# How many litres of gaseous helium would it take to lift 80 kg on Earth?

I have dragons in mind for my world, but I'm trying to find logic in how would a medium-sized anything fly off and still be a danger to humans.

So far, I have a lizard-like cheetah with wings, hollow bones and very strong legs but I still doubt it would be able to fly if it had over 60 kg. So anyway I devised a plan for that, so if anyone has knowledge of physics and such that could answer the question I would be very grateful, thanks.

• Welcome to Worldbuilding! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun! Commented Jan 25, 2019 at 17:06
• We know that there were flying dinosaurs weighting 145kg or more so there's no reason your dinosaur at 60 or 80kgs could not fly without using buoyancy. Commented Jan 25, 2019 at 17:13
• @TimB pterosaurs are not dinosaurs, but that is a good low end mass estimate for Quetzalcoatlus. the largest flying dinosaur aka bird was around 70kg.
– John
Commented Jan 25, 2019 at 17:33
• There is a rather useful calculator here - omnicalculator.com/everyday-life/helium-balloons Commented Jan 25, 2019 at 20:45
• This seems like a pure real-world physics question to me. Since we already have a site for those, I feel like this question should be migrated there. Commented Jan 25, 2019 at 21:11

For a back-of-the-envelope calculation the rule of thumb is that you need one cubic meter (1000 liters) of hydrogen or helium to lift one kilogram; so for 80 kilograms you need about 80 cubic meters (80,000 liters) of lifting gas.

For a more detailed calculation:

• The average density of air at sea level is about 1.2 kg/m³.

• The density of helium at 1 atmosphere pressure and 0°C temperature (that's called "standard temperature and pressure", STP) is about 0.18 kg/m³.

• The lifting force is the difference between the weight of a volume of helium and the same volume of air, or 1.02 kg/m³.

• For 80 kg you need 80 / 1.02 = 78.4 m³ of helium.

For a quick comparison, that's quite comparable with the capacity of a large railroad tank wagon, and about 15% more than the internal volume of a standard 40-foot (12.2 meters) intermodal container:

A stack of standard 40-foot (12.2 meters) intermodal containers. Each container has an internal volume of about 68 cubic meters. Photograph by Martini171, available on Wikimedia under the CC BY-SA 3.0 Unported license.

I still doubt it would be able to fly if it had over 60kgs

Helium has a lifting force of one gram per liter. For a 60kg person or animal, you will therefore 60,000 liters. That is 15,850.323 gallons for americans.

The first thing that comes to my mind when I think about 60,000 liters is that it's the usual volume capacity for yhese trucks:

Also notice that flying this way may be very hard. Larry Walters comes to mind.

• Thatsalottahelium....
– JBH
Commented Jan 25, 2019 at 17:22

it would theoretically take 115920 cubic feet of helium to lig=ft 80 kilograms. Because the difference in the up and downforce is 0.069 pounds. each cubic foot of helium could lift 0.069 pounds. In order to lift a single kilogram, you would need 1449 cubic feet of helium. Now we have to transfer the cubic feet to liters. 1 cubic foot= 28 litres 115920x28 Theoretically, it would take 3245760 liters to lift 80 kilograms.