27.7 inches of water ~ 1 psig. Easy to do. Learned it in ME and the boiler business (dealing with flue draft) many, many years ago. It’s one of those things that just sticks with you.
Even if memory fails me, there’s always http://www.onlineconversion.com/pressure.htm to assist.
I thought one atmosphere was 14.7 psi or 1013 millibars.