Average number of injections per day (out of treatment)
There hasn't been a study that has looked in depth at the frequency of injecting in the UK, but drug injectors who are dependent on heroin consistently report that they typically inject somewhere between 2 and 5 times per day.

Those injecting crack cocaine may inject at the same frequency, but some inject many more times than that in a day - as many as 8, 10, 12, 15 times on some days. However, dependent crack cocaine use with high frequency injecting is often characterised by binges of heavy use lasting several days or a week, followed by a period of no, or reduced, use - so in those cases the average will be lower than those high figures.

The default average in the calculator of 3 injections per day is a best guess for the UK. The figure is editable, and it is interesting to see what happens to the coverage percentage if all the other figures remain constant, and the average injecting frequency is altered up or down. In particular, it is important to note that if injecting frequency is going up, distributing the same amount of injecting equipment will represent a significant reduction in coverage.