How to Measure for Internal Doors in the UK

Here is the short answer. Measure the structural opening inside your door frame, not the old door. Take the width in three places, the height in three places, and the frame rebate, groove, depth for thickness. Use the smallest width and smallest height you find, and plan for clearances of roughly 2 to 3 mm at the top and sides and 8 to 10 mm at the bottom, adjusting for your flooring and any fire door requirements.

Everything below explains that simple method in detail, including standard internal door sizes in the UK, how to handle uneven frames, trimming and lipping, and where bespoke is the better choice.

Measure the opening, not the old door

Old doors sag, warp and get trimmed over time. Measuring a door that is out of shape risks repeating an error. You want the size of the space the new door must work within, which is the inside of the frame, often called the lining or casing.

Quick terminology: the jambs are the vertical sides of the frame and the head is the horizontal top piece. The rebate is the stepped groove in the frame that the door closes against, and its depth governs door thickness compatibility. Lipping is the solid timber edge on many engineered doors that allows limited trimming without exposing the core.

When you measure, put the tape on timber, not on plaster or architrave. You are interested in the structural opening that the new door will occupy.

The five step measuring method

1. Prepare the opening
   Remove the existing door if possible. Take off any draught seals or surface trims that could distort a reading. Note the current flooring and what it will be after you finish the room. A new tile or engineered board can change the required bottom clearance.
   
   

2. Measure the width at three points
   Measure inside the frame at the top, middle and bottom. Record all three. The smallest figure is the one that matters, because the door must pass the tightest point. If your smallest reading is 762 mm, you would normally choose a 762 mm nominal width, then fine tune by trimming within the door’s allowance.
   
   

3. Measure the height at three points
   Measure from the finished floor or the future finished level, up to the underside of the head on the left, centre and right. Again, use the smallest number. If the floor is not yet laid, add the final floor thickness to your measurements so the door will not scrape once the covering is down.
   
   

4. Measure the rebate depth for thickness
   Hook the tape into the rebate and measure the distance from the stop to the inside edge of the frame. Most UK internal doors are 35 mm thick. Some metric ranges are 40 mm. FD30 fire doors are usually 44 mm. The rebate needs to match, or your door will bind or rattle.
   
   

5. Check squareness and diagonals
   Use a long level to see if jambs are plumb. Measure the diagonals of the opening corner to corner. A big difference means the lining is out of square. A door can be planed a little to suit minor variation, but a badly skewed lining often needs attention before a new door will operate smoothly.
   
   

Tools for accurate measurements

A rigid metal tape measure, a pencil and paper or phone notes, a 1 m or longer spirit level, and a small square for corners are ideal. A few photos of each opening with the tape in place also help when you review the numbers later.

Recommended clearances and why they matter

Aim for a consistent working gap so the door swings freely and compresses any seals properly. Around 2 to 3 mm at each side and at the top is typical for standard internal doors. At the bottom, 8 to 10 mm is common, but adjust to suit your floor covering, with carpet and underlay usually needing more and hard floors such as tile or engineered wood needing less. Fire doors follow the manufacturer’s installation instructions. FD30 assemblies often specify about 3 mm at the sides and head and a controlled bottom gap that may be smaller or tied to a threshold or drop seal. If intumescent strips are required, include them in your plan.

Clearances are not decoration, they protect the door and the frame. Too tight, and seasonal movement causes binding. Too generous, and you see daylight or lose acoustic performance. The bottom clearance catches people out most often, especially where a hard floor is upgraded late in the project.

Standard internal door sizes in the UK

Most UK houses mix imperial and metric. Common imperial sizes include 1981 mm high paired with widths of 457, 533, 610, 686, 711, 762 or 838 mm at 35 mm thickness, with 78 x 30 inches being 1981 x 762 mm and 78 x 33 inches being 1981 x 838 mm. Metric options often use 2040 mm heights with typical widths of 526, 626, 726, 826 and 926 mm, and many of these ranges are 40 mm thick. FD30 fire doors follow the same width and height combinations, but the leaf is usually 44 mm thick.

