For our doors with frames that are fitted into the brickwork (or the structural opening for timber constructions) and not for frames fitted in check (behind the brickwork). The image shows a frame with a cill but much the same methodology is employed when measuring a frame without a cill.
Please First measure the height of the actual brickwork sizes (shown as H1, H2 & H3 in the image).
Now measure again for the width of the actual brickwork at the top, middle & bottom (shown as W1, W2 & W3 in the same image).
Use the average of each set of measurements to ascertain the overall structural opening.
When ordering a frame it is usual to allow a fitting clearance to ensure it goes into the structural opening without having to force anything into position as this can knock the frame out of square and make operation difficult. With this in mind, you need to deduct a clearance. This should normally be around 10mm (0.4″).
Deduct your required clearance off the structural measurements you calculated above. This is the measurement you should use when ordering doors with frames from us.