Why? :-) Assuming it's a question asked by a doorknob manufacturer, we actually want to count the "real" doors that have doorknobs, not the moral doors that y

Why? :-) Assuming it's a question asked by a doorknob manufacturer, we actually want to count the "real" doors that have doorknobs, not the moral doors that you may conceptually have at the entrance of some shack. We could also evaluate entrances and such. Let's take the population per continent: http://www.ecology.com/world-population-continent/ http://www.ecology.com/world-population-continent/ Current World Population by Continent Population estimates as of 2010. Source: Population Reference Bureau Asia 4157300000 Africa 1030400000 Europe 738600000 North America* 461114000 South America 390700000 Australia** 36700000 Antarctica*** 0 Now, we'll estimate the number of houses per person, for each continent (related to family size, existence of work office and administration buildings), along with the number of doors per house/office. #| Asia |# (+ (* 4157300000 (+ 1/4 1/12) 4) #| Africa |# (* 1030400000 (+ 1/6 1/30) 4) #| Europe |# (* 738600000 (+ 1/3 1/2) 6) #| North America* |# (* 461114000 (+ 1/3 1/2) 6) #| South America |# (* 390700000 (+ 1/4 1/12) 4) #| Australia** |# (* 36700000 (+ 1/3 1/2) 6) #| Antarctica*** |# (* 0 (+ 1/6 1) 8)) Which gives an estimated total of 13,070,390,000 doors, about 1.9 door per person. (The number of doors in Antartica is grossly underestimated, sorry). PS: If you want to know if I'm right, you'll have to pay USD 2200.00 ;-) : http://www.chinamarketresearchreports.org/report/global-and-china-door-knob-industry-2014-market-research-report.htm http://www.chinamarketresearchreports.org/report/global-and-... (don't forget to divide the number of doorknobs by two to get the number of doors (ignoring revolving doors)).