Film development temperature calculator: time at another temperature
Data sheets give black-and-white development times at 20 °C (68 °F). When your tap water or room is warmer or cooler, enter the 20 °C time and the temperature of your developer, and you get the adjusted time.
Temperature compensation chart and how the time is calculated
Ilford publishes a film development time/temperature compensation chart (April 2002) and calls it a guide for all film and developer combinations. It is a lookup table: find your 20 °C time, read across to your temperature. This calculator uses one smooth curve fitted to the chart:
time = time at 20 °C × e^(−0.103·d + 0.00207·d²), d = temperature − 20 °C
I fitted the two numbers by least squares to the chart's rows that start at 6 minutes or more. Checked against every cell of the chart away from 20 °C, 278 of 292 come out within 15 seconds (the chart rounds to 15 seconds) and the largest difference is 22 seconds. Ilford's own example on the chart, 8 minutes at 20 °C becoming 5 minutes 30 seconds at 24 °C, comes out at 5:29.
Ilford's chart covers 18–27 °C. Its powder developer sheet gives the 10% per °C rule for 20–24 °C only. The curve here keeps going beyond 18–27 °C, and the calculator accepts anything from -50 to 150 °C. Outside 18–27 °C there is no Ilford figure to check it against, so the warning appears and the number is an extrapolation of my fit, not Ilford's data.
So treat the result as an approximation of Ilford's chart. If your film's data sheet lists times at several temperatures, use those: Kodak's HC-110 sheet, for example, gives most films a time at 18, 20, 21, 22 and 24 °C.
The 10% per degree rule
Ilford's powder developer fact sheet (ID-11, Microphen, Perceptol, August 2004) gives a shortcut: add 10% to the time for each 1 °C below 20 °C and take off 10% for each 1 °C above, or use its time/temperature graphs. Its examples turn 6 minutes at 20 °C into 4½ minutes at 23 °C and 9 minutes at 16 °C. The curve here gives 4:29 and 9:22; 16 °C is outside the chart, so the second one is an extrapolation. The same sheet gives 20–24 °C as the working range for those three developers.
Times from the curve
| 20 °C | 18 °C | 19 °C | 21 °C | 22 °C | 23 °C | 24 °C | 25 °C | 26 °C | 27 °C |
|---|---|---|---|---|---|---|---|---|---|
| 5:00 | 6:12 | 5:33 | 4:31 | 4:06 | 3:44 | 3:25 | 3:09 | 2:54 | 2:41 |
| 6:00 | 7:26 | 6:40 | 5:25 | 4:55 | 4:29 | 4:06 | 3:47 | 3:29 | 3:14 |
| 8:00 | 9:55 | 8:53 | 7:14 | 6:34 | 5:59 | 5:29 | 5:02 | 4:39 | 4:18 |
| 10:00 | 12:23 | 11:06 | 9:02 | 8:12 | 7:29 | 6:51 | 6:18 | 5:48 | 5:23 |
| 12:00 | 14:52 | 13:20 | 10:51 | 9:51 | 8:59 | 8:13 | 7:33 | 6:58 | 6:27 |
Computed with the formula above, to the second. Ilford's chart has its own rounded values.
Color film
The curve is for black-and-white developers around room temperature. C-41 color kits run much warmer (CineStill recommends 102 °F, about 39 °C, for its Cs41 kit) and come with their own time and temperature instructions, so use the kit's table for color film. For a C-41 kit you reuse, the developer reuse calculator works out the time for each roll. Ilford's own times for HP5 Plus, FP4 Plus and other films in Ilfosol 3 are in the Ilfosol 3 development times table, Kodak's D-76 times at 18 to 24 °C are on the D-76 development times page, and the dilution calculator gives the mL of concentrate for your tank. For CineStill's per-roll times, see how many rolls a Cs41 kit does.