It means the door is 30 inches wide.
There is not a quote "standard" door width as doors can be purchased in 2" increments. There are a few building code restrictions for minimum width of certain doors. Like an egress door is required to be 36 inches in width for a residence. There can also be minimum widths for handicap accessibility. These doors in a house need to have a clear opening width of 30 inches.
Given an inch is 2.54 centimeters a 30 inch door (or anything else 30 inches long) will be 30x2.54 centimeters long
There is actually no actual measurment you will have to measure it for yourself to fin out
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The dimensions of an average American front door is 36 inches wide and 80 inches high (3' x 6'-8").
width of car door
The most often chosen width of a front door is 36".
For a 30" door, the rough opening would be 32" in width and 82" in height. Check with your local building inspection department just in case a larger door is required.
As an Architect, I use a minimum 28" door for all bedrooms. The typical sizes are 24", 28", 30" 32", 36"
The standard width for the average entry door in America is 36". That is not including the frame. ADA requires 32" clearance when door is open 90 degrees , which results in using a 36" door due to the thickness of the door and projections from the jambs. Interior doors range in width from 24"-32" in even-number increments. Older homes often have custom widths falling between these numbers. Of these sizes, 30" is the most common in America.
Width (excluding door mirrors): 66.1 in. (1.68m) Width (including door mirrors): 76.4in. (1.94m).
If you mean width, you would use a tape measurer and get the answer in feet or meters. If you mean thickness again using a tape measurer get the answer in inches or centimeters.