Playground surfacing depth calculator

Work out the depth

Calculator

Loose fill depth and volume

Fall height (feet) Highest designated play surface to the surfacing below.

Loose fill material Unitary systems are specified by the manufacturer instead.

Use zone area (square feet) Total surfaced area, including every use zone.

Settling allowance Loose fill compacts as soon as it is used.

Tested critical height at that depth
Finished volume
Order with settling allowance

How the depth figure is produced

A surfacing material is dropped on in a lab: an instrumented headform falls onto a sample at increasing heights until it exceeds either 200 G peak deceleration or a Head Injury Criterion of 1000². The greatest height at which it stays under both limits is the critical height, and that number has to be equal to or greater than the fall height of the equipment it sits under.

The handbook's table gives tested critical heights for common loose fill at 6, 9 and 12 inches uncompressed, plus a 9 inch compressed column¹. Uncompressed is how the material behaves when it has just been delivered and raked. Compressed is how it behaves after a season of use, which is the condition most playgrounds are actually in. The gap between the two columns is the reason a site that passed on opening day fails two years later.

What the calculator does not know

Your base. A hard, flat, impermeable base under loose fill reduces performance at the bottom of the layer. Drainage and base design are covered in drainage, base and containment.
Temperature and moisture. Frozen or saturated loose fill does not attenuate the way the test sample did.
Wear patterns. Depth is not uniform. The spots under swing seats and at slide exits lose material first, and those are exactly where falls happen.
Accessibility. Depth and impact performance are one requirement, firmness and stability under ASTM F1951 are another. See accessible routes and surfacing.

Ordering and installing

Loose fill is sold by the cubic yard, delivered. The calculator gives you the finished volume and the volume to order after settling. Two more things to check before the truck arrives:

Step	What to confirm
Excavation	The area is dug to depth so the finished surface is flush with the surrounding grade, not mounded above it
Containment	Borders hold the material in without creating a trip edge or an obstruction inside the use zone
Drainage	Water leaves the area, through a free-draining base or a perimeter drain
Depth marking	A visible depth line on a post or a gauge stick, so the maintenance crew knows the target

The four things that decide whether the depth you paid for survives its first year.

Then put the top-up in the operating budget, not the capital budget. Loose fill is a consumable. The surfacing upkeep guide sets out the raking and top-up cycle, and the cost calculator turns it into an annual number.

What to do next

If the depth this returns is impractical, the answer is usually to lower the equipment rather than to deepen the surfacing: fall height is the input you control. If you are comparing materials rather than confirming one, the surfacing cost comparison works through ten year ownership cost, and the surfacing section covers each material in detail.

Frequently asked questions

Is 6 inches of mulch enough for a 7 foot fall height?

Not for every material. In the CPSC tested figures, 6 inches of engineered wood fiber is rated to about 6 feet, so a 7 foot fall height needs 9 inches. Six inches of wood mulch tested to about 7 feet, which leaves no margin once it compacts. Use the calculator above, then confirm against the supplier's own test report.

Why do I have to order more material than the finished depth?

Loose fill settles as soon as it is walked on. A 9 inch finished depth typically needs roughly 25 percent more material delivered than the raw volume suggests, and more if the base is uneven. Ordering short is the most common way a new installation ends up under depth on opening day.

Does this calculator work for poured rubber or tiles?

No. Unitary surfaces are specified by tested system thickness from the manufacturer, not by a generic depth table, because performance depends on the exact formulation and base. Ask for the ASTM F1292 report for the exact system and fall height you are buying. The poured-in-place guide explains what to ask for.

What depth do I have to maintain, not just install?

The installed depth is the minimum working depth, not a target to decay from. Loose fill scatters fastest under swing seats and at slide exits, sometimes losing several inches in a season. Mark the required depth on a post or a gauge stick and rake and top up whenever it drops below the line.

Surfacing depth calculator