Not every style is available in every size. If your smallest width is 756 mm and your preferred design is only stocked at 762 mm, check the lipping and trimming allowance to confirm whether you can plane a few millimetres off each side. If your opening is far smaller or far taller than any standard path, a made to measure solution or a new lining may be the sensible route.

Door thickness, frame rebate and fire doors

Thickness must match the frame rebate. A 35 mm door in a 40 mm or 44 mm rebate will sit loosely. A 44 mm door in a 35 mm rebate will not close. Check the rebate depth measurement before you order.

For FD30 doors, the leaf is typically 44 mm thick and heavier, the frame and seals must be compatible with the fire certification for the door set including intumescent strips and rated hardware, and gaps are controlled tightly, often around 3 mm at the sides and head with a defined bottom gap that may need a threshold or drop seal. Do not mix and match parts on a fire door without checking the data sheet. Certification applies to an assembly. If you are replacing only a leaf within an existing rated set, confirm compatibility before making any change.

Flooring, thresholds and bottom gaps

Flooring changes alter the bottom clearance you need. The correct approach is to measure to the finished floor, or add the final floor thickness to your figures. Carpet and underlay often benefit from a little more clearance so the pile does not snag and the door glides. Tiles and stone tend to demand level thresholds and modest clearances. With laminate and engineered boards, account for perimeter expansion gaps and any transition strips between rooms. In an ensuite or utility with a step or threshold, measure on both sides and across the width of the doorway. A local high spot can be the difference between quiet operation and a scrape.

Out of square frames and uneven openings

Older properties, moved studs and past repairs can create frames that are not square. Your three width readings and three height readings capture some of this. For minor variation, trim the leaf within the lipping allowance to suit the tightest points. If you see a significant taper or skew, correct the frame, shim the lining plumb and square, and re-trim architraves as needed. This takes more time now, but doors will latch reliably, look even and resist seasonal movement. A big difference between the two diagonal measurements implies a parallelogram. Shaping a leaf to match is rarely a sound long term fix.

Trimming allowances, lipping and when to go bespoke

Most engineered doors have a solid timber lipping around the edges, a stronger edge that lets you trim a little without exposing the core. How much you can remove varies by product. Some ranges allow only around 3 mm per edge, others 10 mm or more. There is no universal figure.

Always check the product specification. Confirm the lipping size on all edges, the maximum recommended trim per edge and in total, and any special rules for glazed leaves or fire doors, which often have stricter limits.

A quick worked example: your smallest width reading is 756 mm. You like a style supplied at 762 mm width with a 10 mm lipping and a 6 mm maximum trim per side. You could remove 3 mm per side, reaching 756 mm while staying within the manufacturer’s trim guidance. If the numbers do not add up without breaching trim limits, go up to the next standard size and reline the frame, or commission a made to measure leaf. Over trimmed edges are weaker, can telegraph the core under paint and void warranties.

Swing direction, hinge side and ironmongery checks

Measure first, then confirm handing and ironmongery before you order. Handing language varies, so use a simple on site test. Stand on the side where the door opens towards you. If the hinges will be on your left and the door pulls toward you, that is left hung on that room face. If the hinges will be on your right, it is right hung on that face.

Now consider how the handle clears any nib wall or radiator, whether the door will swing over a rug, mat well or threshold strip, if you plan to reuse hinge positions or prefer the new leaf unprepared, and how glazed leaves will align visually where you want consistent light lines between adjacent doors. A little planning here prevents patching work and filled screw holes later.

Recording measurements and choosing the right nominal size

Write your three widths and three heights clearly, then circle the smallest for each. Label each with the room name. Next, compare the smallest width and height to the nominal sizes available in your chosen style.

If your smallest width and height both match a standard size, that is simple. If the smallest figures are a few millimetres under a standard, plan to trim within the leaf’s allowance. If they are a little over, ask yourself whether the lining can be adjusted, or if the next size up, carefully trimmed down, is the better path.

