The arc is the whole problem
People size a gate by its opening and then find out about the arc afterwards. A twelve foot single leaf, hinged on one post, sweeps a quarter circle roughly twelve feet in radius, and every square foot of that quarter circle has to be clear of ground, plants, parked vehicles, snow, firewood and the slope of the driveway. Half of it being clear is not useful. The gate stops at whatever angle the first obstruction sits at, and a gate that stops at sixty degrees leaves noticeably less than a gate that stops at ninety.
Two smaller things eat the opening even when the arc is perfect. The hinge pin sits outboard of the leaf, and the leaf has thickness. At a full ninety degrees the leaf stands square, and what remains in the opening is exactly the pin offset plus the leaf thickness — on each side, for a double gate. It is only two or three inches a side, which sounds like nothing until you are reversing a wide trailer through a twelve foot gap at night.
Slope is the thing nobody measures
A driveway gate is usually installed on a driveway, and driveways are rarely level across the direction the gate swings. If the ground climbs into the swing, the gate is fine at the closed position and grounds out somewhere in the middle of the arc. The arithmetic is unforgiving. Take a twelve foot opening with four inches of clearance under the closed leaf. At a two percent cross fall the far corner still gets round, with about an inch of clearance left at ninety degrees — which is nothing. Double it to four percent, an ordinary driveway crown, and the leaf touches down around forty six degrees into the swing, because by then the corner has travelled eight and a third feet out and the ground has climbed the whole four inches.
The usual fixes are to raise the closed clearance, which leaves a bigger gap that dogs and small livestock go under, or to cut the ground flat across the whole arc, which is real earthwork on a driveway you have to keep using. A third answer is to hang the gate to swing the other way, which is free if you notice before the posts go in and expensive afterwards. Measuring the cross fall with a level and a long straightedge before ordering anything takes ten minutes.
Which way it opens
Swinging in toward the property is the common choice, because a leaf that opens out over the road is in traffic while it moves and while it stands open. Swinging in means the arc lands on your own ground, where you can control what is in it. It also means the gate needs its whole radius of driveway inside the entrance, which pushes the gate further back from the road, which is a separate question with its own arithmetic.
| Opening angle | What that means in practice |
|---|---|
| 90 degrees | Leaf square to the opening. Full drive-through less the pin offset and thickness. |
| Around 100 to 110 | Leaf folded slightly back along the fence, out of the way of a wide load. Needs ground behind the gate line. |
| Under 80 | Leaf still leaning into the opening. Noticeably narrower gap and the corner people scrape. |
| Under 60 | Usually a sign the arc is blocked rather than a design choice. |
When a swing gate is the wrong answer
If the entrance is cut into a bank, if the driveway climbs away from the road, if the swing side is where vehicles have to park, or if the leaf would be long enough to sag, the ground on the swing side is the constraint and no amount of good hardware fixes it. A slide gate trades that arc for frontage along the fence line, which some sites have and some do not. The slide gate backspace calculator gives that number, and the two pages together usually settle the argument in about five minutes.
Whatever the gate does, the leaf itself is a separate build. Frame sizes, the direction a diagonal brace has to run, and the outward pull on the top hinge are on the gate sag and brace calculator. If the entrance has masonry pillars either side, the unit count for those is on the brick column calculator, and the holes for timber or steel posts down the fence line are on the fence post calculator.
The parts this page will not touch
A powered gate is a machine. It moves a heavy leaf under motor power across ground where children, pets and people on foot are, and the ways it injures and kills are well documented: a person or a limb caught between a closing leaf and a post, a child riding or climbing a leaf, a hand in the path of a rolling gate at a roller post. Which safety devices an installation requires, where they go, how the operator is set up and how any of it is tested belong to the operator manufacturer's instructions and the safety standard the operator is listed to, and to the installer who does the work. None of that is on this page and this page will not summarise it. If you are considering a powered gate, that conversation starts with a gate automation installer and the manufacturer's manual, before any of the arithmetic here matters.
The strip between the travelled road and your property line usually is not yours. Whether a gate, a pillar, a post or a culvert may sit there at all, how far back from the pavement edge it has to be, what sight lines have to stay open, whether the entrance needs a permit or an encroachment approval, and who is liable for what afterwards are decided by the road authority — a city or county public works department, a state highway department, or a homeowners association on a private road — and they differ between adjacent jurisdictions. Ask them before anything is built. A gate or a column standing where a vehicle can hit it is a hazard to the driver as well as to your wallet.
Questions people ask
How much room does a swing gate actually need?
Its whole leaf length, as a quarter circle on the side it opens to, plus a couple of inches for the pin offset and thickness. A twelve foot leaf sweeps a quarter circle of about twelve feet radius — roughly 113 square feet of ground that has to stay clear, level and unobstructed. That is the number people underestimate, because on a plan it is a thin arc and on the ground it is a large piece of driveway.
Why is the opening narrower than the gap between the posts even when the gate is fully open?
Because the hinge pin sits outboard of the leaf face and the leaf has thickness. At ninety degrees the leaf stands square across the swing side, and the pin offset plus the thickness is still inside the opening. On a double gate it happens at both ends. Two or three inches a side is typical, which the calculator shows separately so you can see whether it matters for what you tow.
My gate opens fine at the closed position and jams halfway. Why?
Almost always the ground rises across the swing. The bottom rail keeps a constant height above the pivot, and the ground does not, so the clearance you set at the closed position shrinks as the leaf travels out. Enter the cross fall as a percentage and the calculator gives the angle at which the corner touches down. Frost heave, a settled gravel drive and a season of grass growth all push the same way.
Is opening past ninety degrees useful?
Sometimes, and it is not free. Past ninety the leaf folds back behind the gate line and runs along the fence, so anything within that distance of the hinge post is in the way. What you buy is a leaf that is fully out of the path of a wide or long load rather than standing square at the edge of it. The calculator reports how far behind the line the corner goes so you can check what is there.
Does this tell me whether my gate is legal or safe?
No, and it is not able to. Whether a gate may stand where you want it, how far back from the travelled way it has to be, what sight lines must stay open and whether the work needs a permit are set by the road authority for your entrance and vary between neighbouring jurisdictions. If the gate is powered, the safety devices, their placement and the setup belong to the operator manufacturer instructions, the safety standard it is listed to, and the installer. This page is geometry only.