Measure Your Door First

Before you start

• Have a tape measure with a proper hook, a phone for photographs, and somewhere to write numbers that is not a scrap of paper.
• Do one door at a time and label the numbers by door. Front and back doors in the same house often differ.
• If the door already has a deadbolt, take one screw out of the strike plate before you start. You will want its length.

The job

1. Measure the thickness at the lock heightOpen the door and measure across the edge, face to face, level with the existing lock. Doors are not always uniform, so measure at the height the hardware will sit rather than at the top corner. Write it down to the nearest sixteenth.
2. Measure the backset to the center of the boreFrom the edge of the door to the middle of the large round hole. The most common error in this whole exercise is measuring to the near edge of the hole instead of its center, which is out by half the bore and enough to buy the wrong lock. If there is no existing hole, this is a number you will be choosing rather than measuring.
3. Measure the bore and the cross boreThe bore is the big hole through the face of the door; the cross bore is the smaller one drilled in from the edge where the bolt sits. Both decide whether a new lock covers what is already there. A new lock that is smaller than the old hole is a filling job, not a fitting job.
4. Establish the swing and the handStand outside the door. Note whether it pulls toward you or pushes away, and which side the hinges are on. Handle sets are handed, and inward-only devices such as flip latches are simply not an option on an outward-swinging door.
5. Measure the existing strike screwsTake one screw out of the strike plate and measure it end to end. Short screws mean the strike is anchored in the jamb board only, which is the cheapest weakness in the whole door to fix. Put the screw back before you forget.
6. Photograph the edge, the face and the frameThree photographs: the door edge with the latch visible, the inside face around the lock, and the frame where the strike sits. When a specification sheet asks a question you did not think to measure, the photograph usually answers it without another trip to the door.

Stop and call somebody if

• Two measurements of the same dimension disagree by more than a sixteenth. Measure again before deciding the door is unusual.
• The door is metal-skinned or a patio slab rather than a conventional timber or composite door. Those take specific hardware and a locksmith will save you money here.
• The frame is out of square enough that the door binds. Hardware fitted to a distorted frame will not sit right whatever the numbers say.

What goes wrong

• Measuring backset to the edge of the bore rather than its center.
• Assuming the back door matches the front because the house is one age.
• Trusting a listing bullet point over the manufacturer specification sheet for the exact model number.

Where this applies

• The Front Door — The jamb, not the lock: on most homes the strike plate is held by screws that stop inside a thin board nailed to the framing behind it.
• The Back Door — Privacy: a back door is usually the only approach to the house where somebody can work unobserved for as long as they like.
• The Garage Side Door — Specification: the side door is almost always the cheapest door on the property, with the cheapest lock, and on an attached garage it opens into the house.

Hardware this job may need

• Deadbolts & Keypad Locks — controlling who has a key, and what happens when the battery dies

Sources

• Builders Hardware Manufacturers Association — standards for builders hardware
• American National Standards Institute — how American National Standards work