It's a simple enough task or so you would think but lots of customers do manage to get their sizes wrong, the usual mistake is when they mix or convert centimetres to millimetres and even worse when the conversion is from the aforementioned to feet & inches, the best way...........measure in "millimetres" only, it's more accurate and less likely to cause an issue, some sizes that customers have insisted they have for a door to their lounge would give the worlds limbo dancing champion a real challenge to get through.
How to Measure
Width: Measure each door (not the door itself) between the frame at the top, the middle and the bottom.
Tell us or choose a door(s) to suit the widest size, always remember that you are responsible for making sure that the widest size can be adjusted by you to suit the narrow area of any part of the opening without compromising the stability or affecting the look of the door.
Height: Measure from the carpet/floor covering to the underside of the timber lintel.
Thickness: Now and only now do you measure the door to find out how thick it is at present, with Interior doors you may have to compromise and fit a slightly thinner door, this does not normally present any problems.
Special Advice for Measuring Frames;
The above advice may be different if ordering any door frames or door and frame combinations, always check the overall outside sizes of the frame against the overall inside sizes of the frame and choose the correct combination of those sizes to suit the opening and to ensure the frame will go into the opening, this may involve you ordering a frame that is slightly smaller than the current frame.
Now measure the frame breadth; The best way to do this? Always start measuring from the inside face to the outside face, you can always add a sub frame or concealed plates to make a frame fit the width or height but it can be very difficult and sometimes impossible to adjust the frame if you have ordered it too big in terms of the sizes from inside to outside across the breadth.
Beware: Do not assume all internal doors in your house are the same size or thickness, check them all.