading a third full.
- 100% free
- No signup
- Gallons and pounds
- Tank duration
- Cold-weather draw limit
What is connected, and how long it runs
Take the input rating off each appliance’s rating plate — the figure marked input, not output and not the room’s heat loss. Hours are burner hours, not hours the appliance is switched on: a furnace sized for a design day spends most of a mild one off. Tick the ones that can be firing at the same moment.
| Appliance | Input BTU/h | h/day | Peak | gal/day | Remove |
|---|---|---|---|---|---|
| 6.99 | |||||
| 1.31 | |||||
| 0.71 | |||||
| 0.24 |
The fuel, and the container it is in
Default 91,500. Liquid propane at 60 °F; about 21,500 BTU per pound. A gallon of propane carries about 27% less energy than a gallon of heating oil.
4.202 lb per gallon at 60 °F, and 21,776 BTU per pound out of the heating value beside it.
The size it is sold as. Filled to the 80% limit it holds 400 gallons of propane, never 500.
A float gauge reads liquid volume as a percentage of the container, so a freshly filled one reads 80 and not 100.
Your supplier’s number rather than a code figure — ask what percentage triggers an automatic delivery, because running a container to zero means a leak test and a relight before it comes back on.
20 °F · -6.67 °C — 64 °F above the point where propane stops boiling altogether.
Consumption and duration
Per day
9.26 gal
38.9 lb
Per year
3379 gal
3,092 therm
Container lasts
21.6 days
200 gal usable at this gauge reading
Two routes to a pound of propane disagree by 1.28%, and the page prints both rather than picking one. The specific gravity above gives 4.2 lb per gallon; dividing the heating value by the 21,500 BTU per pound the fuel tables also quote gives 4.26. That gap is the width of the composition and temperature variation in commercial propane, and no third decimal place anywhere on this page is worth more than it.
What the container can actually deliver
Everything above assumes the vapor is there when the burner calls for it. In cold weather that is the assumption that fails, and it fails with fuel still in the tank.
Peak draw
5.51 lb/h
1.31 gal/h · 35.2 kW
Heat the wall must pass
1,008 BTU/h
5.51 lb/h × 183 BTU/lb of latent heat
At the peak, the fuel lasts
152.5 h
everything ticked, running flat out, which nothing does for long
1,008 BTU/h is the number to take to a vaporization table. It is not heat the appliances use — it is heat that has to arrive through the wetted wall of the container just to turn liquid into gas at the rate the burners are consuming it, and it gets harder to supply as the tank empties and the wetted area shrinks. Air at 20 °F is 64 °F above propane’s -44 °F atmospheric boiling point, and that difference is the only thing pushing the heat in.
What a specific container can vaporize continuously at a given temperature and liquid level is a table, and this site does not reproduce it. It is indexed by the container's wetted surface area — the tank wall actually touching liquid, which shrinks as the tank empties — and by the temperature difference between the air outside and the boiling liquid inside, and it lives in the annexes of NFPA 58 and in the data the container's manufacturer publishes. Take the BTU/h figure this page computes for your peak simultaneous draw and read your own row out of that table. If the draw is larger than the row allows, the answers are a larger container, two containers manifolded together so their wetted areas add, or a powered vaporizer — not a bigger regulator, which cannot make gas that is not there.
US Energy Information Administration published average heat contents, except electricity, which is the exact unit conversion.
Averages. Natural gas heating value is set by the composition your utility delivers and changes through the year; propane and heating oil vary with temperature and blend. Where a bill states a heating value, that figure supersedes these. All four are higher heating values, which is what US utilities bill against and what AFUE is measured against — using an efficiency measured on a lower heating value with a higher heating value fuel figure overstates the result by about 10% on natural gas.
The relative density of commercial propane against water at 60 °F, from the LP-Gas physical property tables that NFPA 58 and the Gas Processors Association publish. HD-5 propane is at least 90% propane by liquid volume and may carry up to 5% propylene, and the rest of the mixture moves this figure; a non-specification blend with butane in it is denser again. It also moves with temperature far more than water does — see the expansion note below.
