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

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
**d**rinking, each day~~(liters)~~ - `w_c` = the amount of water each person needs for
**c**ooking, 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)

- If you burn large gas canister (230g) for an hour a day, it will last 5 days.
- an 8 ounce (~220g) canister will burn for ~3 hours
- more articles: [1] [2] [3]
- more threads: [1]

Canisters come in 100g (3.5 ounces) and 230g (8.1 ounces) sizes.

**gas calculator**- 0.25 ounces fuel / 1 pint water
- 1 ounce / pint = 28 grams / 0.47 liters = 59.91 grams / liter
- 0.25 ounces / pint = 15 grams gas / liter water

- boiling 2 cups of water (starting at room temperature) consumes 0.25 ounces (7g) of gas
- this agrees exactly with the figure from above, so this is also
__15 grams gas / liter water__

- this agrees exactly with the figure from above, so this is also

- 3 cups per person per day for cooking
- drinking water
- assume we hike 10 hours a day
- so that's 56 cups a day