Firstly,you need to understand that the manhole covers sizes are divided into the cover size and clear opening size. Please check the below photo to know what is the cover size,and what is the clear opening size? Tell your supplier that you need to purchase the manhole cover with cover size or clear opening size.
When you are measuring an existing manhole cover, It’s very easy for you to lift out the cover by using a manhole cover hook and measure the actual manhole cover dimensions(Please be very carefully,and don’t drop a cover on your toe…it hurts!). Also,you can measure over the top of an existing cover unless it cannot be removed.
When you are measure an underground chamber with missing cover,you need to measure the height of brickwork. Please see the below photo with green part,which is the height of brickwork. A suitable measuring device or tape measure should be used to take a reading of the two opposing faces of the chamber wall or brickwork.