The atmospheric boiling point of propane, −44 °F or −42 °C. Below it, liquid propane exerts no pressure above the atmosphere and a tank open to a regulator delivers nothing at all.
Latent heat of vaporization of propane near 60 °F, from the LP-Gas physical property tables. This is the heat that has to enter the liquid for a pound of it to become vapor. Rises as the liquid gets colder — near the atmospheric boiling point it is about 184 BTU/lb — and falls away to nothing at the 206 °F critical temperature, which no tank you will meet gets near.
The volumetric expansion of liquid propane with temperature, from the LP-Gas physical property tables — roughly seventeen times that of water over the same range. This is the whole reason the filling limit exists: a container filled liquid-full on a cold morning has nowhere to put the expansion when the sun comes out, and the relief valve is what is left. It is also why propane is metered through a temperature-compensating register — a gallon delivered at 90 °F carries fewer pounds and less heat than a gallon delivered at 40 °F.
The maximum permitted filling limit NFPA 58 sets for an ordinary above-ground LP-Gas container, which is why a fixed maximum liquid level gauge stops a fill at that point and why a 500 gallon tank holds 400 gallons of propane. NFPA 58 varies the limit with the container's type, its volumetric capacity, whether it is above or below ground and the temperature of the liquid going in, and the fill is measured by weight or by the fixed liquid level gauge rather than by a percentage read off a float. Take the limit for the specific container from the code and from the container's own data plate.
Gas piping is not on this page and is not an afterthought: the pipe between the regulator and each appliance is sized on its own length and on the simultaneous input above, under the fuel gas code the jurisdiction has adopted, and an undersized run starves an appliance exactly the way a cold tank does. Anything involving a container, a regulator, a relief valve or a leak test belongs to a licensed gas fitter.
How to work out what a propane system burns and how long it lasts
The first two steps give you consumption, which is easy. The third is the one that decides whether the fuel can actually get out of the tank.
Take input ratings off the plates, not from a heat loss calculation
Every gas appliance carries a rating plate stating its input in BTU per hour — 80,000 for a mid-size furnace, 40,000 for a tank water heater, 65,000 across the burners of a range. That is what it burns when it is firing, and it is the figure this schedule wants. What the appliance delivers into the room is smaller by its efficiency, and confusing the two undercounts fuel by twenty percent on an older furnace.
Count burner hours, not hours the appliance is switched on
A furnace sized for a design day runs a fraction of the time on a mild one, and a water heater fires for minutes at a time between long idles. The honest way to get this figure is backwards from a delivery: gallons since the last fill, divided by days, divided by gallons per hour, is the burner hours the season actually ran. Guessing forwards is where these estimates go wrong by half.
Tick what can fire at once, and read the latent heat line
The peak draw is not the annual total and it is not the sum of everything you own — it is whatever can call at the same moment on the coldest morning. The page turns that into pounds per hour and then into the BTU per hour that has to come through the tank wall, which is exactly what a container vaporization table is indexed by. Take that number and your ambient temperature to the table for your container.
