How to Measure for Aluminium Windows & Doors
Accurate measurements are important when ordering aluminium windows and doors. A small measuring error can result in a frame that does not fit the opening correctly, so measurements should always be checked carefully before placing an order.
Whether you are measuring to order new aluminium windows, folding doors, sliding doors, hinged doors or replacement window frames, the basic rule should always be to: determine the actual opening size, check that the opening is square and level, and allow for the correct installation clearance.
Tools You Will Need
Before you start, have the following tools ready:
- Tape measure
- Spirit level
- Pencil or marker
- Notepad or printed window and door schedule
- Calculator
- Straight edge
- Ladder where required
- Smartphone or camera for taking reference photographs
For larger openings, a laser measuring device can also be useful.
Step 1: Identify Each Window or Door Opening
Give every opening a reference number before measuring, for example:
- W01 – Kitchen Window
- W02 – Main Bedroom Window
- D01 – Patio Sliding Door
- D02 – Entrance Door
Record the measurements against each reference so that windows and doors are not confused when placing your order.
Step 2: Measure the Width
Measure the opening horizontally in at least three places:
- At the top
- In the middle
- At the bottom
Do not assume that the opening is perfectly straight.
Record all three measurements and identify the smallest measurement.
Step 3: Measure the Height
Measure vertically in at least three positions:
- Left side
- Centre
- Right side
Again, record all three measurements and identify the smallest measurement.
Measurements should normally be written as:
Width × Height
For example:
Step 4: Check That the Opening Is Square & Level
Measure diagonally from one corner of the opening to the opposite corner, then measure the opposite diagonal.
If both diagonal measurements are similar, the opening is reasonably square. A significant difference may indicate that the opening is out of square and should be checked before the window or door is manufactured.
Use a spirit level to check that the bottom sill and head of the opening are level and that the sides are plumb. Uneven, sloping or out-of-square openings may require correction before installation.
Step 5: Apply the 10mm Deduction Rule
As a general Sigmadoors measuring rule, deduct 10mm from the smallest opening width and 10mm from the smallest opening height to determine the window or door frame size.
For example:
Measured opening: 1210mm wide × 1510mm high
Deduct: 10mm from width and 10mm from height
Window size to order: 1200mm wide × 1500mm high
This allowance provides installation clearance around the aluminium frame for positioning, levelling, fixing and sealing.
Important: The 10mm deduction should be made from the smallest measurements recorded, not the largest.
Note that all standard windows and doors sold by Sigmadoors online are supplied already with the 10mm deducted.
If not certain or for unusual openings, replacement projects, large doors or situations where the opening is significantly out of square, contact Sigmadoors before ordering.
Step 6: Confirm the Window or Door Configuration
Measurements alone are not enough. You also need to confirm how the window or door must operate.
For aluminium windows, identify whether you require:
- Top-hung windows
- Side-hung windows
- Horizontal sliding windows
- Fixed windows
- Combination windows
For aluminium doors, identify whether you require:
- Hinged entrance doors
- Sliding patio doors
- Folding doors
- Pivot doors
- French or double doors
- Doors with sidelights
- Custom aluminium doors
Also indicate which panels must open and which panels must remain fixed.
Step 7: Confirm the Viewing Direction
Opening directions can easily be reversed if the viewing direction is not clearly stated.
When supplying a sketch or window schedule, clearly indicate whether the configuration is shown as viewed from the inside or outside. Just note that Sigmadoor will always ask you for views from outside.
For hinged doors, also confirm:
- Left-hand or right-hand opening
- Inward or outward opening
- Handle and lock position
For sliding doors, clearly indicate which panels slide and which panels remain fixed.
Step 8: Check the Sill, Floor and Wall Finishes
Before finalising your measurements, check whether any finishes will change the final opening size.
These may include:
- Floor tiles
- Screeds
- Timber flooring
- Window sills
- Stone cladding
- Plaster
- Wall tiles
- Skirting
- Threshold finishes
This is especially important when measuring aluminium doors because the finished floor level may affect the final door height and threshold position.
Where the door is supplied with a bottom frame, it is important to ensure that the finished floor level (FFL) is correctly aligned and flush with the frame threshold. This helps provide a smooth transition and reduces the risk of people tripping over the threshold.
Step 9: Measure Each Opening Individually
Never ever assume that a few similar looking openings are exactly the same size.
Even openings that appear identical may differ because of brickwork, plastering or construction tolerances.
Measure and record each and every opening separately.
Step 10: Double-Check Before Ordering
Before submitting your order, check:
- Width
- Height
- 10mm deduction
- Window or door reference
- Opening configuration
- Opening direction
- Frame colour
- Glass type
- Quantity
- Installation location
For custom aluminium windows and doors, always check the measurements again before manufacturing begins.
Aluminium Window & Door Measuring Tips
1. Measure Twice – Order Once
Follow the old building rule: “Measure twice, cut once.”
Take every measurement at least twice before placing your order. A few extra minutes checking your dimensions can prevent an incorrectly sized aluminium window or door from being manufactured.
2. Always Measure in Millimetres
Record all aluminium window and door dimensions in millimetres (mm). Avoid mixing centimetres, metres and millimetres on the same schedule.
3. Always Write Width × Height
Record dimensions in the following order:
Width × Height
For example:
1800mm W × 1500mm H
Clearly labelling the dimensions can help prevent the width and height from being reversed.
4. Remember the 10mm Deduction Rule
After measuring the opening in several places, use the smallest width and smallest height and deduct 10mm from each to determine the required frame size.
Example:
Opening: 1810 × 1510mm → Order size: 1800 × 1500mm
If the opening is irregular or significantly out of square, contact Sigmadoors before applying the standard deduction.
