Developing warm: Ilford's temperature chart and the five-minute floor

Developer works faster as it warms, and in summer the tap rarely runs at 20 °C. Ilford publishes a one-page chart that converts any black-and-white development time at 20 °C into the time for the temperature you actually have, and it repays a close look.

How to read it

Find your normal time in the 20 °C column, follow the row to your temperature and read off the new time ¹. Ilford's own example: 8 minutes at 20 °C becomes 5 minutes 30 seconds at 24 °C ¹. The chart runs from 18 °C to 27 °C, covers times from 4 to 25 minutes at 20 °C, and is rounded to the nearest 15 seconds ¹. Ilford presents it as a guide for all film and developer combinations ¹, so it works for other makers' films as a starting point, though their own sheets win where they give a time.

At 20 °C	18 °C	22 °C	24 °C	27 °C
6:00	7:15	5:00	4:00	not given
8:00	9:45	6:30	5:30	4:15
10:00	12:30	8:00	7:00	5:30
12:00	14:45	9:45	8:15	6:30

All from Ilford's chart ¹.

The five-minute floor

Printed across the chart in capitals is the line that matters most: development times below 5 minutes are not recommended because of the risk of uneven development ¹. Short times leave little margin for the first seconds of filling the tank and the first agitation, when developer reaches one end of the film before the other. Read the table again with that in mind. At 24 °C anything shorter than 7 minutes at 20 °C falls under the floor; at 27 °C anything shorter than 9 minutes does ¹. Ilford leaves the 27 °C column blank for the shortest times altogether ¹.

So the chart is also an argument: on a hot day, a developer and dilution with a longer normal time keeps you above the floor. HP5 Plus in ID-11 at stock strength is 7½ minutes at 20 °C ²; at 24 °C the chart gives 5:15 ¹, still above the line, while a faster combination might not be.

Colder works too

The chart runs the other way: at 18 °C, 8 minutes becomes 9:45 ¹. Cooler and longer is the safer error; it gives you more time, not less.

Try it

Before your next summer roll, take the temperature of the developer in the tank, not the tap, and read your time from the chart, or from our temperature tool, which reads the same table.