Backset, bore, thickness and handing explained

Why measuring matters more than the brand name

A lock is only useful if it fits the door it is meant for. Two locks that look identical on a shelf may have different latch lengths, different body sizes or a different distance between the handle and the deadbolt. The wrong one may not install at all, may install loosely or may require cutting into the door in a way that weakens it.

People often shop by appearance and finish. That is reasonable, but the numbers come first. Once you know your door's dimensions, you can choose among the many locks that suit it, and the visual choice becomes easy. If you skip this step, a lock that arrives and does not fit costs you time, and returning bulky hardware is a hassle.

This guide is general. Doors in Miami-Dade vary: older homes have wood doors in odd sizes, newer ones have steel, fiberglass and impact-rated doors, and commercial doors follow their own conventions. The terms below apply across most of them, but when a door is unusual, a technician's measurement is more reliable than a guess.

Door thickness

The thickness is the distance from one face of the door to the other, measured at the edge. Most residential entry doors are a common standard thickness, but many older doors, custom doors and some interior doors are thinner or thicker. Locks are built for a range of thicknesses. A latch and spindle that are too short will not reach through a thick door, and parts that are too long may poke out the other side.

Measure at the edge of the door where the latch goes, with the door open. A tape measure or a ruler works. If the door has raised panels or decorative trim, measure the solid stile near the latch rather than the thickest decorative part. Write down the number in inches and fractions, and note whether the measurement is close to a common size or somewhere in between.

If the thickness falls outside the normal range for a lock, check whether the manufacturer offers an extension kit. Some locks adapt to thicker doors with longer spindles. Others do not. A technician can tell you quickly which approach works for an unusual door.

The bore: the big hole in the door face

Most cylindrical locks and handlesets fit into a large round hole drilled through the face of the door, called the bore. The diameter of this hole is a standard size in most modern doors. Older doors and some custom doors may have a different size, or an irregular hole enlarged by earlier hardware.

To measure the bore, remove the old lock if you can, or look at the hole while the lock is out. Measure across the widest part of the circle. If the hole has been enlarged or damaged, a new lock may not cover it with its standard plate. In those cases, a wider rose or an escutcheon plate can hide the damage, or a technician can repair the door and drill a clean hole.

Do not drill a bore yourself unless you have the right tools and experience. A hole drilled off center or crooked is difficult to correct and can split the door. If your door has no bore, because it is a new door that came without a lock hole, a technician can prepare it, and a drilling template ensures accuracy.

The edge hole and the latch faceplate

A second, smaller hole is drilled into the edge of the door, meeting the bore at a right angle. The latch slides into this hole. Around it, a shallow rectangular cutout holds the latch faceplate, which sits flush with the door edge. Latch faceplates come in different shapes, such as rounded corners or square corners, and in different sizes.

The faceplate shape matters for appearance and fit. A square-corner plate in a cutout made for a rounded plate leaves gaps. Many latches include both types or have adjustable corners, but not all. Look at the plate on your old latch and note its shape and approximate length.

Deadbolts also have a latch bolt and a faceplate on the edge, with their own edge hole. If you are replacing a deadbolt, check that hole as well. When the door has had several locks over time, edge holes can be enlarged or scarred, and a technician may need to fit a plate that covers the damage.

Backset: the number most people miss

Backset is the distance from the edge of the door to the center of the bore. It is one of the most important numbers for a cylindrical lock, because the latch must line up with the center of the lock body. Two backsets are common in homes, and many locks include an adjustable latch that can switch between them. Others are fixed at one distance.

To measure backset, place a ruler or tape against the edge of the door and measure to the center of the bore. If the old lock is out, this is easy. If it is in place, you can measure to the center of the knob or lever on the face of the door as an approximation, but the bore itself is more accurate. Write the number and double check it.

If the backset on your door is not one of the standard sizes, you may need a lock designed for it, or the door may need modification. Mortise locks use a different measurement, since the lock body sits inside the door edge. The page on mortise lock service for older Miami homes explains how that differs.

Hole spacing for handlesets and deadbolts

If your door has a handleset and a separate deadbolt, there are two bores, one above the other. The distance between their centers is the hole spacing. Many handlesets are designed for one common spacing, and a replacement needs to match unless you are prepared to repair and redrill the door.

Measure from the center of one hole to the center of the other. Use the actual holes in the door, not the visible hardware, since the plates may hide small differences. Note whether the deadbolt is above or below the handle.

If the spacing on your door is unusual, you have options: a handleset made for that spacing, a lock with an adjustable mount or a repair that plugs one hole and creates a new one. A technician will tell you what makes sense. The page on entry handleset replacement and fit covers this in more detail.

