How to measure for uPVC windows and doors

How do I measure a brick opening for a replacement window or door?

Measure the brick opening itself rather than the frame in it. Take the width at three heights and the height at three positions across, and use the smallest of each. That tightest figure is the one the new window or door has to fit inside.

If the old frame is out, measure the brick reveals directly. If it is still in, measure the brickwork from the outside where you can see it, or have your fitter confirm the opening once the old frame is stripped. Either way, the brick opening decides the size, and the old frame is a guide to it. Write down all six figures, so that you can see how far the opening varies.

Why use the smallest of the readings?

Because a window that fits the tightest point fits everywhere else. Brick openings vary: mortar joints, old plaster returns and slightly leaning reveals all move the figures by a few millimetres, and a standard size has fixed dimensions.

Standard sizes come in fixed steps. Take the smallest reading and choose the largest listed size that fits within it, with room to spare.

Note which room and which wall each set of figures is for, and keep them for your fitter. For a French or patio door, also check the floor is level across the opening, because the door sits on its cill and the cill has to be level.

How much smaller than the opening should the window be?

Allow a gap for packing and sealant. The common trade practice is to take about 10mm off the tightest width and about 10mm off the tightest height, and to check what your fitter wants before you order. Fitters differ, and what is right depends on how the frame will be packed and sealed.

For example, if the tightest width is 1210mm and the tightest height is 1060mm, deducting 10mm from each gives 1200mm by 1050mm. The largest listed size that fits inside that is a 1190mm by 1040mm window, so that is the one to look for. Window sizes are the overall height including the cill, so do the sum with the cill in mind.

How do I check the opening is square?

Measure both diagonals, corner to corner, and compare them. If they match, the opening is square. If they differ by more than a few millimetres, the opening is out of square, so tell your fitter before you order.

A standard size suits an opening that is slightly out of square; the difference affects how the frame is packed and how much gap you need around it. Take extra care over the tightest width and height, and leave the larger allowance. For a larger difference, ask the fitter to look before you buy, or consider the made to measure window and door designer.

Do I measure from the inside or the outside?

Measure the brick opening, and do it from whichever side lets you see the brickwork clearly. For a replacement, that is usually from the outside, with the old frame removed or the reveals visible. For a door, measure the height from where the cill will sit to the underside of the lintel. Product heights include the cill, so the cill is part of the figure.

How do I read left and right hand?

Read handing from the outside. Stand outside facing the window or door. A left hand side hung window is hinged on your left, and a right hand one on your right. A top opener or top hung opener hinges along the top, so handing applies to the side hung styles.

Names differ across the range. The double glazed windows spell out Left Hand Side Hung, while the triple glazed names shorten it to LH, RH and Top. Single doors say left hand or right hand and Open In, meaning they open inwards. French doors are listed left hand master. Patio doors are named by the direction they slide, left to right or right to left. For help choosing the hand, ring 0151 525 4488 before you order.

What sizes can I pick from?

Choose the nearest standard size that fits inside your opening once you have taken off the allowance. These are the sizes that exist in the range.

Product	Widths in mm	Heights in mm
Windows, double and triple glazed	440, 610, 820, 905, 1040, 1190, 1770	610, 820, 965, 1040, 1115, 1190
Single doors	840, 890, 920	2090
French doors	1190 to 1790 in 100mm steps	2055, 2090
Sliding patio doors	1490, 1790, 2090, 2390	2090
GRP composite doors	920	2055

Each window width comes in the heights shown on its listing, so check the listing. Start from windows or doors, or open a sample such as the 1190mm x 1115mm top hung window, the 920mm single door or the 1790mm sliding patio door. For any other size, the window and door designer prices a made to measure window as you choose sizes.

What happens once I have chosen?

The window or door is delivered direct by the manufacturer to your address. Delivery is priced by postcode and shown on the product page and at checkout. It is supplied for you or your installer to fit. Replacement windows and doors are notifiable work, so use a registered installer or tell building control. Before you order, check the colour (white, or grey 7016 outside with white inside), the glass (clear or obscure), the glazing (see double or triple glazing) and the hand. Ring 0151 525 4488 if anything is unclear.