Remember to check thickness by comparing your rebate depth to the leaf thickness. If you need a 44 mm leaf for a fire door, do not try to squeeze a thinner stock door into the set, as it will not form the rated assembly.

Measuring for pairs and wider openings

If you are fitting a pair of doors in one opening, measure as above, then decide how the pair will meet. A rebated pair uses a stepped meeting stile that overlaps, so the overall pair width is not simply two equal leaves, the rebate changes the visible margins and you must follow the pair specification. With square meeting stiles, two square edges meet in the middle, often with a pair maker or flush bolts, and the overall width is the sum of both leaves plus your planned centre gap. For example, a 1524 mm opening might suit a pair of 762 mm leaves, if the product is designed to pair at that size. Always consult the range guidance for pairing details, hardware and any trimming rules for the pair configuration.

Common measuring mistakes to avoid

It is easy to lose accuracy by a few millimetres. Pitfalls include measuring over carpet that is not yet stretched, forgetting to add future floor thickness, writing down the largest of your three readings instead of the smallest, and assuming all doors in a hallway share the same size. A final sweep through the house with your notes to confirm each room’s set of figures is time well spent.

Finishing, prefinished leaves and on site adjustments

Your measurement method does not change if you choose a prefinished leaf or an unfinished one, but your fitting approach does. Prefinished leaves arrive sealed. Any trimmed edges usually need sealing on site with the approved finish to maintain the warranty and protect against moisture changes. Unfinished doors need to be fully finished on all faces and edges before they are hung and while the ambient conditions are stable. If speed and factory finish appeal, browse our prefinished options, including oak prefinished internal doors. If you prefer to apply your own coating, see our unfinished oak doors.

If you intend to stain a veneer, test on an offcut or the back of a lipping where a minimal trim has exposed fresh edge timber. Veneered faces and solid lippings can take stain differently. Clear lacquers provide a more uniform appearance if you want to keep a natural oak or walnut look.

When made to measure or a new lining is the better choice

There is a point where trimming a standard leaf is no longer sensible. Strong clues include needing to exceed the trim limit on more than one edge, a frame that is so far out of square that even margins are unrealistic, or an opening that falls far outside any standard stock size for your chosen design. In those scenarios a new lining, sized to a standard leaf, is often economical and brings the whole doorway back to square. For listed buildings or unique openings, a bespoke leaf made to your exact sizes will preserve proportions without over trimming.

Example, putting it all together

Imagine a hallway door where you note the width as 763 mm at the top, 762 mm in the centre and 760 mm at the bottom, with 760 mm the smallest. Height reads 1982 mm on the left, 1981 mm in the centre and 1979 mm on the right, with 1979 mm the smallest. The rebate depth is 35 mm.

You like a style available in 1981 x 762 x 35 mm. Width is fine, as you can trim 1 mm per side if needed for even margins, or you can adjust the lining slightly. Height needs 2 mm off the bottom for the 1979 mm smallest reading, plus perhaps another 6 mm for a new carpet and underlay. You would therefore trim roughly 8 mm off the bottom, then check the margins and swing across the full arc. This is within typical trim allowances for many engineered doors, though you must check the specific range data sheet.

What to do before you order

• Re check each opening with your notes and confirm the smallest width and height.

• Confirm the rebate depth against the thickness of the door you plan to buy.

• Decide on finish, prefinished for speed or unfinished if you want to apply your own coating.

• Confirm trim allowances in the product specification if you need to reduce the leaf.

• If buying a fire door, read the installation guidance for the rated assembly, including seals and hardware.

Once you are satisfied with sizes and clearances, you can compare styles and finishes with confidence. If you are ready to choose a leaf, explore Newland Floors and Doors’ full range of designs, materials and finishes. You can browse the complete selection in our internal doors collection, where you will find oak, pine, white primed and walnut options in popular sizes, along with matching FD30 variants in many ranges. If you are leaning towards a classic timber look, our oak internal doors show the breadth of designs available.

