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There are two types of jet fuel used in commercial aviation: Jet A (and its version Jet A-1) and Jet B. They have slightly different densities. Jet A / Jet A-1 is the most common type.
According to Chevron, at 15 ºC the densities are
Jet A / Jet A-1: 0.775 to 0.840 kg/LJet B: 0.751 to 0.802 kg/L
So, 1 liter of jet fuel can weigh between 0.75 and 0.84 kg. If you need one specific number, you can use 0.81 kg, which is roughly the average for Jet A / Jet A-1 type.
References:Chevron Products Corporation - Aviation Turbine Fuel SpecificationsWikipedia - Jet fuel
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