# What is the surface area of a roll of 35mm or 120 film (for calculating the amount of fixer to use)?

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:

1. What is the (ballpark) surface area of a 36 exp roll of 35mm film.

2. What is the surface area of a 24 exp roll of 35mm film?

3. What is the surface area of a 120 roll of film?

• If you're developing in a tank, fixer mixed to the manufacturer's instructions will do fine if you pour in enough to cover the reel. Aug 22, 2015 at 11:41
• @Alex, do you need more precision ? Aug 22, 2015 at 15:00
• Thanks @Olivier. It doesn't have to be too precise, so I can assume about 0.05 m^2 so fixer that can fix 1 m^2 of film can do about 20 films. However, I think it is is interesting that Kodak has given specifications, which I found very hard to find elsewhere on the web. If anyone has any other calculations please let do post them as answers!
– Alex
Aug 22, 2015 at 20:15
• I used to test the strength of what I had before re-using it by dumping a scrap of exposed film in a glass of the stuff and seeing how long it takes for it to turn transparent. That's the fix time at the current strength. How much it gets depleted varies with how much silver is being removed from the film. Aug 22, 2015 at 21:10
• You are welcome @Alex. I think we gave you the best answer we could regarding your question. If you are looking for the best way to use a specific Kodak fixer for a given film and want to know about temperature, timing, dillution or other settings (equipements,...), you should probably post a new question... and I won't answer this new one as I would be playing out of my league :) Aug 22, 2015 at 21:11

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.

• In reality, the fixer will also "fix" the orange area around the sprocket holes. Kodak always said 80 sq. inches for a 36 exposure roll of 35mm. 24 exposures would be about 2/3 of that. For medium film, it is easy to just measure length and width of the hanging developed film strip. This area concept is only about predicting the life of the fixer, not about measuring usage in the film tank. Aug 22, 2015 at 14:11
• @WayneF, thanks for the value of 80 sq inches. It is about 0.052 m², close to my "crude" estimation of 0.046 (13 % difference) but surely a better estimate, coming from Kodak Aug 22, 2015 at 14:51
• @WayneF can you please post that as an answer with a reference if you have one?
– Alex
Aug 22, 2015 at 20:13
• Alex, 80 sq inches is quite universally known (8x10 inch print equivalent). Just search google.com/search?q=80+sq.+in.+35mm+film+kodak and many references will tell you the same 80 sq in. Aug 23, 2015 at 5:49
• @Lanti see my edited answer for 220 roll (last line) Oct 11, 2016 at 19:13