Start With the Right Tools and a Clear Head

Replacing a garden gate is one of those jobs that looks straightforward until the new gate arrives and refuses to fit. Almost every disappointment comes down to one thing: the measurements were taken from the old gate rather than the opening. A gate that has swollen, dropped or been trimmed over the years is a poor template, so put it to one side and measure the gap it came out of.

You will need a steel tape measure long enough to span the opening comfortably (a 5m one is ideal), a spirit level, a pencil, and something to write on. A helper is genuinely useful, particularly for holding the tape level at height. If you can, take a photograph of the opening with the tape measure in shot — it saves a return trip to the garden centre and gives you a record if you need to query an order later.

Work in millimetres throughout. Timber gates are almost always sold in metric sizes, and mixing feet and inches with millimetres is a reliable way to introduce errors. Write every measurement down as you take it, even the ones you think you will remember.

Measure the Opening at the Top, Middle and Bottom

Posts move. Timber posts lean, concrete posts settle, and a gate that has been hanging and swinging for fifteen years will have pushed the hanging post outwards. This is why a single width measurement is not enough.

• Measure the clear width between the posts at the top, at the midpoint and at the bottom, close to ground level.
• Measure the height on both sides, from the highest point of the ground beneath the gate to the underside of any crossbar or the top of the post, whichever governs.
• Take the smallest width and the smallest height as your working figures. A gate must fit through the tightest point, not the widest.

If the three widths vary by more than about 15mm, the opening is noticeably out of square and a standard-sized gate may bind. Note the variation down; it may mean trimming is required, or that you should consider a made-to-measure gate.

Check Whether the Frame Is Square

Widths alone do not tell you whether the opening is a true rectangle. Measure corner to corner, diagonally, in both directions. On a square opening these two figures will match. If they differ by more than roughly 10mm, the posts are out of plumb or the ground has shifted.

Rest the spirit level against each post to see which way it leans, and check whether the ground between the posts slopes. A gently sloping path is common and perfectly manageable, but it changes the clearance you need, so it needs to be recorded rather than discovered on fitting day.

Note Hinge Positions, Swing Direction and Latch Side

Hinges are where replacement gates most often catch people out. Even a gate of exactly the right width will not hang if the hinge positions do not line up with the existing holes or brackets.

• Record which post the gate hangs from, and which side the latch closes against.
• Measure the distance from the top of the hanging post to the centre of the upper hinge, and from the bottom of the post to the centre of the lower hinge.
• Confirm whether the gate opens inwards or outwards, and whether it needs to swing back flat against a wall or fence.
• Note the hinge type — T-hinges, strap hinges, adjustable hooks and bands, or bolt-on brackets — and the gap the old hinge occupied.

Taking a photo of the hanging stile with the tape against it is quick and removes all doubt later. If you are changing hinge types, allow for the extra gap between post and gate that a different hinge may create.

Ground Clearance and What Sits Under the Gate

A gate needs room to swing. Aim for a minimum of 20mm clear of the ground on a firm, level surface such as paving or concrete, and 40mm or more over gravel, bark or rough grass so the gate does not scrape or drag. Where the ground slopes across the opening, measure from the highest point the gate passes over, not the lowest.

Also look at what is in the way: drain covers, raised slab edges, a step, or a kerb. If you want a self-closing mechanism, a drop bolt or a lockable latch, mention it now — some of these affect the required clearances and the stile width you should order.

Work Out the Size to Order and Allow for Clearance

Take your smallest opening width and subtract the total clearance you want — typically 10mm to 15mm at each side, so 20mm to 30mm overall. This allows for seasonal swelling of timber, a coat of paint or stain, and the swing of the gate within the frame. Take your smallest height and subtract your chosen ground clearance.

If you are buying a made-to-measure gate, the supplier usually wants the opening size rather than the gate size, and will make the deductions themselves — so state clearly which figures you are giving. For off-the-shelf gates, you are ordering the gate size, so do the sums yourself.

Finally, check the condition of the posts before you order. If they are rotten at the base or wobbling in the ground, no gate will hang well against them. Replacing a post changes the opening, sometimes by 50mm or more, so it is far better to renew the posts first and measure afterwards than to measure twice and order the wrong gate.