How long a 100lb propane tank lasts depends entirely on the propane heater that you connect to it. While a small heater can run for many weeks, a very strong industrial-grade heater will only last one day.
Quick answer: A 100lb propane tank lasts anywhere between 13 hours to 576 hours depending on the heating power. On an average household 9,000 BTU heater, it will last for 192 hours of heating. On a (very) strong industrial-grade torpedo heater it will last 13 hours only.
Let’s have a look at where these numbers come from. Additionally, I prepared a long table with all heater sizes (in BTU) where you can look up your heater’s heating duration on a 100lb propane tank.
What’s the maximum fill level of a 100lb propane tank?
Propane tanks are never filled to 100% of their capacity. The reason is that varying temperature causes propane to expand and shrink a lot.
Temperature changes affect the volume of propane gas about 17 times as much as water vapor.
This causes a propane tank filled to 80% in spring to be at 85% (or higher) in the summer, given that none of the propane was used up.
The entrapped propane gas in a propane tank can’t change its volume. The volume is fixed to the tank’s volume.
Because the propane can’t expand inside the tank, the internal pressure increases with increasing temperature.
To prevent an explosion due to too high pressure, propane tanks are filled only to 80%. The 80%-rule is a safety measure.
The remaining 20% of the volume inside a propane tank is air. Air compresses very well (that’s why it’s used in suspension parts). So, you can think of the remaining 20% of air as suspension for high-pressure propane.
Therefore:
A 100lb propane tank contains up to 80lb of propane.
How much energy is in a 100lb propane tank?
One pound of propane contains 21,594 BTU of heat energy (source).
A 100lb propane tank filled to 80% capacity (maximum) contains
100 lb × 21,594 BTU per lb × 80% = 1,727,520 BTU
of heat energy.
How much energy do different propane heaters use?
There are many different propane heater sizes. Accordingly, they all release different rates of heat energy.
For example, a small-sized propane heater consumes 3,000 BTU of heat per hour (denoted in BTU/h).
A moderately sized propane heater such as the Mr. Heater Buddy and Big Buddy heaters (which I always recommend, click to see my review) consume around 9,000 BTU to 18,000 BTU of heat per hour.
Tank top heaters consume an average of 30,000 BTU to 45,000 BTU per hour.
Large patio heaters are usually 45,000 BTU/h.
Industrial-grade torpedo heaters consume up to 125,000 BTU/h.
Because no two propane heaters are equal, I have prepared a long table of all possible BTU ratings and their respective tank capacity (in hours) below.
| Heating power (in BTU/h) | Tank capacity (in hours) |
| 3,000 BTU/h | 576 hours |
| 6,000 BTU/h | 288 hours |
| 9,000 BTU/h | 192 hours |
| 12,000 BTU/h | 144 hours |
| 15,000 BTU/h | 115 hours |
| 18,000 BTU/h | 96 hours |
| 21,000 BTU/h | 82 hours |
| 24,000 BTU/h | 72 hours |
| 27,000 BTU/h | 64 hours |
| 30,000 BTU/h | 58 hours |
| 33,000 BTU/h | 52 hours |
| 36,000 BTU/h | 48 hours |
| 39,000 BTU/h | 44 hours |
| 42,000 BTU/h | 41 hours |
| 45,000 BTU/h | 38 hours |
| 48,000 BTU/h | 36 hours |
| 51,000 BTU/h | 34 hours |
| 54,000 BTU/h | 32 hours |
| 57,000 BTU/h | 30 hours |
| 60,000 BTU/h | 29 hours |
| 70,000 BTU/h | 25 hours |
| 80,000 BTU/h | 22 hours |
| 90,000 BTU/h | 19 hours |
| 100,000 BTU/h | 17 hours |
| 110,000 BTU/h | 16 hours |
| 120,000 BTU/h | 14 hours |
| 130,000 BTU/h | 13 hours |
As you can see, the smallest heater lasts for at least 576 hours on a 100lb propane tank. Meanwhile, the largest heater would only last for 13 hours.
An average indoor propane heater of 9,000 BTU/h lasts 192 hours on a 100lb propane tank. These are the kinds of heaters used in RVs, cabins, garages, or in your living space.
How long will a 100lb propane tank last for regular everyday usage?
The table above shows the number of hours you can run your heater continuously on a 100lb propane tank.
However, you likely won’t run a heater for 24 hours a day continuously.
So, a more realistic approach would be to assume 4 hours to 8 hours of continuous heating a day. This, of course, depends on the weather.
For the table, I assume a heater running on 9,000 BTU/h, which is enough to keep a (very) large living space or a large uninsulated cabin or RV heated:
| Daily heating duration (in hours) | How long does a 100lb propane tank last? (with a 9,000 BTU/h heater) |
| 2 hours | 96 days |
| 4 hours | 48 days |
| 6 hours | 32 days |
| 8 hours | 24 days |
| 10 hours | 19 days |
| 12 hours | 16 days |
| 24 hours | 8 days |
As you can see, your daily heating duration impacts the propane tank’s longevity a lot.
How to increase a propane tank’s durability
You can look up the heating duration of your 100lb propane tank in the table above. All you need to know is your heater’s BTU rating.
But how can you increase the time your propane tank lasts?
Here are a few quick and simple tips:
The greatest direct impact on a propane tank’s durability is the amount of gas your heater draws from it.
So, the simplest way to increase your propane tank’s durability is to turn down the heating setting.
Outputting half of the heat doubles the heating duration you get out of your tank.
Also, you should check your propane tank, your heater, and the gas hose for leaks. Leaking gas is gas that you can’t use for heating.
And if you are unlucky, a leakage will drain gas even if the heater is not running!
You can check this by connecting your heater to the propane tank, and then listening for a hissing sound (without the heater turned on).
Conclusion
A 100lb tank lasts for 192 hours on a strong 9,000 BTU/h indoor heater. That’s 8 days of continuous heating (which is usually way more than you need).
You can expect a 100lb propane tank to last you for 3-4 weeks of usage when you heat it for 8 hours a day.