The same as the nominal.
Unless stated otherwise and in the vast majority of cases, the size you give us will be taken to be the nominal, i.e. the size of the aperture into which you will be fitting the grille, diffuser or louvre.
From this size HVC shall deduct a tolerance, permitting the neck of the unit to fit into the hole comfortably. This tolerance is normally between 9 – 15mm, however, does vary by product type, therefore please download the specific product PDF for further details.
There are notable exceptions to the above:
If you have any more questions about grilles, diffusers or louvres, please do not hesitate to contact HVC Supplies today. Our dedicated customer service team will be more than happy to answer any queries.