HIC, Critical Fall Height and Impact Attenuation Explained for Playgrounds
Critical fall height is the highest drop a surface can absorb while keeping the Head Injury Criterion under 1,000. Here is how AS 4422 testing works and how to use the numbers when specifying surfacing.
Critical fall height is the highest drop from which a playground surface can protect a child's head, defined in AS 4422 as the height at which the Head Injury Criterion (HIC) reaches 1,000. Every piece of equipment has a free height of fall, and the surfacing beneath it must have a critical fall height equal to or greater than that number. Reviewed September 2026.
The three terms, in plain English
Free height of fall is a property of the equipment: the vertical distance from the highest point a child is expected to stand, sit or hang to the surface below. AS 4685 caps it at 3 metres for public playground equipment.
Critical fall height is a property of the surface: the highest drop it can absorb while keeping the impact under the injury threshold. It comes from a test certificate, not from a brochure.
Impact attenuation is the surface's ability to absorb energy. HIC and Gmax are the two ways it is measured.
Match them like this: the surfacing's critical fall height must be at least the equipment's free height of fall.
What HIC actually measures
The Head Injury Criterion is a single number derived from how hard and how long a head-form decelerates when it strikes the surface. A test rig drops an instrumented metal head-form onto the surface from increasing heights and records the deceleration curve. HIC weighs both the peak force and the time it acts, because a sharp short spike and a longer softer push do different damage.
- HIC under 1,000: the internationally accepted threshold below which a life-threatening head injury is unlikely.
- Gmax: the peak deceleration in multiples of gravity. Some standards also cap Gmax at 200 g. AS 4422 uses HIC as the governing value.
The critical fall height reported on a certificate is the drop height at which HIC crosses 1,000.
How AS 4422 testing works
- Samples of the surfacing, at the installed depth or thickness, are conditioned to specified temperatures.
- The head-form is dropped from a series of heights and HIC is recorded at each.
- The critical fall height is interpolated from the results.
- The certificate states the depth or thickness tested and the critical fall height achieved.
The depth or thickness on the certificate is the number your installer must achieve on site. Thinner rubber or shallower mulch than the tested sample is not compliant, regardless of the certificate.
Why surfaces fail in the field
Test certificates describe new material in a laboratory. On site:
- Loose fill compacts, migrates and thins under swings and slide exits, so its real critical fall height drops month by month.
- Wet-pour rubber hardens with UV exposure and age, raising HIC.
- Rubber tiles lift at joins and expose hard edges.
- Synthetic turf shock pads can be substituted with thinner pads during installation.
This is why AS/NZS 4486.1 recommends annual comprehensive inspections, and why on-site impact testing is worth budgeting for on ageing surfaces.
Matching surfacing to fall height
| Surface | How depth relates to fall height | Indicative cost per m² |
|---|---|---|
| Certified mulch or sand | 300 mm plus, deeper for higher equipment | About $60 |
| Synthetic turf on shock pad | Pad thickness set by fall height | About $110 |
| Wet-pour rubber | Build-up thickness set by fall height | About $160 |
Rates are Kidzspace indicative figures, supplied and installed, ex GST. Ask your supplier for the AS 4422 certificate for the exact depth they are quoting.
Frequently asked questions
What HIC score is acceptable for a playground?
Below 1,000. Critical fall height is defined as the drop at which HIC reaches 1,000.
What is the maximum fall height for playground equipment in Australia?
AS 4685 limits the free height of fall to 3 metres for public playground equipment.
Does grass count as impact-attenuating surfacing?
No. Natural turf and compacted soil are not certified surfaces and are only acceptable where equipment has no significant fall height.
How often should surfacing be impact tested?
Test when installed if the depth or product differs from the certificate, and consider re-testing wet-pour and tiles after 5 to 8 years or whenever an inspection finds hardening or compaction.
Planning a playground?
Tell us about your site and we'll design something your community will love.
