How to Measure External Doors & Windows

Before You Start Measuring

Getting the right measurement is the single most important step in ordering a new external door. Read these golden rules before you pick up a tape measure.

We Cannot Accept Responsibility for Incorrect Measurements

External doors are manufactured to the exact dimensions you provide — we do not make any fitting allowances on the sizes given to us. If you are unsure at any point, please consult a professional fitter, joiner, or builder before ordering. You can also call us on +44 7472 828257 and we will gladly help and advise.

Measure the Brickwork Opening

Always measure the raw brick-to-brick aperture — never the existing door, frame, or architrave. Old frames are rarely true and will give misleading readings.

Take Every Measurement 3 Times

Measure width at the top, middle, and bottom of the opening. Measure height at the left, centre, and right. Always use the smallest of the three readings.

Always Deduct 10mm

Subtract 10mm from both your final width and height measurements. This creates a 5mm fitting clearance on each side, allowing the frame to be set plumb and square.

Include the Frame & Cill

Your measurements must include the full door frame, the cill (sill), and any frame extenders you require. Do not measure the door leaf alone.

Metric Millimetres Only

All dimensions must be supplied in millimetres (mm). We do not accept imperial measurements or convert from feet and inches.

View Everything Externally

All design decisions — handing, opening direction, glazing position, side panel location — must be described as seen from outside the building, looking in.

What You'll Need

Gather these items before you begin. Having the right tools makes the job quicker, safer, and more accurate.

Tools & Materials

Measuring a Standard External Door

Follow these two steps for any standard external door — whether composite, upvc, french, patio or bifold doors. Both width and height must be measured separately and independently.

Measure the Opening Width (Brick to Brick)

• Near the top of the opening (25–50mm below the lintel)
• At the middle of the opening
• Near the bottom of the opening (25–50mm above the floor)

Measure the Opening Height (Brick to Brick)

• On the left side (near the left brickwork)
• At the centre of the opening
• On the right side (near the right brickwork)

Smallest of 3 width readings, minus 10mm

Smallest of 3 height readings, minus 10mm

Always describe the door as seen from outside the building

Millimetres only — no inches, no conversion

Door with a Side Panel

A side panel (also called a sidelight) is a fixed glazed panel that sits beside the main door within the same frame. You will need to measure the total opening width plus the individual side panel width — these are two separate measurements.

Measure the Total Opening Width (Full Brick to Brick)

Measure the Side Panel Width (Mullion Centre to Outer Frame)

What to Provide When Ordering a Door with Side Panel

1. Total frame order width (full brick-to-brick width minus 10mm) · 2. Side panel width (mullion centre to outer frame edge) · 3. Frame order height (brick-to-brick height minus 10mm) · 4. Side panel position — left or right as viewed externally

Door with a Top Light

A top light (also called a fanlight or transom light) is a fixed glazed panel that sits above the main door, separated by a horizontal transom bar. You need the total opening height and the transom drop — the depth of the top light.

Measure the Total Opening Height (Full Brick to Brick)

Measure the Transom Drop (Top Light Depth)

What to Provide When Ordering a Door with Top Light

1. Frame order width (brick-to-brick width minus 10mm) · 2. Total frame order height (brick-to-brick height minus 10mm) · 3. Transom drop (top of frame to centre of transom bar)

Door Handing & Opening Direction

Once you have your measurements, you need to specify how the door opens. Getting this wrong means the door will be manufactured the wrong way around and cannot be corrected after manufacture.

How to Determine Handing — Always View Externally

• Left-hand hung — the hinges are on the left as you face the door from outside.
• Right-hand hung — the hinges are on the right as you face the door from outside.

• Inward opening — the door swings toward you when you open it from inside (i.e., into the building). This is the most common for UK external doors.
• Outward opening — the door swings away from you when opening from inside (i.e., out from the building). Often used for back doors or outbuildings.

Measuring a UPVC Window

The same brick-to-brick principle applies to windows as to doors. Always measure the raw aperture in the brickwork — never the old frame, glazing bead, or existing window unit.

Measure the Window Opening Width (Brick to Brick)

• Near the top of the opening (just below the lintel)
• At the middle of the opening
• Near the bottom of the opening (just above the cill)

Measure the Window Opening Height (Brick to Brick)

Measure the Mullion Width (Multi-Pane Windows)

Measure the Transom Drop (Horizontal Dividing Bar)

What to Provide When Ordering a Window with Mullion or Transom

Standard window: order width + order height · + Mullion: also provide mullion position (inner frame edge to mullion centre) · + Transom: also provide transom drop (top of frame to centre of transom bar) · Equal panes: simply state this and we will calculate the positions automatically

Smallest of 3 width readings (top / middle / bottom), minus 10mm

Smallest of 3 height readings (left / centre / right), minus 10mm. Always measure to underside of cill.

If applicable: inner frame edge to centre of mullion bar (mm)

If applicable: top of frame down to centre of transom bar (mm)

Measuring a Square Bay Window

Square bay windows consist of three window panels and two 90° corner posts. They require a combination of internal width measurements and an external height measurement — different from standard windows.

Internal Back Span, Side Panel Widths & External Height

• Example: internal back span = 1,800mm → external order width = 1,800 + 140 = 1,940mm

What to Provide When Ordering a Bay Window

1. Internal back span (we add 140mm for corner posts) · 2. Left side panel width (internal frame-to-frame) · 3. Centre panel width (internal frame-to-frame) · 4. Right side panel width (internal frame-to-frame) · 5. External height (brick-to-brick, smallest of 3, minus 10mm)

Measuring a Bow Window

Bow windows are more complex than bays. They consist of multiple panels at shallower angles, creating a curved projection. Three key measurements are required: the internal back span , individual facet widths , and the projection depth .

Back Span, Individual Facet Widths & Projection

What to Provide When Ordering a Bow Window

1. Internal back span (straight across, edge to edge) · 2. Width of each individual facet (measured internally) · 3. Projection depth (from back span face to front frame face, internally) · 4. External height (brick-to-brick, smallest of 3, minus 10mm) · 5. Number of facets (typically 3 or 5)

Pre-Order Checklist

Before you submit your order, confirm every item in this checklist. A missed detail is the most common cause of a door not fitting or being manufactured incorrectly.

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