17mm or 21–23mm: Matwell Frame Depth for UK Fitters

Use a 17 mm internal depth for standard entrance mats, and step up to 21–23 mm internal depth for thicker contract or coir mats. The rule governing every job is that internal height should equal your measured mat thickness plus 2–3 mm clearance, never the nominal thickness printed on the mat’s packaging. Backing type and pile compression can shift that figure more than most fitters expect.

TL;DR:

Standard entrance mats typically require a recess depth of 17 mm, while thicker contract or coir mats need 21 to 23 mm for a proper fit.

Accurate measurement includes backing layers and pile compression, with a recommended 2 to 3 mm clearance based on footfall and load conditions.

Confirm the recess is level and free of screed inconsistencies before cutting, and dry-fit components to avoid mismatches and door clearance issues.

Solid metal frames with consistent thickness and border width provide better durability and stability over time, especially in high-traffic areas.

For mats thicker than 20 mm, consider ramps or surface-mounted profiles rather than forcing a deeper recess, to prevent damage or fit issues.

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What is the standard matwell frame depth?
How do you measure mat thickness and calculate the recess allowance?
Preparing the subfloor and recess for the correct depth
Choosing a frame that keeps that depth accurate over time
Installation checklist: getting depth, fit, and door clearance right
Why these figures reflect how Quality Carpet Trims builds and supports its frames
What actually matters when you set a matwell depth
Get the right matwell frame depth
Sources
FAQ

What is the standard matwell frame depth?

Manufacturers present matwell dimensions in a fixed order: internal height (the space your mat sits in), overall depth, and frame thickness. Get that order wrong when reading a spec sheet and you will order the wrong frame. Nuway’s product specification shows this clearly with an example listed as 16 (13) x 21 x 3 mm, meaning a 16 mm internal height, 21 mm overall depth, and a 3 mm frame thickness.

Across the trade, a few figures come up again and again:

16–17 mm internal height for standard entrance mats, the most widely stocked size.
21–23 mm internal height for thicker contract or coir mats that take heavier footfall.
11 mm internal height for slimline mats in low-traffic domestic settings.
Frame thickness around 3 mm, with a frame border width of roughly 42 mm on most retail profiles.

Romus’s technical tables list frame options at 11 mm, 17 mm and 22 mm internal heights precisely so installers can match frame to mat gauge rather than guess. Worked example: a mat labelled “17 mm nominal” but measuring 18 mm once compressed under a boot needs an 18 mm to 20 mm recess, not a bare 17 mm slot copied straight off the box.

How do you measure mat thickness and calculate the recess allowance?

Get the measurement wrong and you either end up with a mat that rattles in its frame or one that will not sit flush no matter how hard you press it down. The fix is a short, repeatable process rather than a single tape reading.

Measure the mat’s thickness at three or four points using a rule or callipers, not just at the edge.
Include the backing layer in that measurement, since rubber or PVC backing adds bulk the pile alone will not show.
Press down gently to check pile compression under load, because deep-pile coir often compresses several millimetres once walked on.
Record the largest of your readings and specify the recess to that figure, not the average.
Add clearance of 2–3 mm on top, a margin Slip-Not’s product guidance treats as standard practice for absorbing minor site irregularities.

Add extra allowance where the mat has a heavy rubber backing or the entrance sees constant heavy footfall, since both compress and wear differently over time.

Pro Tip: If you can get the mat and frame together before cutting the recess, dry fit them on a flat bench first. It takes five minutes and catches a mismatch that would otherwise only show up once the floor is already cut.

Preparing the subfloor and recess for the correct depth

A frame spec is only as good as the recess it sits in, and screed inconsistency is the most common reason a correctly specified matwell still ends up uneven. Before cutting anything, confirm the screed is flat and level across the whole recess area, not just at the edges where it is easiest to check.

Decide early whether tiles or existing screed need removing to reach the correct base level, since retrofitting into an already-finished floor changes your depth maths.
Calculate outside frame dimensions by adding the frame border allowance (around 42 mm per side on standard profiles) to your internal mat dimensions, as retail product specs consistently confirm: quoted measurements are the inside of the frame, not the cutting line.
Leave clearance at mitred corners, since a tight mitre that binds during a dry fit will only get worse once bedded in.
Choose adhesive bedding for a rigid, permanent fit on solid screed, or mechanical fixings where the frame needs to be removable for future refurbishment.
At external entrances, factor in drainage and moisture risk separately, since a recessed well designed for internal use will not cope with standing water.

Choosing a frame that keeps that depth accurate over time

Frame thickness and border width do not just affect how the recess is cut. They dictate how the finished floor reads once the mat is in daily use. A thicker frame profile, or one with a mounting lip, often needs a slightly deeper recess than a slim flush profile, so check the frame’s own thickness figure before finalising your cut.

The material matters just as much as the numbers. Many trims on the market can split, bend, or dent under repeated foot traffic and trolley wheels, and once one section deforms, the whole matwell loses its flush finish.

