How long does a 420lb Propane Tank last?

The longevity of a 420lb propane tank varies significantly based on the heater you’re using it with.

For a smaller heating system, it might last you almost an entire season, but if you’re powering a strong industrial-scale heater, a 420lb propane tank can run out of gas in just a matter of days.

Quick answer: A 420lb propane tank can last from roughly 60 hours up to an impressive 2,419 hours, depending on the heater’s BTU rating. With a standard household 9,000 BTU heater, it lasts for 805 hours (33 days) of uninterrupted heating. On the other hand, with an industrial-grade torpedo heater (120,000 BTU), it would last just about 60 hours.

Let’s dive deeper to understand these durations.

What’s the maximum fill level of a 420lb propane tank?

Much like the 100lb tanks, 420lb propane tanks are not filled to their maximum capacity due to the properties of propane.

As the temperature fluctuates, propane expands or contracts.

To accommodate this change in volume and to ensure safety, propane tanks are typically filled only up to 80% of their total capacity.

Hence, a 420lb propane tank would contain up to 336lb of propane when filled to this recommended level.

How much energy is in a 420lb propane tank?

Using the known heat energy of propane, which is 21,594 BTU per pound:

420 lb × 21,594 BTU per lb × 80% = 7,252,992 BTU

This value represents the total energy stored in a typically filled 420lb propane tank.

How much energy do various propane heaters consume?

Just as we detailed in the 100lb tank post, the BTU ratings of propane heaters can be vast. From petite personal heaters to gigantic industrial units, the range is wide.

The table below provides a quick reference of various heater sizes and how long they would last with a 420lb tank:

To calculate how long a propane tank will last given a heater’s BTU/hr rating, we can use the formula:

Tank Capacity (in hours) = Total BTUs in Tank ÷ Heater’s BTU/hr


  • The “Total BTUs in Tank” represents the energy content of the propane in the tank. Heater’s
  • “BTU/hr” is the rate at which the heater consumes propane.

Given the total BTUs in the 420lb propane tank is 7,252,992 BTU (as derived earlier), let’s have a look at the table:

Heating power (in BTU/h)420 lb propane tank capacity (in hours)
3,000 BTU/h2,418 hours
6,000 BTU/h1,209 hours
9,000 BTU/h805 hours
12,000 BTU/h604 hours
15,000 BTU/h483 hours
18,000 BTU/h402 hours
21,000 BTU/h345 hours
24,000 BTU/h302 hours
27,000 BTU/h268 hours
30,000 BTU/h242 hours
33,000 BTU/h220 hours
36,000 BTU/h201 hours
39,000 BTU/h186 hours
42,000 BTU/h173 hours
45,000 BTU/h161 hours
48,000 BTU/h151 hours
51,000 BTU/h142 hours
54,000 BTU/h134 hours
57,000 BTU/h127 hours
60,000 BTU/h121 hours
90,000 BTU/h81 hours
120,000 BTU/h60 hours
150,000 BTU/h48 hours

I rounded the above figures for simplicity. As you can see, the tank capacities decrease as the heating power of the device increases, which makes sense because the more gas you draw, the shorter a 420lb propane tank lasts.

For everyday use, you’ll most likely employ a heater with a rating around 9,000 BTU/h. Such a heater will run for 805 hours on a 420lb propane tank.

One example is the Mr. Heater Buddy portable propane heater.

Larger heaters, such as vented propane heaters for garages and larger spaces consume up to 50,000 BTU per hour and more. 

Practical daily use of a 420lb propane tank

Though the table illustrates how long a propane tank lasts for continuous running times, it’s rare for heaters to operate non-stop every day.

The following table shows how long a 420lb propane tank lasts, if you run a 9,000 BTU/h heater for just a few hours a day:

Daily heating duration (in hours)How long does a 420lb propane tank last? (with a 9,000 BTU/h heater)
2 hours402 days
4 hours201 days
6 hours134 days
8 hours100 days
10 hours80 days
12 hours67 days
24 hours34 days

How to make your 420 lb propane tank last longer

Increasing your propane tank’s life can be achieved through a few measures:

  • Adjust the Heating: The most direct approach is to set a lower heat setting. Reducing the heating output will naturally extend the life of your propane tank.
    For example, my Mr. Heater portable propane heater has two different heat settings, corresponding to 4,000 BTU/h and 9,000 BTU/h.
    So, on the low setting, my propane tank lasts twice as long as on the high setting.
  • Check for Leaks: Regularly inspect your propane tank, heater, and the connecting hose for potential gas leaks.
    Also, I recommend getting a new propane gas hose every couple years to ensure there are no micro-leaks which you can’t detect.
    I recommend getting this PatioGem propane gas hose (click here to see it on amazon).
    This not only ensures your safety but also makes certain that you aren’t losing gas unnecessarily.

Also, I highly recommend you have a look at this article: Why is my propane heater using so much gas?

There, I list 8 different causes of high gas consumption and how to fix them.

And additionally, you get more tips on further minimizing propane consumption through insulation.


For a commonly used 9,000 BTU/h indoor heater, a 420lb tank can provide a full 805 hours of heating, translating to around 34 days of continuous heating.

In practical terms, with about 8 hours of daily heating, a 420lb propane tank can serve you for nearly three months.

You can maximize your tank’s longevity by ensuring you use a new hose without leaks and running your heater on moderate settings.

Also, check out the other posts in the series: