Great question – I had to look this one up, and I unfortunately don’t think I can give you an exact answer. Most of the fuel needed for space missions is used at the very beginning of the mission, to get out of the Earth’s atmosphere and into space. Once in space, the route of the mission to Mars can be specially chosen to make the most of the gravity of the Earth and Moon, to reduce the amount of fuel needed. Rocket fuel is normally given by a mass rather than a volume, as many of the calculations look at the mass of fuel needed per unit mass of payload (i.e. the spacecraft being launched). Also, some types of rocket fuel can be solid rather than liquid too. The launch vehicle used to launch the Perseverance rover, an Atlas V-541, had a mass of over 500000 kg once it was fully fuelled and carrying the rover. Bearing in mind that this was just to get to Mars, and not to come back as well, it would take many hundreds of tonnes of fuel for a full journey there and back. Scientists are exploring the idea that if astronauts go to Mars, they can use materials on Mars (such as methane gas) to generate the fuel needed for the journey back.