Fixer & Stop Bath Capacity Calculator (Rolls per Litre, Fixing Time, When to Discard, Wash)
Track how many rolls or sheets a litre of working fixer and stop bath will process, how the fixing time grows as the fixer is used, when the clearing test says to discard it, and the wash sequence for archival negatives and prints.
How it works
capacity = rolls per litre × litres (rapid fixer 1+4: 24 rolls film, 12 T-grain, 40 8×10 sheets; 1+9: 12 / 6 / 40; sodium thiosulfate: 15 / 8 / 30) · fixing time ≈ 2 × clearing time (≥ 1 min); estimated time grows ≈ ×(1 + fraction used) · discard when clearing time ≥ 2 × fresh · stop bath ≈ 15 rolls or sheets per litre
Fixer dissolves the undeveloped silver halide out of the emulsion; as it works it accumulates silver and slows down, until it can no longer clear the film in a reasonable time and starts leaving compounds that later stain. The manufacturers publish capacities per litre, and the darkroom test is simpler still: time how long a snip of unexposed film takes to turn clear, and fix for twice that — discard when the time has doubled. T-grain films carry more silver and exhaust fixer twice as fast; paper needs a weaker dilution and short times so the base does not soak up thiosulfate. A stop bath is cheap insurance for both fixer life and consistent development.
Worked example
One litre of rapid fixer at 1+4 handles about 24 rolls of conventional film. After 8 rolls it is a third used: the fixing time has stretched from 3:00 fresh to about 4:00, and a leader that clears in 45 s (versus 30 s fresh) confirms it is still good — fix for 1:30 minimum by the clearing rule, and discard when clearing reaches 60 s. The matching indicator stop bath has about 7 rolls of life left.
Frequently asked questions
How do I do the clearing test?
Drop a snip of undeveloped film leader into the fixer and time how long until it turns fully transparent (look through it at a light). Fix for at least twice that time. When it takes twice as long as it did in fresh fixer, replace the fixer.
Can I fix film and paper in the same fixer?
Keep them separate. Film fixer at 1+4 is stronger than paper needs and carries silver and anti-halation dye into paper; paper fixing at film strength soaks thiosulfate into fibre base and lengthens the wash. Use 1+9 for paper and 1+4 for film, each with its own bottle.
Is longer fixing safer?
Not much. Over-fixing bleaches fine highlight detail in film slowly, and in fibre paper it loads the base with fixer that is hard to wash out. Fix for twice the clearing time (film 3–5 min, RC paper 30–60 s, fibre 1 min in fresh rapid fixer) and wash properly.
Related calculators
Darkroom figures are starting points. Film, developer batch, agitation, water and thermometer accuracy all shift results — run a test roll or strip, keep notes, and adjust from your own results.