How to estimate of the amount of gas that you need for backpacking with the Primus Eta Spider?

fuel consumption formula

The variables involved are:

• p = number of people
• d = number of days
• f_p = the amount of fuel needed to heat water for purification tablets (grams of fuel / liter of water)
• f_b = the amount of fuel needed to boil water, usually for cooking (grams of fuel / liter of water)
• w_d = the amount of water each person needs for drinking, each day (liters)
• w_c = the amount of water each person needs for cooking, each day (liters)

The amount of fuel you need is: $$p \cdot d (f_p w_d + f_b w_c)$$

Some notes:

• f_p and f_b will vary based on your stove, as well as whether you're melting snow or not
• w_d varies a lot based on the person.
• The f_p w_d term is only needed if the water is very cold / frozen to start with. If it's warm enough, you can ignore this part.
• you may need to cook one fewer day than you need water for (because you usually can eat breakfast before leaving, and eat dinner after returning)