You cannot guess the accurate size of an oven. You must measure to make sure that your oven will fit in the appropriate spot between your cabinets, or in the wall cabinet opening. Most manufacturers design ovens that are 30 inches, 27 inches or 24 inches wide. The opening where you will install the oven must be slightly larger than the dimensions of the oven, usually less than 1 inch to achieve a snug fit.
Stretch a measuring tape horizontally across the front of the oven to record the outside width, and open the oven door and measure the opening of the oven horizontally to record the inside width of the oven.
Measure the front of the oven vertically to record the outside height of the oven, and measure the inside opening vertically to obtain the interior oven height.
Open the oven door and measure the cavity of the oven from the front to the back of the appliance to record the interior depth of the oven. Measure from the front to the back on the outside of the appliance to obtain the exterior depth of the oven. If you are measuring for a replacement wall oven and do not want to remove the appliance from the wall, measure the depth of the cabinet instead of the oven to obtain the maximum depth your cabinet will accommodate.
Don't forget to measure your door openings and hallways before purchasing a new oven.