Trampoline Anchor and Ground Calculator

Anchors: an arithmetic problem wrapped round a wind problem

Spacing anchors evenly round a frame is straightforward. Divide the perimeter by the number of anchors you have and that is the distance along the rim between them. On a round frame the straight-line distance between neighbours is shorter than that, because it is a chord rather than an arc: the diameter times the sine of pi over the count. A fourteen foot round frame has a 43.98 foot perimeter, so four anchors sit 11.00 feet apart round the rim and 9.90 feet apart in a straight line, at ninety degrees to each other.

What that arithmetic cannot tell you is how many anchors the trampoline needs, what kind they should be, or whether they will hold in the ground you have. Those depend on the trampoline, the exposure of the site, and whether the soil is sand, clay, made ground or six inches of topsoil over rubble. They come from the manufacturer instructions and from the anchor kit specified for that model, and this page has no view on them whatsoever.

The reason the subject deserves any attention at all is that a trampoline is a sail. It is large, light, and presents a lot of surface, and gusts well short of a named storm are enough to move one. They travel: across a garden, through fencing, into a neighbouring property or a road. Even setting aside anything worse, that is a serious property loss for the sake of an anchor kit.

The strap angle trade

Once you know where the anchors go round the rim, the remaining question is where the stake goes relative to the frame, and that is set by the strap angle. It is straight trigonometry: the horizontal run from the attachment point to the stake is the attachment height divided by the tangent of the angle from the ground, and the strap length along the line is the height divided by the sine.

With an attachment two and a half feet up and a strap at forty-five degrees, the stake sits 2.50 feet out and the strap runs 3.54 feet. At thirty degrees the stake goes out to 4.33 feet; at sixty it comes in to 1.44. The page shows all three so the trade is visible, because there is one.

A shallow strap takes up a lot more ground, and every foot the stake moves out is another foot of the garden with something in it to catch a foot or a mower. It also directs most of the pull sideways across the stake. A steep strap keeps things tidy and close to the frame but pulls almost vertically, and straight-up pull is the direction a driven stake resists worst. There is no universally correct angle, which is exactly why the field is an input. The kit instructions specify one that suits the anchor they supply, and that is the number to use.

Sunken installs: it is an excavation, not a job

The sunken option exists on this page mostly to make one number visible early. A fourteen foot frame with a foot of working space each side means a sixteen foot circle. Three feet deep, that is 201.06 square feet times three, which is 603.19 cubic feet, or 22.34 cubic yards sitting in the ground.

Soil bulks when it is dug — 25 percent is a middling figure and clay does worse — so 22.34 in the ground becomes 27.93 loose. That is fourteen level pickup loads, or a hundred and fifty-one wheelbarrow trips, and every one of those barrows is coming up out of a hole. The question of where it all goes is not a detail to sort out later: spread across a thousand square feet of garden it would raise the level by nine inches, which is not a thing anybody actually wants.

The volume figure also assumes vertical faces, which is optimistic. Any real dig in soil that is not firm gets battered back or benched, and that increases the volume, sometimes by a lot. This is worth saying without dressing it up: excavations collapse without warning, they do it in ground that looked entirely stable, and people are killed by them every year. There is no method on this page for digging one, no batter angle and nothing about shoring, because a hole of this size is work for someone who does them for a living.

What a pit does that a lawn does not

Two things change when a trampoline goes into the ground and neither is obvious from the volume. The first is water: a pit is a hole and holes fill, so drainage is part of the design rather than something to add if it turns out to be a problem. The second is air. A trampoline above ground has air moving freely under the mat; sunk into a sealed pit it does not, and the mat behaves differently as a result. Sunken systems deal with this deliberately.

The retaining structure around the pit is a third thing entirely, and the area of face to be built — perimeter times depth — is shown in the results so it is at least on the list. How it is built, what holds the sides, and whether the trampoline is even intended to be installed below grade are questions for the manufacturer instructions. Plenty of trampolines are not, and installing one that way can be outside what it was designed and tested for.

The hazard that anchoring does not touch

It is worth being clear that anchoring a trampoline addresses wind and nothing else. Trampolines cause serious injuries — fractures, head and spinal injuries — when used exactly as intended, and the risk rises sharply with more than one person on the mat at once. No spacing, strap angle or dig volume on this page has any bearing on that, and nothing here should be read as saying any arrangement is safe.

For the rest of the ground work: the play area yard fit calculator checks the frame plus whatever clearance your instructions specify against the boundaries you actually have, the excavation volume calculator handles digs of other shapes, the topsoil calculator covers reinstatement, and the gravel calculator gives the tonnage if a drainage layer goes in.

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