Supplied by Trade Door Handles, trusted UK supplier of architectural ironmongery since 1976, with quick nationwide delivery.

If you've ever ordered a "2.5 inch" or "3 inch" tubular latch and wondered exactly what that measurement refers to, you're not alone. It's one of the most common questions we're asked, and it's an easy one to get wrong if nobody's explained it properly. The short answer is that the inch size refers to the case depth of the latch, not the backset — and mixing the two up is the single biggest reason handles end up off-centre or latches don't fit the door at all.

In this guide we'll cover what a tubular latch actually is, the difference between case depth and backset, a full size chart in both imperial and metric, how to measure your own door correctly, and which latch type suits your project.

A tubular latch is a spring-operated latch mechanism that is mortised (recessed) into the edge of a door, most commonly on internal passage doors where no locking function is needed. It's called "tubular" because the latch case is a simple round or square tube, rather than the flat rectangular case used on mortice latches and locks.

Tubular latches are the standard choice behind almost every internal lever handle and door knob in the UK. A few key points worth knowing:

• Most door handles on backplates use a 2.5" tubular latch
• Most doors fitted with a round or square rose use a 3" tubular latch, though this can vary depending on the joiner or the door construction
• Larger sizes (3" and above) are typically used with mortice knobs, moving the knob away from the door casing so knuckles don't catch on the frame
• A bathroom version — the tubular deadbolt — adds a square bolt operated by a turn, for doors that need to lock

If your project needs a locking version rather than a passage latch, our tubular deadbolts range covers bathroom and WC doors.

Before choosing a size, it helps to know the individual parts of a tubular latch and what each one does:

• Forend — the visible metal plate on the door edge, screwed to the door and holding the latch case in place. This is usually the only part you see once the latch is fitted.
• Follower — the 8mm square hole running through the centre of the latch case, which the door handle's spindle passes through. This is a standard size across almost all UK tubular latches, which is why handles and latches from different ranges are generally interchangeable.
• Sprung latch (bolt) — the angled, wedge-shaped tongue that projects from the latch case and engages with the frame. It's spring-loaded so that closing the door pushes the latch in, then lets it spring back out once it reaches the keep.
• Strike plate (keeper) — the metal plate fitted into the door frame, sometimes called the keeper, that the latch bolt engages with to hold the door shut. A good strike plate has a small adjustable tongue so the latch can be tightened up if it becomes loose over time.
• Dust box — the small plastic (usually black) box fitted behind the strike plate, which conceals the mortice cut into the door frame and keeps debris out of the latch mechanism.

Knowing these terms makes it much easier to read a product listing correctly, and to describe exactly what's needed if you're ordering a replacement part rather than a full latch.

This is where most confusion happens, so it's worth being precise:

• Case depth is the overall length of the latch case, measured from the front edge (the forend, visible on the door edge) to the back of the case. This is the "2.5 inch, 3 inch, 4 inch" figure you see on packaging.
• Backset is the distance from the centre of the spindle hole to the front edge of the door. This determines where your handle actually sits.

Manufacturers' backset measurements can vary by a couple of millimetres between suppliers, even for the same nominal case depth, so always check the specific product listing rather than assuming every "3 inch" latch is identical.

Here's how the common imperial sizes translate to metric case depth and backset:

• 2.5" tubular latch — Case depth 64mm, backset 44mm
• 3" tubular latch — Case depth 78mm, backset 58mm
• 4" tubular latch — Case depth 104mm, backset 83mm
• 5" tubular latch — Case depth 129mm, backset 108.5mm
• 6" tubular latch — Case depth 153mm, backset 130mm

It's a legacy of the building trade that these imperial sizes have stuck around even though every catalogue, brochure and box now lists the metric equivalent. Tradespeople will often ask for a latch "in inches" out of habit, so it helps to know both.

An easy way to work out which tubular latch you need is to measure from the edge of the door to the centre of the door stile (the vertical timber section running down the edge of the door). Choose the backset closest to this measurement, and your handle will sit as centrally as possible.

As a general rule:

• A 64mm (2.5") case is usually best on glazed doors, to keep the drilling clear of the glass rebate
• A 76mm (3") case is a good all-round size for most lever handles
• A 102mm (4") case or larger is generally recommended for door knobs, since knobs fitted too close to the frame can trap fingers when the door opens — for this reason, knobs are often fitted into the mid-rail on panelled doors so a longer latch can be used

If you only remember two numbers from this guide, make it these — a tubular latch 76mm (3") case and a tubular latch 64mm (2.5") case cover the vast majority of UK internal doors between them.

