When developing black-and-white film, you may need to calculate the concentrate vs water (dilution) measurements. These measurements change depending on how much solution you’d like to mix.
For example, 300ml or 10oz of developer solution will cover a single 35mm film — but what does that mean for water and concentrate volumes if you’re developing with Rodinal at 1+25 dilution?
To get the exact water and concentrate volumes needed for the 300ml mix, you’ll need to solve this equation:
x = (1/25) × (300 — x)
…Where “x” is the amount of developer needed.
The answer is 11.54ml of developer, which leaves us with 300 — 11.54 = 288.46ml of water. To check the answer, we can divide 288.46 by 11.54, which gives us 25 (one part developer, 25 parts water). This answer should be rounded to the more practical 290ml and 12ml, or it can be converted to 10oz and 12ml for the US/UK beakers.
(Note that the rounding should ideally differ for ounces and millilitres, and it may be prudent to round the concentrate volume up.)
Chemical Dilution Calculator does all of the above. It also automatically switches default measurement units based on your location and lets you see the precise measurements if needed.
The Chemical Dilution Calculator is part of the Chem Log app. To use it, launch this page: analog.cafe/app/chem-log, and click the purple “Dilution Calculator” button.
Dmitri Jan 13, ‘25
eric Jan 14, ‘25