A formula counter does not count anything itself. It takes counters you have already built and does arithmetic on them, so you can show a single derived number — a shortfall, a total, a headroom figure — instead of making people subtract two counters in their head.

Because of this, you must create at least one ordinary counter first. Until you do, the using a formula option in the type dropdown is greyed out.

Go to Admin > Counter settings and click Add. Give the counter a Name, then set Use this setting to count to using a formula.

Set the time frame first. This decides which counters you are allowed to use in the formula. A formula on a session time frame can only use counters built on that same session, so that you are never adding a per-day figure to a per-session one. A formula on a day time frame can use any counter.

Then write the formula in the text box. Type @ to bring up a list of your existing counters and pick one by name. Combine them with +, -, * and /, and use brackets to control the order. Plain numbers are allowed too, so @All Staff + 5 is valid.

The box checks the formula as you type and shows an error underneath if it is incomplete or references a counter that does not fit the time frame. Save stays disabled until the formula is valid.