Matching size, style and trimming to the product specification

Different ranges suit different fitting scenarios. A door with generous solid lippings all round is a sensible choice if you expect to plane a few millimetres to suit older frames. Glazed leaves often have more restrictive trim rules, especially on the top rail. A style available in both metric and imperial heights widens your options if you are juggling a mixture of openings across a project. Fire doors typically restrict trimming to modest adjustments and require appropriate hinges, latches and closers. Use the measuring outcome to filter designs. If you know you need to remove 4 mm per side, focus on products that support that within warranty. If you are fitting several doors and one opening is a problem size, consider whether changing that lining allows the house to use a single consistent stock size across the order.

Acoustic and privacy considerations tied to gaps

Measurement is not just about fit. Gaps affect sound and light transfer too. For bedrooms and home offices, a consistent small side and head gap, a sensible bottom clearance and appropriate seals help reduce noise bleed. Glazed leaves bring light but may not suit every private space. When you measure and plan margins, think about what you want the door to do beyond closing the opening.

Practical notes for apartments and shared corridors

Internal flat doors that open onto a shared corridor may carry specific requirements for fire, smoke and sometimes acoustic performance. The measuring steps here remain valid, but the specification is set by building regulations, your building’s fire strategy and the door set certification. If your hallway door is a rated set with a closer and smoke seals, consult the approved product data before taking any action. Do not assume a decorative stock leaf is acceptable in a rated frame.

Storage and acclimatisation after delivery

Once your order arrives, store doors flat on level supports in a dry, ventilated space. Avoid leaning them against walls. Let the doors acclimatise to the property conditions before you trim and hang, especially in winter or after wet trades. Finish all faces and edges as the manufacturer advises, including any newly trimmed edges on prefinished products, so that moisture changes do not warp the leaf after installation.

When to ask for help

If your opening sizes vary widely across a house, if frames are badly out of square, or if you are fitting rated doors with seals and closers for the first time, getting a professional to check your measurements is sensible. The same applies where a single opening must match adjacent glazed screens or where a pair configuration calls for specific hardware and meeting stiles.

Summary rules you can rely on

Measure the frame opening, not the old door. Take three widths and three heights, then work to the smallest figures. Allow sensible clearances, especially at the bottom where flooring changes bite. Confirm the rebate depth and the leaf thickness, and match them. Check trimming allowances on the exact product before you plan to plane. If a lining is far out of square, consider correcting it rather than forcing a leaf to suit.

Armed with accurate measurements and a plan for clearances, you can select materials and styles with confidence.

Frequently Asked Questions:

What measurements should I send when asking Newland Floors and Doors for sizing advice?

Send the smallest width and smallest height you recorded inside the frame, plus the frame rebate depth to confirm thickness. Tell us the finished floor type and thickness if it is changing. If you know the preferred handing, mention which side the hinges will be on when viewed from the room face.

Can I order a door if my frame is slightly out of square?

Yes, minor variation is common and can usually be handled by trimming within the door’s lipping allowance. If your readings show a pronounced taper or large diagonal differences, it is better to correct or reline the frame so you achieve even margins and smooth operation.

How much can I trim off a door from your ranges?

It depends on the product. Many engineered doors allow somewhere between about 3 and 10 mm per edge, but glazed leaves and FD30 fire doors often have stricter limits. Always check the product specification for the exact lipping size and trimming guidance for that design.

Do you sell FD30 fire doors and what should I measure differently?

Many ranges include 44 mm FD30 variants in popular sizes. Measure the rebate depth carefully, as the frame and leaf thickness must match, and plan for tighter side and head gaps as specified by the manufacturer. Bottom gaps may need a threshold or drop seal. You will find FD30 options within our main internal doors collection.

Do you offer both prefinished and unfinished options?

Yes. If you want a factory finished surface, explore our oak prefinished internal doors. If you prefer to apply your own finish, see our unfinished oak doors.