Solid metal frames resist bending and denting that thinner sections can suffer under commercial footfall.
Powder-coated solid-metal finishes hold up well against abrasive cleaning regimes.
Frames available in multiple finishes let you match various door and hardware styles without compromising on material.

Gradus’s supply and fit information notes that recessed brass frames are commonly rated for mats up to around 20 mm, with ramped or surface-mounted alternatives offered where a full recess is not practical. That 20 mm ceiling is a useful sanity check before you commit to a deep recess cut.

Installation checklist: getting depth, fit, and door clearance right

Work through this in order and you avoid the two failures that cause most callbacks: a mat that sits too high and catches the door, or one that sits too low and gathers grit at the edges.

Measure the mat at its thickest point, including backing and compressed pile.
Mark the recess outside dimensions, adding the frame border width to your internal mat size.
Cut the recess to the calculated depth, checking level across the full area with a straight edge or spirit level.
Dry-fit the frame sections before any fixing, checking mitres sit tight with no gaps.
Check door swing clearance with the mat and frame both in place, not just the empty recess.
Secure the frame using adhesive bedding or pre-drilled mechanical fixings, matching the method to your subfloor.
Fit the mat and test underfoot for rattle, rock, or a proud edge that catches a shoe.

A feeler gauge is useful here for checking gaps around the frame edge that are too fine to spot by eye. If the mat sits slightly high, a thin shim under the frame or a partial re-bed usually solves it faster than recutting the whole recess, but treat that as a fix for a small error, not a substitute for getting the depth right first time.

Why these figures reflect how Quality Carpet Trims builds and supports its frames

The depth guidance here matches what shows up on Quality Carpet Trims’ own matwell product range and its retrofit fitting guidance, both built around solid-metal frames rather than the thinner alternatives common elsewhere in the trade.

Frames are solid metal, hand-finished across 10 luxury finishes, not powder-coated aluminium dressed up to look premium.
Free samples let fitters check thickness and finish against a job before ordering a full run.
UK manufacture means fitting guidance reflects real installer questions, not a translated spec sheet.

What actually matters when you set a matwell depth

Most guidance on this topic treats the frame as an afterthought to the mat, when it should be the other way round. A mat is a wearing item you will replace in a few years. The frame, if it is solid brass and properly bedded, is not. Get its depth wrong and you are recutting a screed recess, which costs far more than the frame itself.

The conventional advice of “match the frame to the mat’s nominal thickness” undersells the problem. Nominal thickness is a manufacturing figure, not a site measurement, and pile compression under real footfall routinely adds a millimetre or two that never appears on the packaging. Measure what actually sits in the recess, not what the label claims.

If there is one priority above the rest, it is this: choose your clearance allowance based on footfall, not on habit. A quiet office reception and a busy retail entrance calling for the same 17 mm frame will not wear the same way, and the fitter who accounts for that difference up front saves the return visit later.

— Matt

Get the right matwell frame depth

There are suppliers that offer solutions to the problem this guide walks through: getting an internal height that matches your mat, without risking a thin profile that will bend or dent under repeat traffic. Some frames are solid metal, hand-finished across multiple finishes, and available with free samples so you can check thickness against your recess before committing to an order.

If your job needs a depth or finish that is not listed on a standard spec sheet, get in touch for bespoke dimensions rather than forcing a mat into the wrong-sized frame. Browse the full matwell frame range to check current sizes and finishes, or start with a sample against your own mat before you cut a single recess.

Sources

FAQ

How deep should a matwell be?

Use a 17 mm internal depth for standard entrance mats, and 21–23 mm internal depth for thicker contract or coir mats. The working rule is your measured mat thickness plus 2–3 mm clearance to account for pile compression and backing.

How thick is entrance matting?

Most entrance mats measure between 11 mm and 17 mm thick, with heavier contract and coir mats reaching 21–23 mm. Manufacturer technical tables list internal heights at 11 mm, 17 mm and 22 mm precisely to match these common gauges.

How do you fit a sunken doormat?

Measure the mat’s actual thickness at its thickest point, mark the recess including the frame’s border width, and cut the screed to the calculated depth. Dry-fit the frame first, check the mitred corners sit tight, then bed it with adhesive or pre-drilled mechanical fixings before laying the mat.

What is a recessed matwell?

A recessed matwell is a frame set into the floor at a calculated depth so an entrance mat sits flush with the surrounding surface rather than sitting on top of it. Quality Carpet Trims supplies these frames in solid metal across 10 luxury finishes, built to hold that flush fit without bending or denting under repeated traffic.

What if my mat is thicker than the frame’s rated depth?

Frames are generally rated up to a maximum mat height, with some recessed brass profiles rated to around 20 mm. Beyond that, manufacturers typically recommend a ramped or surface-mounted profile rather than forcing an inadequate recess.

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