Rated 4.8 Stars by 1000's of Satisfied Customers
UK's No.1 Composite Door Supplier
Step-by-Step Guide
How to Measure for an
External Door
Covers standard doors, side panels, top lights, door handing and opening direction — all as viewed from outside.
Before You Start
These guidelines help you measure for a new composite door and frame. All measurements must be taken from the brickwork opening, not from the existing door or frame. Measurements should be supplied in millimetres (mm).
If you are at all unsure, we strongly recommend contacting a professional door fitter or builder to carry out a site survey before ordering.
Essential Rules
- Measure brick-to-brick — not the old frame
- Measure at least 3 points per dimension
- Always use the smallest measurement
- Deduct 10mm from width AND height
- Supply all measurements in metric (mm)
- View all designs from the outside
- Check every measurement twice
- Include cill & frame extenders in overall size
Standard Door & Frame
Single door, no side panels or top lights
Measuring the Width
Using a steel tape measure, measure the clear brickwork opening horizontally at three positions:
- Top of the opening — just below the lintel
- Middle of the opening — at the midpoint
- Bottom of the opening — just above the sill
Record all three measurements. Use the smallest value as your working width.
Deduct 10mm from the final width. This creates a 5mm fitting gap on each side, allowing the door frame to be plumbed, packed, and sealed correctly.
Measuring the Height
Measure the clear brickwork opening vertically from the floor/threshold to the underside of the lintel at three positions:
- Left side of the opening
- Centre of the opening
- Right side of the opening
Record all three measurements. Use the smallest value as your working height.
Deduct 10mm from the final height. This provides 5mm fitting tolerance top and bottom for levelling, packing, and sealing the door frame.
Door with Side Panel(s)
Measure the full opening width, then calculate the individual panel widths
Measuring Width with Side Panels
First, measure the total brickwork opening width brick-to-brick at three heights. Take the smallest reading and deduct 10mm.
Then find the individual side panel width by measuring from the centre of the mullion (the vertical door frame member separating the door from the panel) to the outer edge of the side panel frame.
- Total opening width — measured brick to brick (smallest of 3), minus 10mm
- Side panel width — from mullion centre to outer door frame edge
- Measure each side panel individually if both sides differ
Door with Top Light
Measure the full opening height, then calculate the transom drop
Measuring Height with a Top Light
First, measure the total brickwork opening height brick-to-brick at three points, take the smallest, and deduct 10mm. This is your total door frame height.
Then measure the transom drop: from the centre of the horizontal transom bar (the door frame member separating the door from the top light) to the top of the door frame.
- Total opening height — brick to brick (smallest of 3), minus 10mm
- Transom drop — from transom centre to top of door frame
Door Handing
Always determined as viewed from the outside — the approach side of the door
Which Side are the Hinges On?
Door handing describes which side the hinges are positioned on, always determined when standing outside looking at the door. Getting this right is critical — an incorrectly handed door cannot be corrected after manufacture.
Stand outside facing the door and identify which side the hinges are on. That side gives you the hand. If replacing an existing door, check the hinge positions from outside to confirm before ordering.
Left Hand (LH)
Hinges are on the left side of the door frame as viewed from outside. The door handle is on the right and the door swings open to the right.
Right Hand (RH)
Hinges are on the right side of the door frame as viewed from outside. The door handle is on the left and the door swings open to the left.
Opening Direction
Does the door swing inward or outward, as viewed from the outside?
Which Way Does the Door Swing?
Opening direction describes whether the door swings towards you (outward opening) or away from you (inward opening) when you are standing outside the property.
The vast majority of UK external front doors are inward opening — they swing into the hallway. Outward opening doors are used where internal space is limited. The opening direction affects the frame rebate, weatherseal positioning, and hinge type, so it must be correctly specified at the time of ordering.
Inward Opening
The door swings away from you into the building as you push it open from outside. Standard for most UK external doors. Hinges are not visible from outside when the door is closed.
Outward Opening
The door swings toward you as you pull it open from outside. Less common for UK front doors, but used where internal hallway space is restricted. Hinges are visible from the outside when the door is closed.
Handing & Opening Direction Combinations
When ordering, specify both the hand and the opening direction. All descriptions are as viewed from outside the property.
| Order Code | Hinge Side (viewed outside) | Opening Direction | Handle Side |
|---|---|---|---|
| LH Inward | Left | Swings into building | Right |
| RH Inward | Right | Swings into building | Left |
| LH Outward | Left | Swings toward you | Right |
| RH Outward | Right | Swings toward you | Left |
Measurement Summary
Quick reference for all door configurations
| Configuration | Measurements Required | Width Deduction | Height Deduction |
|---|---|---|---|
| Standard Door | Total W, Total H | −10mm | −10mm |
| Door + Side Panel | Total W, Total H, Panel W | −10mm | −10mm |
| Door + Both Side Panels | Total W, Total H, Left Panel W, Right Panel W | −10mm | −10mm |
| Door + Top Light | Total W, Total H, Transom Drop | −10mm | −10mm |
| Door + Side Panel + Top Light | Total W, Total H, Panel W, Transom Drop | −10mm | −10mm |
| Door + Both Panels + Top Light | Total W, Total H, Left W, Right W, Transom Drop | −10mm | −10mm |
2013 - 2026 © GRP Composite Doors UK Ltd. All Rights Reserved.
Terms & Conditions. Privacy Policy. Cookie Policy. Sitemap.
Registered in England No. 8384596 | VAT Registered No. 155 4615 09