Most firms in the UK have a gender pay gap, meaning they pay higher average hourly salaries to men than to women.
Unlike pay inequality, which compares the wages of men and women doing the same job, a gender pay gap at a company is not illegal, but could possibly reflect discrimination.
But, from April firms must disclose the percentage difference between average hourly earnings for men and women.
Firms which do not comply face legal action, the government has warned.
The new rules apply to companies which employ more than 250 staff.
Want to find out the gender pay gap at your company? Try the calculator below.