If you’re stranded in your vehicle in the sort of temperatures that much of Canada has experienced of late, it’s not long before things can get life-threatening. At 35 below, frostbite manifests in minutes, and if you’re not fully decked out in expedition-grade winter gear, these temperatures can become lethal.

This is especially true if you’re ill-prepared – not having a proper survival kit or skipping out on essential preventative maintenance – or if the weather conditions or your location mean that you’ll be waiting some time for help to arrive. In such cases, it’s useful to know how long your vehicle can idle (for heat), on whatever amount of fuel happens to be remaining in its tank.

It’s an easy thing to figure out.

Read on for the how-to. You only need to know three numbers, and do two easy calculations. With these complete, you’ll be able to look at the fuel gauge in whatever you drive and visualize a number of possible idling hours based on how full your fuel tank is.

Note that the calculations below only apply to modern fuel-injected gasoline engines, and not necessarily hybrids or diesels – in other words, a gas-powered car built in the past few decades.

Here’s how to do it.

Step 1: Write Down These Three Numbers

The displacement of your engine, in litres

This is commonly indicated right on top of the engine itself (just pop the hood), or available in the owner’s manual, or via a quick search on the internet. Maybe, you know the displacement of your engine, in litres, off of the top of your head.

In most mainstream vehicles, engine displacement falls between about 1.5 and 3.5 litres of displacement. The number of cylinders, and whether or not the engine is turbocharged or supercharged, is irrelevant.

The capacity of your fuel tank, in litres

Check your owner’s manual, the manufacturer website specification page for your specific vehicle, or do a quick search. You need to know how many litres of gasoline your fuel tank can hold.

Note that in some applications (typically pickup trucks), several different fuel tank sizes are possible. Be sure to track down the fuel tank capacity for the specific vehicle you’re driving.

The number 0.6

The number 0.6 is easy, but important. This is the number of litres of fuel your engine burns per hour, per litre of displacement, while at idle. Special thanks to the folks at Natural Resources Canada (NRCan) for helping us with this number.

Step 2: Calculate How Many Litres Your Engine Uses Per Hour of Idling

This is easy-peasy. Just multiply the engine displacement in litres by the number 0.6. The resulting figure is how many litres of gas your engine uses per hour, while it’s idling.

Here are a few examples

If you drive a Ford Explorer with the 3.5-litre engine, then it uses (3.5 litres engine displacement x 0.6) = 2.1 litres of fuel per hour while idling.

If you drive a Hyundai Kona 1.6T, then it uses (1.6 litres engine displacement x 0.6)= 1 litre per hour of fuel while idling.

If you drive a Chevrolet Silverado with the 5.3 litre engine, then it uses (5.3 litres engine displacement x 0.6) = 3.2 litres per hour of fuel while idling.

If you drive a Dodge Viper, then it uses (8.4 litres engine displacement x 0.6)= 5 litres per hour while idling.

Still with me? Good. Just sub in the displacement of your specific engine to the formula above, and you know how many litres of fuel per hour your vehicle’s engine requires when idling.

Step 3: Calculate Full-tank Idling Hours

Now, you know how many litres of fuel your engine uses per hour of idling, and how much fuel its tank can hold. With this information, you can easily figure out how many hours of idling are possible on a full tank.

Let’s use the Ford Explorer from the example above. With the 3.5-litre engine, we know it uses (3.5 x 0.6) = 2.1 litres of fuel per hour at idle. We also know from a quick google search that the Explorer has a 70-litre fuel tank.

Now, simply take the fuel tank size (70 litres, in this case) and divide by your litres-per-hour-at-idle figure (2.1 litres, in this case).

Therefore, 70 litre fuel tank / 2.1 litres per hour at idle = 33 hours.

That’s 33 hours of idling possible on a full tank.

Once you’ve calculated the full-tank idling hours available for your specific vehicle, commit it to memory.

From there, it’s just a matter of adjusting that number based on the amount of fuel in your tank. In our example with the Explorer, 33 hours of idling on a full tank means about 16 hours of idling are possible on a half tank, and that about 8 hours are possible on a quarter tank.

Also, remember that you can stretch your fuel supply by idling the engine for, perhaps, 30 minutes every hour, thereby doubling the amount of idle time possible.

Finally, don’t forget to clear the snow away from your tailpipes if you’re idling while stranded, as this can prevent dangerous exhaust gasses from poisoning you while you wait for help to arrive.