Analog Film Development Time Calculator

Analog film development is a chemical process where exposed film is soaked in developer solution to make the latent image visible. Temperature directly affects how fast the chemical reaction occurs, and even a 1°C difference shows up in the negative.

The Ilford/Kodak temperature adjustment rule:

For every 1°C above 20°C (68°F), reduce development time by approximately 10%. For every 1°C below 20°C (68°F), increase development time by approximately 10%.

More precisely:

Adjusted Time = Base Time × (0.9 ^ ΔT°C) when warmer

Adjusted Time = Base Time × (1.1 ^ |ΔT°C|) when cooler

That is what this calculator runs, and it is worth knowing how well it holds up. Compared against the multiplication factors Ilford publishes, it tracks them closely across the usable range: it gives 0.81× at 22°C where the chart says 0.8, 0.66× at 24°C where the chart says 0.65, and 1.21× at 18°C where the chart says 1.25.

Push much beyond that and it stops being trustworthy. At 30°C the rule asks for 35% of the base time, and no manufacturer publishes data that far out because developer that warm also softens the emulsion and lifts grain. Treat anything outside 18–24°C as a starting point for a test roll, not a number to trust on film you care about.

One quirk of the rule as everyone states it: 0.9 and 1.1 are not each other’s inverse. Warm a bath by 1°C and cool it back and you land at 0.99 of where you started. It is a rule of thumb, and that 1% is far smaller than the error in your thermometer.

Common developer times at 20°C (68°F):

Agitation pattern:

Standard agitation: Invert the tank continuously for the first 60 seconds, then 3–4 inversions every 30 seconds. Less agitation = slightly lower contrast and finer grain. More agitation = higher contrast and potentially more grain.

Temperature stability is critical:

A water bath at your target temperature keeps the developer, stop bath, and fixer all at the same temperature. Fluctuations of more than 1°C during development can cause uneven results.

Stop bath and fixer times are less temperature-sensitive than developer. Typically 1 minute in stop bath and 3–5 minutes in fresh rapid fixer are sufficient across normal temperatures.