Technical specifications
| What goes in | An appliance schedule of input ratings in BTU per hour and burner hours per day, with a tick for the ones that can fire simultaneously. Input is the figure on the rating plate and is what the appliance burns; its output is smaller, and fuel is bought on the input, so no efficiency term appears anywhere in the conversion. |
|---|---|
| Heating value | 91,500 BTU per gallon of liquid propane at 60 °F, the US Energy Information Administration published average, prefilled and editable. A delivery ticket or a supplier that states its own figure supersedes it, and all of these are higher heating values — the basis US billing and AFUE both use. |
| Pounds per gallon | Derived rather than quoted: a specific gravity of 0.504 at 60 °F from the LP-Gas property tables, applied to this site's own water density, gives about 4.2 lb per gallon. The page also prints what the heating value divided by the 21,500 BTU per pound figure implies, and states the disagreement between the two instead of choosing one. |
| Container capacity | Water capacity, the size a container is sold as, times the NFPA 58 maximum permitted filling limit of 80 percent — so a 500 gallon tank carries 400 gallons of propane and a float gauge reads 80 when it is full. Duration runs from the current gauge reading down to whatever percentage your supplier triggers a delivery at. |
| The cold-weather figure | Peak simultaneous draw in pounds per hour times 183 BTU per pound of latent heat — the rate at which heat must cross the wetted wall of the container just to boil liquid into gas as fast as the burners consume it. This is the number a vaporization table is indexed by, and the page computes it so you can go read your own row. |
| Where the table stops | At the container. What a specific tank vaporizes at a given temperature and liquid level lives in the NFPA 58 annexes and in the manufacturer's data, and this site does not reproduce it — the answer to an oversized draw is a larger container, two manifolded so their wetted areas add, or a powered vaporizer, never a bigger regulator. |
| The hard floor | Propane boils at −44 °F at one atmosphere. Below that a container exerts no pressure above the atmosphere and delivers nothing at all, regardless of how much liquid the gauge shows. |
| Where the gauge reading goes | In this tab, unsent. An appliance list and a tank reading say a good deal about a property and who is at home, and none of it is stored — the arithmetic also works standing at the tank with no signal, which is where the gauge is. |
Frequently asked questions
My tank is a third full and appliances are cutting out. What is happening?
The tank is almost certainly not keeping up with vaporization rather than running out. Liquid propane boils to make the vapor the regulator feeds, boiling absorbs about 183 BTU for every pound produced, and that heat has to arrive through the part of the tank wall touching liquid. As a tank empties, the wetted area shrinks; as the weather cools, the temperature difference driving heat through that area shrinks too. Draw harder than the two together can supply and the liquid chills itself, the vapor pressure above it drops, and the regulator eventually cannot hold its outlet pressure. You will often see frost on the tank at the liquid line, which is the boundary made visible. More fuel does not fix it — more wetted area does.
How many gallons an hour does a 30,000 BTU heater use?
About 0.33 gallons an hour at the standard 91,500 BTU per gallon, which is a hair under 1.4 pounds. The arithmetic is division and nothing else: input rating over heating value. What people get wrong around it is the heating value rather than the sum — 91,500 is a published average for liquid propane at 60 °F, real deliveries vary with composition, and a gallon metered on a hot afternoon carries fewer pounds and therefore less heat than one metered on a cold morning, which is why propane is sold through a temperature-compensating register.
Why is a 500 gallon tank only filled to 400 gallons?
Because liquid propane expands roughly 1.6 percent for every 10 °F, about seventeen times as much as water over the same range, and a container filled liquid-full has nowhere to put that expansion when the sun comes out. NFPA 58 therefore sets a maximum permitted filling limit — 80 percent by liquid volume for an ordinary above-ground container — and the fixed maximum liquid level gauge is the device that enforces it during a fill. That vapor space is not wasted capacity: it is the safety margin standing between a warm afternoon and the relief valve. It is also why a float gauge on a full tank reads 80 and not 100, which alarms people who have just paid for a fill.
Should I size the tank on the annual usage or on the peak draw?
On both, because they are different constraints and either can govern. Annual usage and delivery frequency decide how big a container has to be so that a truck comes a sensible number of times a season — a house burning 800 gallons a winter on a 250 gallon tank is a delivery every few weeks. Peak simultaneous draw decides whether the container can vaporize fast enough at your design temperature, and that constraint frequently asks for a larger tank than the fuel budget does, especially where a generator or a shop heater is on the same system. Two smaller containers manifolded together answer the second constraint without answering the first, because their wetted areas add while their delivery interval does not change.
Does a 20 lb barbecue cylinder have the same problem?