Handing: left, right, and why it matters

Some hardware is handed, meaning it is made for a door that swings a particular way. Levers, in particular, often have a bend or a shape that works for one direction. Many modern levers are reversible, and some are sold as left or right. Knobs usually do not care about handing.

To find your door's handing, stand outside the door, on the side where you would normally knock, and look at where the hinges are. If the hinges are on the left and the door swings away from you, it is a left-hand door. If the hinges are on the right, it is a right-hand door. The rules for doors that swing toward you differ, so check the lock manufacturer's instructions or ask a technician.

Getting this wrong is easy and frustrating, since a lever installed backward points the wrong way and may not feel right. It also matters for exterior doors where the outside and inside sides differ. If you are unsure, take a photo of the door from the outside showing the hinges and the swing direction.

The strike plate and the frame

The lock is half of a pair. The other half is the strike plate on the door frame, which receives the latch and the bolt. If you change the lock, the strike plate often needs to match. A new latch may be a different shape or length, and the old strike may not line up.

Look at the strike on your frame and measure the cutout in the wood or metal. Note the height from the top of the frame, the position of the latch hole and the length of the plate. Check that the screws are long enough to reach solid framing. A short screw in thin trim gives little protection.

The page on strike plate and door reinforcement basics explains how much difference a good strike makes. If your frame is damaged, split or soft, replacing the lock alone will not fix the problem. See door frame damage and lock fit for what to do.

Unusual doors: impact, glass, metal and commercial

Not all doors can be measured and drilled the same way. Impact-rated doors have reinforced construction and sometimes have restrictions on drilling or hardware changes. Doors with large glass panels limit where screws can go. Steel and fiberglass doors have internal blocking at the lock area that determines what hardware fits. Aluminum storefront doors use narrow-stile hardware, and commercial doors may follow standards for exit devices and closers.

For those doors, bring the manufacturer's details if you have them. Look for a label on the edge or the hinge side, and take a photo. A technician can recognize many types by sight and know what constraints apply.

The page on lock hardware for impact-rated doors explains the residential case in more detail. For storefronts, see storefront door lock repair for Miami shops.

A simple measuring routine you can follow

Gather a tape measure, a pencil and your phone for photos. Work with the door open and, if safe, with the old lock removed or at least well visible. Take notes as you go and photograph each step, so you can show a technician later.

1. Measure the door thickness at the edge, and note the number.
2. Measure the diameter of the large bore and note whether the edge looks damaged.
3. Measure the backset from the door edge to the center of the bore.
4. If there are two holes, measure the spacing between their centers.
5. Note the shape and length of the latch faceplate on the door edge.
6. Check the strike plate on the frame and note its size and screw condition.
7. Stand outside and note which side the hinges are on and which way the door swings.
8. Photograph both sides of the door, the edge and the old lock, including any markings.

Common mistakes that lead to a wrong order

The first mistake is relying on a measurement from memory or from the old lock's box. Locks get swapped over the years, and a lock in your door may not be the size you assume. The second is measuring only the visible part of the hardware, such as the rose, instead of the bore beneath it. The third is ignoring the strike. A new latch may not fit an old strike, and a mismatch can leave a door that closes but does not lock.

Another mistake is buying hardware designed for interior doors and using it on an exterior door. Interior hardware is built for privacy, not security, and uses lighter parts. A similar mistake is choosing an impressive-looking set made of low-grade materials that will not last in the Miami-Dade climate.

Finally, many people forget that keys and finish are part of the plan. If you want the new lock keyed to match others in the house, tell the technician when you call. The page on lock grades explained for homeowners helps you compare quality, and buying locks online versus professional installation explains the trade-offs.

When to let a technician measure

Measuring a standard door is easy. Measuring an odd one is not. If your door is old, damaged, an unusual size, impact-rated, glass-heavy or part of a storefront, a technician's gauge and experience reduce risk. They can measure on site, check the frame and recommend what fits, often carrying options so the job can be completed in one visit.

If you prefer to order your own hardware, send your measurements and photos in advance. The person on the phone can often tell you whether your choice is likely to fit. It is much easier to correct a plan before the box arrives than after.

Quotes are given only by phone, and complicated jobs may need an on-site visit. There is a service call charge, explained on the phone before dispatch. If you have gathered your numbers and photos and want to talk through the next step, call and describe your door and what you hope to install.

Questions people ask

Related pages

Written by Claude (AI) (AI) for 24 Hour Locksmith Miami.