5. Measure Every Opening Separately
Do not assume that several windows or doors that appear identical have exactly the same opening dimensions.
Measure every opening individually and give it its own reference number.
6. Check That the Opening Is Square, Level & Plumb
Use diagonal measurements and a spirit level. The sill and head should be level, while the sides should be plumb.
Large differences should be investigated before ordering.
7. Check the Finished Floor Level for Doors
When measuring aluminium doors, determine whether tiles, screed, timber flooring or other floor finishes have already been installed.
If not, establish the final finished floor level before confirming the door height.
8. Take Extra Care with Replacement Windows & Doors
The dimensions of an existing aluminium, steel or timber frame do not necessarily represent the structural opening that will remain after it is removed.
Consider how the existing frame will be removed and what opening will remain before ordering the replacement.
9. Don’t Rely Only on Building Plans 
Architectural drawings and window schedules are useful for quotations and planning, but actual site openings can vary during construction.
Where openings have already been built, verify the final dimensions on site before manufacturing.
10. Check Sills, Plaster, Tiles & Cladding
Look for finishes that could reduce or change the opening, including plaster, tiles, stone cladding, window sills and thresholds.
Record anything unusual and take photographs where necessary.
11. Take Photographs of the Openings
Photographs can help identify unusual conditions that measurements alone may not show.
Take clear photographs showing the complete opening and surrounding wall, sill and floor conditions.
12. Check Large Openings Carefully
Large sliding doors, folding doors and oversized windows require particularly accurate measurements.
Check the width and height in several positions and verify that the opening is level, plumb and square.
13. Confirm the Opening Configuration
Before ordering, confirm which window sashes or door panels open, which remain fixed and the required opening direction.
A correctly measured frame with the wrong configuration is still the wrong product.
New-Build vs Replacement Measurements
For a new building, measurements should be taken from the completed or prepared structural opening once the opening is sufficiently complete to establish reliable dimensions.
For replacement aluminium windows and doors, additional care is required because an existing frame may conceal the true structural opening. Removal of old steel, timber or aluminium frames can also affect the final opening dimensions.
Where possible, establish the actual opening that will remain after removal before ordering the replacement frame.
Standard Sizes or Custom Sizes?
After measuring your opening and applying the appropriate installation allowance, compare the required frame dimensions with Sigmadoors’ standard aluminium window and door sizes.
Standard sizes can often provide a more economical solution where the building opening can accommodate them.
If your opening requires a different width, height or configuration, Sigmadoors can manufacture custom aluminium windows and doors to your required dimensions and specifications.
What to Send Us for a Quote
To help us prepare an accurate quotation, send:
- Width × height of each opening
- Quantity required
- Window or door type
- Opening configuration
- Frame colour
- Preferred glass type
- Photographs of the openings
- Project location
- Window schedule or architectural plans, where available
Frequently Asked Questions About Measuring Aluminium Windows & Doors
How Much Should I Deduct From the Opening for an Aluminium Window?
As a general Sigmadoors rule, deduct 10mm from the smallest measured width and 10mm from the smallest measured height.
For example, an opening measuring 1210 × 1510mm would generally require a 1200 × 1500mm frame.
Do I Measure the Window or the Brick Opening?
When ordering a new window for an existing opening, the structural opening is normally the important starting measurement. Measure the opening in several positions before determining the required aluminium frame size.
Should I Measure Width or Height First?
Aluminium window and door dimensions should be recorded as:
Width × Height
For example, 1800 × 1200mm means 1800mm wide and 1200mm high.
Why Should I Measure in Three Places?
Brickwork and finished openings are not always perfectly straight, level or square. Measuring the width and height in several places helps identify the smallest available dimensions.
What If My Opening Is Not Square?
Check both diagonal measurements. If there is a significant difference, the opening may need to be corrected before installation.
If you are unsure, send Sigmadoors the measurements and photographs before ordering.
How Do I Measure for a Replacement Aluminium Window?
Take additional care with replacement windows because the existing frame may conceal part of the structural opening. Establish what opening will remain once the old frame has been removed before confirming the replacement window size. In the case of clip-on aluminium window replacement, it is advisable to get the assistance of experienced Sigmadoors professionals.
How Do I Measure an Aluminium Door?
Measure the width at the top, middle and bottom and the height at the left, centre and right. Also check that the opening is square, level and plumb.
For doors, it is particularly important to establish the finished floor level before confirming the final height.
Do I Need to Allow for Floor Tiles When Measuring a Door?
Yes. Tiles, screed, timber flooring and other finishes can change the finished floor level and therefore affect the available door opening height.
Can I Use the Dimensions on My Architectural Plans?
Plans and window schedules are useful for quoting, but actual built openings should be checked on site before final manufacture whenever possible.
Can Sigmadoors Manufacture Custom Sizes?
Yes. If a standard aluminium window or door size is not suitable for your opening, custom windows and doors can be manufactured to the required dimensions and configuration.
Important Before You Order
Aluminium windows and doors are manufactured to specific dimensions. Measure twice, check your configuration and confirm your required frame size before placing your order.
If you are unsure about your measurements, the 10mm deduction, opening direction or configuration, contact Sigmadoors before ordering.
Disclaimer: This measuring guide is provided for general guidance only. Site conditions, wall finishes, floor levels, aluminium systems and installation requirements may vary. Always ensure that a professional has verified all measurements before placing an order. Where an opening is uneven, out of square, unusually large or difficult to measure, we recommend having the opening professionally measured. Sigmadoors cannot accept responsibility for products manufactured to incorrect dimensions supplied by the customer. Final manufacturing sizes and installation clearances should be confirmed before ordering.