Volunteering—unpaid, non-compulsory work, according to the International Labor Organization– is big business. Andrew Haldane, the Bank of England’s chief economist, has taken a stab at estimating just how big and his conclusions are striking.

Mr. Haldane helped found an outfit in the U.K. called Pro Bono Economics that matches community-minded economists with charities seeking to measure the impact of their actions. A lecture delivered to fellow economists Sept. 9 was part sales pitch to persuade his audience to sign up and part exploration of the value to society of volunteering.