I think I am quoting some jobs below cost but I cannot tell which ones — how do I work it out?

The error is almost never the hourly rate on paper. It is that a working day contains far fewer billable hours than it feels like: travel between jobs, quoting, buying materials, invoicing and chasing payment are all real hours that no customer pays for directly.

A trade billing what looks like a healthy rate for eight hours a day is often billing five. Spreading the same overhead across five hours rather than eight raises the true cost per billable hour by 60%, which is where an apparently profitable rate quietly becomes a loss-making one.

Work it out from the year rather than the job. Take total overhead — vehicle, insurance, tools, phone, accountant, certification — divide by the number of hours you genuinely bill in a year, and add that to your hourly wage. That is the number a quote has to clear before anything is profit.

Then check fixed-price work specifically. Fixed quotes are where losses concentrate, because the overrun is invisible: the job took eleven hours instead of eight and the invoice did not change.