Three problems, not one
People draw a swing set as a single object and then discover on the day that the three parts of it are governed by different numbers. The beam is a division problem: whatever clear length you have between the frames, minus the gaps you want at each end, divided by one fewer than the number of seats. The chains are a subtraction: start at the top of the beam, take off the beam depth, take off however far the hardware drops the pin below the underside, take off the seat height you want and the thickness of the seat and its shackles, and whatever is left is chain. The frames are a right triangle: the leg angle you pick fixes both how long the legs have to be and how far their feet land from the centreline.
Work the defaults through. Ten feet of clear beam with two feet of gap at each end leaves six feet — 72 inches — and with two seats there is exactly one space, so the hanger pairs land 72 inches apart. The beam top at eight feet is 96 inches; a nominal 4x8 is about 7.25 inches deep, so the underside is at 88.75; three inches of hardware drop puts the pin at 85.75. Ask for an 18 inch seat with two inches of seat and shackle and the chain comes out at 65.75 inches, which is 5 feet 5-3/4. At twenty degrees the legs run 96 divided by the cosine of twenty, which is 102.16 inches or 8.51 feet, and each frame plants its feet 5.82 feet apart.
The number that changes without you touching anything
Chain length is measured from the finished surface, and on a loose-fill play area the finished surface is not a fixed thing. Wood fibre, chips, sand and pea gravel all compact under their own weight over the first weeks and then keep compacting under use. Worse, the material gets scuffed and kicked out of exactly the spot under a swing seat, which is the highest-traffic square yard on the whole structure. The result is a seat that was set at eighteen inches over fresh fill sitting over a scooped hollow a few months later, with the seat height measured from the bottom of that hollow being noticeably more.
Nothing in the hardware has moved. This is a surfacing maintenance problem that presents as a geometry problem, and the fix is raking the material back and topping it up rather than shortening the chains. If you find yourself shortening chains to compensate, that is a signal about the surfacing, not about the swing. The playground surfacing calculator handles the top-up volume, and it is equally blunt about not supplying a depth.
Why there is no seat spacing default here
The obvious missing field on this page is the distance between seats, and it is missing on purpose. That figure is a safety dimension: it exists so that two children swinging at once do not collide with each other or with the frame, and it is published by the people who designed the equipment and by the national playground safety guidance. It is not a number a calculator can derive from a beam length, and putting a plausible-looking default in the field would be inventing one.
So the page works the other way round. You give it the beam you have and the number of seats you want, and it tells you what spacing that arrangement produces. Then you take that number to the instructions and check it. If it is short, the answer is a longer beam or fewer seats, and it is much cheaper to find that out before the timber is cut.
The same logic applies to the end gaps. The field is labelled as your layout choice because that is honestly what it is here; what clearance a swinging seat actually needs to a frame leg is in the instructions for that equipment.
The A-frame triangle
Leg angle is the one genuinely free choice on the frame, and it trades two things off. A steeper leg — a smaller angle from vertical — uses less timber and takes up less ground, and a shallower leg plants the feet wider. Cosine gives you the length, tangent gives you the spread, and both scale straight off the beam height, so raising the beam a foot widens the base as well as lengthening the legs.
The leg length reported here runs to the top of the beam and is measured along the leg. It is a stock length before any of the real work: the top of each leg has to be cut to the angle and notched or bracketed to meet its partner and the beam, and the bottom has to be cut to the same angle if the feet are to sit flat rather than on an edge. Add for those cuts rather than ordering to the number.
What the leg length is not is a size. Nothing here says how thick the legs should be, what fixes them to the beam, or what the whole assembly carries. A swing is a dynamic load — it is not the weight of a child but the weight of a child changing direction — and that is the sort of question that belongs to an engineer and to the manufacturer of the hardware. The header and beam load calculator works out tributary widths and reactions for the span tables, and it stops in the same place.
What sits under and around it
The ground footprint reported at the bottom of the results is the structure and nothing else. It is not the use zone, and the difference is enormous for swings specifically: a child leaves a swing seat travelling forward, so the zone in front of and behind a swing runs far beyond anything the frame occupies. The swing arc sweep calculator shows where the seat itself travels, which is a floor on that distance and nowhere near the whole of it. The actual zone comes from the equipment instructions and the guidance.
For the legs themselves, the concrete footing calculator and the fence post calculator cover the holes and the concrete, and the deck post height calculator is the same family of arithmetic if you are setting the frame off a deck.
Questions people ask
How long should the chains on a swing be?
Chain length is what is left over after four subtractions, not a figure you look up. Start at the top of the beam above the finished surface, subtract the beam depth to get the underside, subtract however far your hardware drops the pin below that, then subtract the seat height you want plus the thickness of the seat and its shackles. An eight foot beam of nominal 4x8 with a three inch hardware drop and an eighteen inch seat leaves about 65-3/4 inches of chain.
How far apart should swing seats be on one beam?
This page will not tell you, and neither should any calculator. Seat spacing exists so that two swings in use do not collide with each other or with the frame, and the figure comes from the equipment manufacturer instructions and the current national playground safety guidance. What the page does instead is take your beam length and seat count and report the spacing that arrangement produces, so you can check that number against the instructions before anything is cut.
Why does the beam height have to be measured above the surfacing?
Because the surfacing is what the child lands on and what the seat height is measured from. Loose fill also raises the ground level by its installed depth, so a beam set eight feet above bare soil is less than eight feet above the surfacing once that surfacing is in. Install the surfacing first, or at minimum set the beam height from where the finished surface will be rather than from where the soil is now.
How do I work out the A-frame leg length and spread?
Both come off the beam height and the angle you choose from vertical. Leg length is beam height divided by the cosine of the angle; foot spread across the frame is twice the beam height times the tangent. A ninety-six inch beam at twenty degrees gives legs of 102.2 inches and feet 69.9 inches apart. Add material for the top cut, the notch or bracket, and the bottom cut before ordering stock.
Can I hang a swing from a tree branch using these numbers?
No. Nothing on this page applies to a tree. Whether a limb, its union with the trunk, and the root system can take a swinging load is a question about the health and structure of one specific living tree, and it changes with decay, drought, storm damage and the season. It belongs to an arborist and an engineer looking at that tree, and a limb that fails does so without warning. The geometry here assumes a beam you built and can inspect.