• A 76mm tubular latch is the standard choice for most lever handles fitted on a round or square rose, giving a comfortable backset of 58mm
• A 64mm tubular latch is the go-to for glazed doors and narrow stiles, where a deeper case risks breaking into the glass rebate

If in doubt, 76mm is the safer default for a standard timber passage door; drop to 64mm only where the door construction demands it.

Case and forend finishes matter for a consistent look once the handle is fitted. Alongside the standard satin and polished finishes, a black tubular latch is increasingly popular where the visible edge components (hinges, bolts) are also finished in black, matching contemporary black ironmongery schemes. Match the forend and strike plate finish to your handle set wherever the latch edge will be visible once fitted.

Not all tubular latches are built to the same standard, and the right choice depends on how hard the door gets used.

• Light/standard duty latches are common on housebuilder-spec doors and work fine with most basic sprung lever handles, though they can begin to squeak or wear with heavy daily use. These aren't generally recommended for use with door knobs, which put more strain on the mechanism.
• Double sprung latches use a separate soft spring for the latch tongue (giving a gentle door-closing action) and a heavier spring on the follower, so unsprung levers and door knobs are reliably returned to horizontal. This makes them a solid mid-range choice for most residential and light commercial doors.
• Heavy sprung architectural latches are built for solid, unsprung hardware such as heavyweight stainless steel levers or solid brass and bronze door knobs, and typically include a quality removable forend and a strike plate with a dust box to neaten the finish and hide chisel marks in the frame.

Most tubular latches, aside from budget economy ranges, come with bolt-through fixing holes either side of the follower. These allow the fixing bolts supplied with many door handles to pass through the door and latch mechanism so the handle set can be bolted together securely, rather than relying on grub screws alone.

This matters most with handles fitted on a rose. A rose handle set is usually supplied with a couple of small screws, but the real job of holding the handle onto the door is done by a pair of male and female through-bolts. These clamp the two roses together directly through the door, front to back, so the weight of the handle is carried by the bolts rather than the door timber. Without this bolt-through fixing, a rose handle relies far more heavily on the screws alone, which is a particular risk on lightweight internal doors (hollow-core or thin panel doors), where the timber has little to bite into and a handle can work loose or even pull away from the door over time.

This is exactly why the latch case has holes either side of the follower — they let the male and female bolts pass straight through the latch mechanism itself on their way through the door, so the bolts, the latch and the handle all end up clamped together as one solid fixing rather than three separate, weaker ones. A latch without these holes (as is common on economy ranges) can't be used with a bolt-through handle set, so it's always worth checking this before ordering a passage set for a lightweight door.

A well-specified latch should also include a strike plate with a small adjustable tongue that holds the latch bolt firmly in the frame, and ideally a dust box to conceal the mortice cut into the frame.

Square-edged latches are the most common and can be fitted with a chisel, which is why joiners tend to favour them on site. Radius-edged latches need a router and jig (or factory pre-machining), so they're less common as a retrofit but do give a neater finish.

Tubular latches aren't the only option for internal doors, so it's worth knowing where they sit against the alternatives:

• Tubular latch vs mortice latch — a mortice latch uses a flat rectangular case rather than a round tube, and is typically specified where a flat, more traditional latch profile is preferred, or to match an existing mortice lock case on the same door.
• Tubular latch vs mortice flat latch — our mortice flat latches are a flat-case alternative often used on older doors or where a specific case profile is required by the ironmongery being fitted.
• Tubular latch vs upright mortice latch — upright mortice latches are fitted vertically rather than through the door edge horizontally, and tend to appear on period or heritage door styles.
• Tubular latch vs tubular deadbolt — visually near-identical, but a tubular deadbolt has a square bolt operated by a turn for doors that need to lock, such as bathrooms, rather than the wedge-shaped spring latch tongue.

Whichever latch type you land on, our full locks and latches range covers every case style, size and finish, and pairs with handles on both a rose and a backplate.

A tubular latch is a small, easy-to-overlook component, but it's the part of a door set that gets used thousands of times a year — so it's worth buying the right one rather than the nearest one. General hardware and DIY retailers typically stock one or two standard sizes in a single duty rating, which is fine for a straightforward passage door but leaves little room to match an unusual backset, a heavier door, or a specific finish.

As a specialist ironmongery supplier, we carry the full 2.5" to 6" size range, in light, sprung and heavy-duty options, so you can match the latch to the door rather than the door to whatever latch happens to be on the shelf.