Shooting a film boxed at ISO 400 while the meter sits at ISO 1600 means you pushed it two stops. Enter the box speed and the speed you actually set and the tool returns how many stops you moved and how many times the exposure differs. It is the figure to quote when you hand the roll over and say how many stops to push.
The stop difference is the log base two of the shot speed divided by the box speed, straight from the definition that doubling equals one stop. The development time factor raises the per-stop change to the power of that stop count. The per-stop change has no official value because it varies with the developer and film pairing, so it comes from your own input. Current as of October 2026.
Exposure index and the aperture and shutter pairings behind it are not covered. How much grain coarsens and how much shadow separation is lost differ by emulsion, so no figure is given for either. Treat the results as a reference and run a test roll before anything that matters.
Frequently Asked Questions
No. There is no official standard that assigns a film speed to a situation. This calculator only converts the gap between the speed printed on the box and the speed you actually set into stops.
It differs with every developer and film pairing, so no single official figure exists. Read the percentage a one-stop push adds from the time chart the developer maker publishes and enter that.
A decimal appears whenever the ratio of the two speeds is not a power of two. Real development is usually done in half-stop or whole-stop steps, so check the nearest whole stop line as well.