When designing your raft you must know the weight you intend to float.
If you work in kg it makes your calculation easier.
As a rule of thumb you must have 1.5 times the weight of your team in displaceable water.

It sounds complicated but it’s not – here is an example calculation:

• You have two heavy team members each weighing 80kg.
• This is a total weight of 160kg.
• So as a rule of thumb you want 160 x 1.5 = 240 litres of displacement.
• So your barrels must have a combined volume of 240 litres. It sounds a lot but this is only two big water drums.

For more advice, here's a raft-building website