(hourly rate × multiplier)
Overtime pay is calculated by multiplying the regular hourly rate by an overtime multiplier, then multiplying that by the number of overtime hours worked, and adding it to regular pay.
Regular pay = hourly rate × regular hours. Overtime rate = hourly rate × the chosen multiplier. Overtime pay = overtime rate × overtime hours. Total gross pay = regular pay + overtime pay. "Gross" means before tax, National Insurance, or any other deductions.
In the US, the Fair Labor Standards Act (FLSA) sets a minimum overtime rate of 1.5× (time-and-a-half) for non-exempt employees' hours over 40 in a workweek, though employers may offer higher rates. In the UK there is no statutory overtime rate — employers set their own rate (or none at all, if overtime isn't paid), which is why the Custom multiplier option exists alongside the common 1.25×, 1.5× and 2× ("double time") presets.
This calculator computes gross pay only — it doesn't model income tax, National Insurance, Social Security, Medicare, or any other deductions, and it doesn't check whether your employer's overtime rules or your employment status (exempt vs non-exempt in the US) actually entitle you to overtime pay. Check your contract or local employment law for what applies to you.
US federal overtime rules are set by the Fair Labor Standards Act, as summarised by the US Department of Labor.