I am planning to buy the Dell E1910 monitor.
The data in various reviews are misleading - Dell website says it is 19" but some other review sites say it is 18.5".
What is the actual viewable area measurement? 18.5" or 19"?
Dell's specifications give the panel size as "19-inch (482.6 mm viewable area)". The metric value corresponds to exactly 19 inches. They may be cheating with the conversion, but if the viewable area turns out to be 0.1 mm less than 19 inches, you would have a legitimate grievance that Dell is publishing inaccurate specifications.
We can cross-check with the pixel pitch, which is listed as 0.2835 mm with a native resolution of 1440x900. sqrt((1440*0.2835 mm)^2 + (900*0.2835 mm)^2) = 18.95 inches. Not exactly 19, but a lot closer to 19 than 18.5.
It's neither, because no one can make something that's exactly 18.5" or 19" across. The "correct" answer will vary from monitor to monitor, change continually, run to dozens of digits, and be impossible to measure. To how many significant figures do you want an approximate answer? To two significant figures, I expect it's 19". To three significant figures, I expect it's 18.5".