This mead-making calculator tool guides you on how much honey you will need to add for any given volume of water to reach your desired alcohol level.

Note that this does not include the honey used for backsweetening at the end of fermentation, which will vary based on your taste. For a reasonably sweet mead, this will be around an extra fifth of the honey you used at the start. For example, if you used 2.5kg of honey to make a small batch, you would usually need another 0.5kg to backsweeten (after stabilising), see our homebrew stabilising calculator.