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Frequently I see specs on fixers like: 1L working solution will fix 1 m^2 of film. This is not terribly helpful for me as I don't know the area of 35mm or 120 film. So, can someone please summarise:
What is the (ballpark) surface area of a 36 exp roll of 35mm film.
What is the surface area of a 24 exp roll of 35mm film?
What is the surface area of a 120 roll of film?
A 35mm film has an image size (exposure area) of 24*36 mm. Generally, you have a 2 mm gap between two consecutive image and about 5 mm on each side for the perforation. As the fixer will react on all the film, all the surface has to been accounted for. The holes should be taken into account too but you probably won't see a difference if you don't.
So for our purpose, the area of an "image" is (36+2)*(24+5+5) = 38*34 = 1292 mm² (or about 0.001292 m²).
1) The surface area of a 36 exp roll of 35mm film is about 36*0.001292 = 0.0465 m²
2) The surface area of a 24 exp roll of 35mm film is about 24*0.001292 = 0.0310 m²
Now for the 120 film roll, the film is about 61 mm large and 760 mm long (which allows 8 exposures as a single exposure is about 56*84 mm + gaps, assuming 2*3 format). It gives a surface of 0.061*0.760 = 0.04636 m².
3) The surface area of a 120 roll is about 0.04636 m².
It's only mathematics (and conversion form inch or millimeters to meters), you should be able to apply this with any film you want.
Don't use those number to compute the fixer to water ratio. For film, a mix of 1 + 4 is a classical number. So if you need 1 L, 200 ml of fixer + 800 ml of water. Using 1), one could say : I have 0.0465 m² to develop, so I will use 0.0465 L of the fixer and add as much water as needed => nope, don't do that :)
Edit: a 220 roll is twice as long as a 120 roll so it shouldn't be hard to compute its area.
