This calculator helps you to work out what level of sales you will need to cover your fixed costs or overheads, assuming that you know your cost of sales (i.e. the direct costs of generating a given level of sales excluding overheads etc).
For example, you estimate that for every £120,000 of sales your direct cost of sales (eg labour and materials) is £37,500 (incidentally, that would be a gross margin of 68.75%). You know that your fixed overhead costs are £90,000 a month. The calculator will now tell you that you need at least £130,909 of monthly sales to break even.