Calculator assumes employee is not subject to Student loan contributions. If applicable these are typically 9% of salaries above £20k (Plan 1) and 9% of salaries above £27k (Plan 2). Also assumes worker is not of state pension age (after which employee NI is no longer due, but employer NI remains payable).
Directors have an annual earnings basis for national insurance purposes and so the calculations for a director may differ in some cases.
To use the calculator to work out the required gross pay to arrive at a required net pay, simply play around with the gross pay until the net pay result is produced.