How deep should swing set legs be set in concrete?

The short answer

Set the legs as deep as your manual says, and measure that depth from the bottom of the surfacing, not from the lawn. Where the ground freezes in winter, keep digging: the concrete goes down to the local frost depth, which your building department publishes. The extra depth is concrete below the leg, not a leg sunk lower.

The two numbers people find disagree for a reason. Kit manuals give a hole sized to stop a light frame lifting and tipping: 5 to 20 inches in the home manuals compared here. The frost advice on the anchoring page, up to 48 inches or more in northern states, is about something else: frozen soil lifting anything shallow. Neither number is wrong. The deeper one governs, and in a cold yard that is the frost depth.

Ten installation documents, side by side

Three things stand out.

• The kit range is 5 to 20 inches, not 14 to 20. The shallowest figure belongs to a drawing used in at least three metal set manuals: a 5 inch deep, 12-5/8 inch wide pad of about 45 pounds of concrete per leg, with the leg end standing on 2 inches of brick or gravel³⁴⁵. Those manuals make concrete compulsory only on sandy soil and leave the choice to "your local contractor" everywhere else³.
• Home manuals measure from the lawn; commercial ones from the bottom of the surfacing. Swn0403 tells you to keep the concrete no higher than ground level and cover it with soil¹; ALEKO fills the hole "level with ground surface"². Willygoat prints "does not include safety surfacing depth" against every footing size it lists⁸, and SportsPlay draws its 3 ft 6 in footing underneath a 12 inch layer of surfacing⁹.
• Only one maker mentions climate. SportsPlay says its footings suit "most applications", that "varying soil & climatic conditions may require deeper and heavier footings in some areas", and to check local building codes⁹. Nobody else says anything about frost.

Where to measure the depth from

Start at the base of the protective surfacing. The CPSC public handbook asks for concrete footings to sit "below ground level and beneath the base of the protective surfacing material", both so nobody trips on them and so a child who falls does not land on the concrete¹¹. The home handbook says the same in fewer words: anchors buried or covered with surfacing¹².

That turns into a measurement in two common cases.

1. Surfacing set into a dug pad. Site prep and leveling suggests excavating the pad so the finished loose fill sits near grade. Measure the hole from the pad floor. A 20 inch ALEKO hole dug from the old lawn level before a 9 inch pad is excavated leaves 11 inches of concrete in the soil, and a concrete collar standing in the fall zone.
2. Surfacing built up on top of the lawn behind a border. Measure from the lawn, which is now the base of the surfacing. Nothing has to change in the hole, but the beam and every seat are now closer to the play surface by the depth of the fill.

That second effect is why the commercial makers measure from below the surfacing. SportsPlay says that if the surfacing will be less than 12 inches, "additional pipe should be put in concrete" to keep the frame at its proper height⁹. Willygoat tells installers to dig the nest swing footing deeper so the top rail finishes 7 feet above the play surfacing⁸. On a home set, the check that matters is the seat: the 5 inch kits ask for at least 8 inches between the seat and the ground surface³, and how high a swing seat should be sets the target at 12 to 20 inches above the compacted surfacing. Backyard surfacing gives the depth of fill to plan for.

Maker depth or frost depth: which one wins

In ground that freezes, frost depth wins. The manuals size a hole to hold the frame down against a swinging child; none of them sizes it against the ground moving. Wet soil expands as it freezes and lifts what is shallow, then leaves it slightly higher when it thaws. Winterizing a playset describes what that does to footings that stop short of the frost line: they lift and tilt, and a frame lifted unevenly is a frame out of square.

Building codes deal with this for houses. The International Residential Code puts exterior footings at least 12 inches below undisturbed ground, and protects them from frost by taking them below the frost line the local jurisdiction sets¹³. Minnesota's rule shows how far apart those lines can be inside one state: 3 1/2 feet in the southern counties and 5 feet in the northern ones, for building footings, unless a soils engineer says otherwise¹⁴.

A swing set is not a building, and no manual compared here asks for frost depth. But SportsPlay's instruction to check local codes for climate⁹ points the same way as the site's anchoring guidance: ask the building department for the frost depth, and treat it as the footing depth wherever it is deeper than your manual's figure.

The depth for your yard, worked through

On a 4 x 4 wooden leg the question changes. Burying timber in concrete traps water at the line where it leaves the footing, which is why the anchoring page prefers a pier with a galvanized post base, and why DIY playset builds need ground contact lumber for anything that touches soil. Timber rot and repair covers what happens when that line goes soft.

How wide, how much concrete, and how long to wait

Width comes from the same documents. The kits want about 12 inches (ALEKO, the 5 inch drawing) or 350 mm (Swn0403)¹²³. Jensen sets the most useful rule of thumb: at least 4 inches of concrete around the end of the pipe, in a 2 foot round hole, using two 80 pound bags per hole and topping up with dirt⁶. The 5 inch kits put about 45 pounds of concrete in each leg³. The site's anchoring page budgets two or three 60 pound bags for a typical footing.

Cure times vary more than they should. Jensen says 24 hours, then a test without a child on the seat⁶. The 5 inch kits say 6 to 7 days³. ALEKO says to wait until the concrete is fully set and cured, by the concrete maker's instructions, and Swn0403 until it is completely dry¹². When in doubt, use the longest figure, because the first hard swing pulls up on the leg that is least set.

Where this stops applying

The depths in the table are what each maker printed for its own frame. They do not transfer between products: a 5 inch pad designed for a light A-frame says nothing about a 10 foot commercial beam with six seats. Taller beams, heavier riders and more seats load the footings harder, which is what swing set weight limits works through.

Frost depth is a local number. Minnesota is one example, not a national figure, and your building department may also have rules about permits or setbacks for play equipment: permits and HOA rules covers those. ASTM F1148, the home playground standard, is a paid document and was not read for this page.

What to do next

Read the anchoring section of your manual and write down its depth and width. Ask the building department for the frost depth. Dig to the deeper of the two, from the base of the surfacing you plan to lay. Then follow assembling a playset yourself for the order of work, since most frames are assembled and levelled before the concrete goes in, and budget the fill with the mulch quantity calculator.