Which Size Door Latch

With numerous sizes available, determining the correct size door latch to buy can be confusing for those who haven’t fitted door furniture before. We sell two sizes of mortice door latches in several finishes, all of which you shall find the dimensions are detailed within the descriptions along with a line drawing (diagram).


An important dimension you shall need to know before you can select the correct size door latch for your doors is the ‘backset’. The backset is the distance from the edge of the door to the centre of the door handle or knob spindle hole, or where the spindle shall pass through the door should you be fitting new doors.


In the UK there are two common backset sizes carpenters use for residential properties, these are 57mm and 45mm. While selecting the mortice latch you will also find that the overall case length is detailed, these are 76mm (3 Inch) and 63mm (2 ½ Inch).


Whilst fitting door handles and knobs the following recommendations may be of help.


If fitting door handles to new doors which haven’t previously had latches fitted then the 3 Inch case (57mm backset) is usually preferred, this will position the handle slightly further away from the door frame than the 2 ½ Inch door latch, within the centre of most surrounding edging of standard size panel doors.


When fitting door knobs then again the 3 Inch case (57mm backset) door latch is recommended as it positions the knob further away from the frame preventing your hand from getting caught or scraping between the frame and knob.


If you are replacing existing door latches it is important you buy the size that is currently in place to prevent filling or plunge routing. Determining the backset and overall length can be done by removing the existing latch and taking the dimensions. If the door handle or knob is still in place and you’d rather not remove it then you can measure from the edge of the door to the centre of the spindle hole (centre of the handle/knob).


Please also see the diagram above, showing measurement lines X indicating backset and Y overall case length.