How to Measure for French Doors: Width, Height & Handing

Measuring for French doors requires more than recording one width and one height. A complete doorset needs to fit inside the structural opening with the manufacturer’s required fitting tolerance, while the threshold, cill, final floor levels and master-leaf handing all need to be confirmed before ordering. This guide explains how to measure for French doors in a UK property.

Our French doors are complete uPVC systems, so use the exact overall frame dimensions from the individual Crystal product page rather than trying to calculate the size from a single door leaf.

What Are You Measuring?

For a complete French doorset, the key measurement is the structural aperture that will receive the full frame. If an existing patio or French-door frame is still in place, decorative trims may hide part of the opening. The visible glass width or distance between frame edges is not necessarily the structural size.

If possible, have the installer identify the true opening and confirm which finishes will remain. This is especially important where render, plaster, cavity closers or old timber subframes are present.

Tools You Need

• Long tape measure marked in millimetres.
• Long spirit level or laser level.
• Straightedge.
• Notebook or phone for recording each reading.
• Second person for wide openings.

Write each figure down immediately and label it width top, width centre, width bottom, height left and so on. This prevents transposed measurements.

Step 1: Measure the Width at the Top

Measure between the structural sides at the top of the opening. Keep the tape horizontal and do not include decorative trims that will be removed. Record the figure in millimetres.

If the opening is still occupied by the old frame, note that the measurement is preliminary unless you can identify the actual structural faces.

Step 2: Measure the Centre Width

Repeat the measurement around the midpoint of the opening. A difference from the top reading can indicate that one side is out of plumb or that masonry/plaster varies.

Do not average the figures to create an order size. The exact doorset must fit through the narrowest relevant point while leaving the manufacturer’s required installation space.

Step 3: Measure the Bottom Width

Measure across the bottom above the threshold or structural base, depending on the installation detail. External paving and old cills can make this area difficult to read, so make sure you know which parts are staying and which are being removed.

Three width measurements give a much better picture than a single centre measurement.

Step 4: Measure the Height at Three Points

Measure from the agreed bottom datum to the underside of the head at the left, centre and right. The bottom datum might be the structural sill, finished threshold level or another point specified by the installer.

If the new doorset includes a cill, confirm whether the product’s listed height includes that cill. Do not assume all manufacturers use the same dimension convention.

Step 5: Check the Diagonals

Measure from top-left to bottom-right and then top-right to bottom-left. Similar readings suggest the opening is reasonably square. A significant difference means the frame must be carefully packed or the opening may need remedial work.

The uPVC frame should be installed square; it should not be twisted to match an irregular aperture because that can affect how the two leaves meet and lock.

Step 6: Check Level and Plumb

Use a level on the base, head and both sides. Wide openings can disguise a slight slope that becomes important when a pair of doors needs to meet accurately in the centre.

If the head or threshold is badly out of level, ask the builder or installer to correct the opening before final dimensions are confirmed.

Step 7: Confirm the Fitting Tolerance

Check the exact manufacturer or installer requirement for space around the frame. Do not deduct an arbitrary 10mm or another online figure from your aperture. The correct tolerance depends on the system and fixing method.

For high-value doorsets, having the fitter survey the opening before ordering is often the safest approach.

Step 8: Account for Floor Finishes

New tiles, adhesive, screed, engineered flooring or underfloor-heating build-ups can change the internal level. External paving and drainage can also change the threshold relationship. Establish final levels before the doorset height is fixed.

Do not raise external paving later in a way that blocks drainage or compromises the cill detail.

Step 9: Identify the Master Leaf

The master leaf is the one used for normal opening and locking. Our Crystal French doors include left-hand master configurations, so confirm whether that suits the layout. The viewing convention used to define left and right must come from the product documentation.

Think about the route through the opening. The master leaf should not clash with an island, wall, radiator, furniture or external feature.

Step 10: Check Opening Direction

Confirm whether the specific doorset opens inward or outward. Measure the clear swing area for both leaves and consider curtains, blinds and exterior steps. An opening can be dimensionally perfect but still impractical if the leaves have nowhere to swing.

Example French Door Sizes

Our Crystal range is available in multiple widths. The Crystal UPVC French Door White Clear Triple Glazing Left Hand Master has several size variants, demonstrating why you should choose from exact metric dimensions rather than a generic “standard French doors” label.

See our French door sizes UK guide for a fuller explanation of common frame sizes and structural openings.

Replacing Existing French Doors

If you are replacing an old complete frame, do not assume the new system should have identical outer dimensions. Old installation gaps, subframes and trims may differ. Survey the structural opening and compare it with the new product.

Removing internal trims can sometimes reveal packing gaps and help confirm the true size without taking the whole frame out before the replacement arrives.

Replacing Sliding Patio Doors With French Doors

Sliding patio frames can have different frame profiles and thresholds. The structural opening may still accept a French doorset, but the new frame size, cill and swing space need to be checked. A sliding door does not need swing clearance, so furniture and patio layouts can become an issue after conversion.

Measure both the opening and the usable space into which the leaves will open.

New Extensions and New Openings

If the wall opening is not yet built, choose the French doorset early. Give the structural designer and builder the exact overall frame dimension and required aperture. This is much more reliable than forming an arbitrary opening and searching for a product later.

Where a new opening is being cut into an existing wall, structural support above must be designed appropriately before measuring for the final frame.

French Door Measuring Checklist

• Structural width at top, middle and bottom.
• Height at left, centre and right.
• Diagonals.
• Head and base level.
• Jambs plumb.
• Finished internal floor level.
• External paving and drainage level.
• Cill included and its dimension convention.
• Exact frame width and height.
• Required fitting tolerance.
• Master-leaf handing.
• Opening direction and swing clearance.

Common Measuring Mistakes

• Measuring the old glass area instead of the opening.
• Taking only one width reading.
• Using decorative trim as the structural edge.
• Forgetting the cill.
• Ignoring planned floor finishes.
• Ordering before checking diagonals.
• Assuming left-hand master from a generic definition.
• Forgetting that hinged leaves need swing space.

Frequently Asked Questions

Do I measure brick to brick for French doors?

The structural opening is important for a complete frame replacement, but visible brickwork or render may not show the full aperture. Confirm the true opening and fitting detail.

How much smaller should the frame be than the opening?

Use the exact fitting tolerance specified by the doorset manufacturer or installer. Do not use a universal deduction.

Should I measure before the new floor goes down?

You can, but you must know the final floor build-up and threshold level so the installed frame height remains correct.

What does left-hand master mean?

It identifies the primary operating leaf according to the manufacturer’s defined viewpoint. Use the specific product diagram to confirm it.

Can French doors replace a sliding patio door?

Often they can if the structural opening and frame dimensions work, but swing clearance, threshold and installation details need to be assessed.

Choose the Correct French Doors

Once the measurements have been verified, compare our uPVC French doors by exact size, colour and glazing. For the wider selection process, read our French doors buying guide.