Calculating how much weight of fuel can seem a bit confusing, and certainly lethal if done incorrectly. In this article I will show you how to make an estimated calculation so that if you need to calculate the actual weight, you’ll have a rough idea to begin with.

The reason we have to calculate the weight of fuel instead of just taking an average figure per gallon or litre is that the volume of fuel changes depending on its temperature. If you fill a tank to the brim on a cool evening and the next morning the temperature rises as the sun beats down on the wing, the fuel will start to spill out of the vent. This is because the density of the fuel is greater when it is cold than hot as it expands in the heat. The density of water also changes with temperature but the amount is negligible, however it is at its densest at 4°C. When we talk about the specific gravity of fuel (or specific density), we are comparing it to a sample of water at 4°C. Water weighs 1 kilogram per decimetre (which is 10cm x 10cm x 10cm/ or 1 litre at 4°C). If we take a litre of fuel and it has a ‘specific gravity’ of 0.8, that means it will weigh 80% of the weight of water, so 0.8kg.

Calculating the weight of fuel in litres to kilograms is simple. In my head, if I added 78 litres of fuel to the tank, if it was water it would be 78 kilograms, and as it is fuel (hopefully not water!) we can multiply it by the Spg (in the UK in winter try using 0.8 and in the summer 0.70 as safe numbers—— it is in fact 0.71 at 15°c).   78kg X 0.8 = 62.4kg

UK gallons to pounds also has a similar hack to work out the weight, because 1 (UK!!) imperial gallon of water weighs 10 pounds.

So to add Imperial 45 gallons and the weight is 45 UK gals X 10(if it was water)  X 0.8 (as it is fuel)= 360lbs. Be careful when using gallons because you fill up in the UK