Why a Few Millimetres Matter

Ordering a garden gate should be straightforward, but it often goes wrong at the very first step: measuring the opening. A gate that is even a few millimetres too wide will not close, whilst one that is too narrow will look untidy and leave gaps that wind and rain can exploit. The most common mistake is measuring the old gate rather than the gap it sits in. Old gates warp, swell and sag, so their dimensions are rarely a true reflection of the opening. Always measure the clear opening between the posts or walls. That is the space your new gate must fill, and getting it right saves you the hassle of returns, re-drilling hinges or, worse, a gate that never hangs properly.

Gather Your Tools and Clear the Opening

Before you start, you will need a few basics: a reliable tape measure (a metal one is best, as fabric tapes stretch), a spirit level, a pencil and paper, and ideally a helper to hold the other end. Clear away any vegetation, pots or debris that might get in the way. If your existing posts are rotten, loose or out of plumb, replace or repair them first. A new gate hung on failing posts will never stay true, and your measurements will be meaningless once the posts shift. Check that the ground beneath the opening is firm and level enough to support the gate. If you are measuring between walls, look for render, pipes or uneven brickwork that could affect the fit.

Measuring the Width and Height Correctly

Start with the width. Measure the distance between the inside faces of the posts or walls at three points: at the top, in the middle, and at the bottom. Write each figure down in millimetres. The smallest of these three measurements is the one that matters most, because it represents the tightest point the gate must pass through. If the top is 900mm and the bottom is 890mm, your gate must be no wider than 890mm minus your clearance. Next, measure the height. Take readings from the ground to the top of the opening at the left, centre and right. Again, use the smallest height as your guide, unless you plan to raise the ground level later. Remember to allow for ground clearance: a wooden gate typically needs 25–50mm above the ground to avoid soaking up moisture, whilst a metal gate can sit closer, perhaps 10–20mm. Hinges and latches also need space, so factor in a few millimetres on each side.

Checking the Opening for Square

An opening can be the right width and height but still be out of square, which means the corners are not true right angles. This is a common problem with older fence posts that have leaned over time. To check, measure diagonally from the top left corner to the bottom right corner, then from the top right to the bottom left. If the two diagonal measurements match within a few millimetres, the opening is square. If they differ by more than 10mm, the frame is skewed, and a standard rectangular gate will either bind or leave a wedge-shaped gap. You can sometimes compensate by ordering a gate slightly smaller and using adjustable hinges, or by packing out the hinges. But if the difference is large, it is better to straighten or replace the posts. Also use your spirit level to check that the posts are plumb and the top of the opening is level. A post that leans inwards will reduce the effective width at the top.

Other Things to Consider Before Ordering

Think about how the gate will be used. Which way does it need to swing? You will need clear space behind it for the gate to open fully, so check for walls, plants or slopes. If the ground slopes across the opening, a straight-bottomed gate may catch on the higher side. You might need a gate with a sloped bottom rail, or you could set it higher and accept a larger gap underneath. Consider the material: wooden gates swell in wet weather, so allow extra clearance on the width—often 10–20mm total. Metal gates expand less but still need a little room. Also check for obstructions such as downpipes, raised beds or bin stores that could block the swing or the latch. If you are fixing to masonry, ensure the wall is sound. Finally, decide whether you want the gate to sit between the posts or overlap them. Most garden gates are designed to fit between the posts, so measure the clear gap accordingly.

Double-Check and Record Your Measurements

Once you have your figures, measure everything again. It sounds tedious, but a second pass catches mistakes. Use the same reference points and keep the tape measure straight and taut. Write down all measurements in millimetres, clearly labelled: smallest width, smallest height, diagonal difference, and any notes about slope or obstructions. Take a few photos of the opening with a tape measure in place—they are invaluable if you need to discuss the job with a supplier. If anything feels uncertain, ask someone experienced to check your work. A few extra minutes with a tape measure now will save you hours of frustration later. With accurate numbers, you can order your gate with confidence, knowing it will fit, swing freely and last for years.