Gender Pay Gap Calculator, as "Working for Free" days, by the Washington Post

The median salary for women working full-time is about 80 percent of men’s. That gap, put in other terms, means women are working for free 10 weeks a year. So, if you’re a woman ...... you started working for free 15 hours ago...

Well, that is a little blunt — there are gradients on that difference. The pay gap varies depending on the occupation, working hours, education attainment, experience, and geography.

Look at when women in varying occupations would start working for free, based on the wage gap in that field (US data). 

Published 26/10/17