Door Knobs and Backset Explained

Author: Carl Benson

Carl is the founder of Suffolk Latch Company, with a background in engineering and over 20 years’ experience in home hardware. He works closely with suppliers and designers alike to develop quality ironmongery that stands the test of time. Read my full bio.

Introduction

Door handle backset is the distance from the edge of the door to the centre of the square spindle hole in the latch or lock. It determines where your handle or knob sits on the door. The spindle, also called a mortice bar, passes through that hole to operate the mechanism.

Backset also matters when measuring bathroom locks, deadlocks and euro profile locks, although the relevant centre can be different. Here is how to identify it, separate it from case length and record the measurement before ordering.

What does backset mean?

The starting point is the door edge where the latch or lock faceplate sits. On the mechanism itself, use the faceplate reference shown in its drawing. Measure to the centre of the operating hole, rather than its nearest edge.

Which centre you need depends on the hardware:

Some mechanisms place the handle and keyhole at different distances from the door edge. Do not assume they share one backset. Cylinder length and the distance between operating centres are separate measurements.

Backset and case length are different measurements

Case length, also called case depth, describes how far the latch or lock body extends into the door. It tells the carpenter how deep the mortice needs to be cut, subject to the clearance and method stated in the fitting instructions. Backset is the distance from the front face of the faceplate to the centre of the relevant mortice opening.

You will often see an inch size in a product name and millimetres on its drawing. This is an industry anomaly that we have inherited. The useful rule is that the size used to sell the latch describes its nominal case size; it does not establish the backset. Always confirm the true backset measurement on the product drawing before ordering.

How to measure backset

On an existing door

1. Open the door so you can see the latch or lock faceplate.
2. Identify the spindle centre. On a lever handle, this is its pivot, not the middle of the lever grip or backplate.
3. Measure straight across the door face, at right angles to the closing edge, from that edge to the centre.
4. Record the result in millimetres and compare it with the exact replacement drawing.
5. For the most reliable measurement, remove the handle or door knob so that you can see the spindle centre rather than estimating it through the furniture.

If the furniture hides the centre, the visible rose or backplate may be misleading. Check the mechanism directly before relying on an estimated measurement.

On a removed latch or lock

Lay the mechanism flat and measure from the front face of the faceplate to the centre of the appropriate hole. Sash locks and tubular latches may have a fixed faceplate or a loose faceplate fitted separately. If you are measuring from the bare front of a case with a loose faceplate removed, add the faceplate thickness to establish the finished backset. Read the product drawing carefully because its reference point may already include the faceplate. Measure the case separately, without including the projecting latch tongue or bolt.

Is there a standard backset?

There is no single backset for every door handle, knob or lock. For the 3-inch tubular latch illustrated in the centre of the photograph below, the confirmed backset is 56 mm. The 3-inch label refers to its nominal case size.

This is a product example, not a conversion rule for every 3-inch latch or lock. Actual backsets can vary between manufacturers, so do not assume that all 3-inch tubular latches have an identical backset. Check the specification for the exact product you are purchasing.

Checking handle and knob clearance

Backset affects the space around your hand. A knob brings your fingers around the spindle centre; a lever places the grip further along its length. If a door knob has a very short backset, you may strike your knuckles on the door surround when turning it. Check the actual knob diameter, projection, lever movement and nearby frame or architrave.

Also check the door stile—the upright timber beside a panel or glazing—and the space available for the case and fixings. Where the door design allows, aim to place the spindle centre comfortably within the width of the stile so that the hardware looks properly positioned and has room to operate. A traditional four-panelled door often has a wide central lock rail, which provides a suitable area for the door furniture. Think about the door and hardware together before ordering: a deeper backset may improve hand clearance but move the furniture towards a panel or glazing bead. Door style alone cannot establish the correct depth, so the final position still needs measuring.

Can you change the backset?

Changing the handle alone does not move the operating centre in an existing latch or lock. A different backset normally means a different mechanism and may require new holes or alterations to the door.

Check the full replacement dimensions before planning that work. For a fire door, follow the door assembly’s evidence and approved fitting requirements before changing hardware or cutting the door.

Your measurement record

Keep these details with photographs of the existing fitting:

Conclusion

Identify the correct operating centre, measure its distance from the door edge and record the case length separately. These checks establish where the furniture will sit and help you compare the measurements of the handle, knob, latch or lock before ordering.

FAQ

Does a 76 mm latch have a 76 mm backset?

No. If 76 mm describes the case, it does not describe the backset. Read the separate backset dimension for that model.

Is door thickness the same as backset?

No. Thickness runs between the two faces of the door. Backset runs from the closing edge towards the operating centre.

Do door knobs always need a longer latch?

No. They need adequate hand clearance. The required backset depends on the knob, surrounding frame and available door space.

Can I reuse the existing handle holes?

Only if the replacement mechanism and furniture match the relevant positions and fixings. Matching backset alone does not establish that everything will fit. Our guide to door handle compatibility explains the other measurements to check.

Related Blog Posts

• Which Lock or Latch Do You Need?
• How to Fit a Tubular Latch
• Are Door Handles Universal?
• How to Choose Handles for an Internal Door