It has the same physics and much less wall to work with, which is why a small cylinder frosts and fades so quickly under a high-output burner. A 100 lb cylinder feeding a construction heater in freezing weather is the classic case: it works for an hour, the liquid chills, output falls off, and somebody puts a second cylinder on a manifold rather than a bigger regulator on the first. Never apply heat to a cylinder to fix it — an open flame or an electric blanket on a propane container is prohibited by every code that touches the subject, and it is a way to lift a relief valve rather than a way to get more gas.
Is propane cheaper or dearer than natural gas per BTU?
That depends entirely on local prices, but the units make the comparison hard rather than the prices. Propane is sold by the gallon at about 91,500 BTU each; natural gas is sold by the therm, which is 100,000 BTU by definition, or by the ccf, which is a therm only at a stated heating value. Put both on a dollars-per-million-BTU basis before comparing anything. The physical difference that matters more than price is delivery: natural gas arrives continuously at a regulated pressure and propane arrives on a truck into a container you have to keep from running out or running cold, which is the entire subject of this page.
Can I use this to size the gas piping?
No — the simultaneous input figure it produces is one of the inputs to that calculation, and the rest of it is not here. Pipe sizing works from the total connected input carried by each section, the developed length of the run from the regulator, the pressure drop allowed and the pipe material, against the sizing tables in the fuel gas code the jurisdiction has adopted. An undersized run starves an appliance in a way that looks exactly like a cold tank, and telling the two apart in the field means checking the manifold pressure at the appliance while everything else is firing. That work, and anything touching a container, a regulator, a relief valve or a leak test, belongs to a licensed gas fitter.
Boiling a liquid on demand, and why the tank runs cold before it runs out
Consumption is the easy half and every calculator gets it right: an appliance’s input rating divided by the fuel’s heating value is gallons per hour, and nothing about that division is subtle. Two things around it are. The first is that a rating plate states input rather than output, so the figure to divide is what the burner consumes and not what the appliance delivers — an 80 percent furnace rated at 80,000 BTU per hour input puts 64,000 into the house and burns fuel for all 80,000. The second is that burner hours are not clock hours. Equipment sized for a design day idles through most of a normal one, and the only reliable way to get the number is backwards from a delivery ticket: gallons burned, divided by days elapsed, divided by gallons per hour.
The hard half is that a propane container does not store gas. It stores a liquid under its own vapor pressure, and every pound of vapor drawn off the top has to be boiled out of that liquid, which absorbs roughly 183 BTU per pound. There is no heater inside the tank; that heat comes through the wall from the air outside, driven by the difference between ambient temperature and the boiling liquid, across whatever area of wall is actually touching liquid. Both terms move the wrong way in winter — the air is colder and the tank, as the season goes on, is emptier and therefore has less wetted wall. Exceed what those two can supply and the liquid cools itself until its vapor pressure can no longer hold the regulator’s outlet, appliances drop out one after another, and a frost line appears on the tank at the liquid level. The gauge still reads a third full. The failure is thermal, not a shortage, and no regulator can make gas that has not boiled.
So this page computes the latent load your peak draw puts on the wall and stops there, because the rest is a table it will not reproduce: what a specific container vaporizes at a given temperature and level is published in the NFPA 58 annexes and by the tank manufacturer, indexed by wetted area and temperature difference, and inventing a number for it would be worse than sending you to read it. When the draw is too large the fixes are a bigger container, two manifolded so their wetted areas add, or a powered vaporizer. Everything upstream of the burner is a different question: how much heat the building needs at all is a heat loss calculation against the envelope, which is where the assembly R-value of the walls decides more than the fuel price does, and what equipment to put on the end of it is the furnace size calculator. A tank shared with a standby generating set deserves its own line on the schedule, because a set running through a three-day outage burns more in those days than the house does in a month.
Where the appliance list lives
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
An appliance schedule with a tank reading beside it says what a property runs on and how often somebody is there, and it belongs on your phone rather than in a form. Nothing here is sent anywhere. If the reason you are counting gallons is a whole-house standby supply, the electrical half of that job is the electrical load calculator; and a property looking to burn less of everything usually finds the largest single saving in the envelope, then in whatever the solar panel output calculator says a roof could contribute.