How to Measure for Aluminium Doors and Windows (and Common Mistakes to Avoid)

Correct measurements are critical when ordering aluminium doors and windows.
Even small errors can lead to delays, additional costs, or products that simply won’t fit.

Here’s how measuring works in practice, what to be careful of, and when a professional survey is essential.

Why Accurate Measuring Matters

Aluminium doors and windows are made to order.
Once manufactured, sizes cannot be adjusted.

Incorrect measurements can result in:

• Gaps around frames

• Installation issues

• Delays to your project

• Additional manufacturing costs

This is why measuring should never be rushed.

Structural Opening vs Brick-to-Brick

One of the most common mistakes is measuring the wrong opening.

Brick-to-brick measurement

• Taken from the outer brick faces

• Used mainly for rough reference

• Not suitable for final ordering

Structural opening

• The actual opening the product sits within

• Accounts for plaster, render, and finishes

• This is the measurement used for manufacturing

Always confirm which measurement is being used.

Allowing for Installation Tolerances

Aluminium systems require tolerance for:

• Adjustment during installation

• Levelling and alignment

• Sealant and fixing clearance

Typically, a small allowance is deducted from the opening size to ensure the product fits correctly.

Ordering too tight is a common and costly mistake.

Finished Floor Levels Matter

Finished floor levels are often overlooked.

You must consider:

• Final floor build-up

• Threshold height

• Internal vs external levels

• Drainage requirements

Measuring before floors are finished can lead to incorrect door heights if this isn’t planned properly.

Cills, Thresholds & Trickle Vents

Measurements are affected by:

• Whether a cill is included

• Threshold type (standard, low, or rebated)

• Trickle vent requirements

These details change overall sizes and must be confirmed before ordering.

New Openings vs Replacements

Replacements

• Existing openings already defined

• Still need accurate internal measurements

• Hidden structural elements may affect sizing

New openings

• Structural calculations may be required

• Steel supports and lintels affect final sizes

• Measurements should be taken after structural work is complete

Never assume drawings alone are sufficient.

Common Measuring Mistakes

• Measuring plaster instead of structure

• Forgetting floor build-up

• Ordering too tight

• Ignoring threshold requirements

• Not allowing for installation tolerances

These are some of the most common causes of issues.

When a Professional Survey Is Essential

A professional survey is strongly recommended when:

• Openings are large

• Structural work is involved

• Multiple products meet

• Levels are complex

• You want absolute certainty before ordering

A survey removes guesswork and protects your investment.

Final Thoughts

Measuring aluminium doors and windows isn’t complicated — but it must be done correctly.

Careful planning, accurate measurements, and professional guidance ensure:

• Smooth installation

• Correct fit

• No costly surprises

At Home Build Doors, we offer guidance and survey support to make sure every order is right before it goes into production.