Gmax testing is the standard way to measure how hard an athletic field is, and therefore how much impact a player absorbs when they hit the ground. The number is simple to read and easy to misunderstand. A new synthetic field may test in the low 100s and drift above 200 within a few years as infill compacts and migrates, and a grass field can swing from soft to dangerous within a season depending on moisture. Keeping a field in the safe range is as much about how it was built as how it is maintained. The crews we work with treat Gmax as a design target from the first lift of base material, which is what safe turf field installation really means.
What Gmax testing measures
A Gmax test drops a weighted missile fitted with an accelerometer from a fixed height and records the peak deceleration as it stops. A hard surface stops it almost instantly, producing a high reading; a soft surface stops it gradually. Two test methods are in common use.
| Method | Standard | Missile | Typical use |
|---|---|---|---|
| Gmax (F355 Procedure A) | ASTM F355 / F1936 | 20 lb flat-faced missile, 24 inch drop | Synthetic turf field certification and annual testing |
| Clegg Impact Tester | ASTM F1702 | 2.25 kg hammer, 18 inch drop | Natural grass and some professional synthetic field programs |
The two methods use different weights and heights, so their numbers are not interchangeable. When a Gmax value is quoted, ask which method produced it.
A standard F1936 test measures ten points in a defined pattern covering high-wear areas such as midfield, goal mouths and hash marks. Each point is dropped three times and the average of the second and third drops is reported.
The 165 and 200 Gmax thresholds
Two numbers show up in every specification.
- 200 Gmax. The maximum allowed at any single test point under ASTM F1936, the impact attenuation standard for synthetic turf systems. A point above 200 is a failure and the field should be remediated before play.
- 165 Gmax. The Synthetic Turf Council and many owners use 165 as a conservative working limit. A field at 195 is technically passing but one dry summer from failing, so many districts write 165 into specifications and warranties.
Lower is not automatically better; a very soft surface is tiring and affects ball response. Most well-built synthetic fields with a shock pad test between about 90 and 130 when new. Natural grass varies far more, from the 60s on a wet spring field to well over 150 on a hard, dry one in late summer.
Pro tip: ask for the installation Gmax report and the test locations before accepting a new field. That baseline is what every future test is compared against, and it should be part of the warranty file.
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Why Gmax rises over time
A synthetic field is a layered system: stone base, optional shock pad, turf carpet and infill. The infill does most of the cushioning on fields without a pad. Over time, several things make the surface harder.
- Infill compaction. Rubber and sand settle under traffic and rain and absorb less energy. High-traffic zones compact first, which is why goal mouths and midfield usually test highest.
- Infill migration. Cleats, grooming and water move infill out of heavy-use areas. A spot that has lost half an inch of infill is testing on the base rather than the cushion.
- Pad breakdown. Some shock pads lose resilience over many years, especially foam products exposed to heat and water.
- Base problems. An under-compacted base keeps settling, creating scattered high readings rather than a uniform rise.
- Cold. Rubber infill stiffens below about 40°F and Gmax climbs, which is why testing is done above a minimum temperature.
Natural grass fields harden from soil compaction, low moisture and thin cover. Aeration and irrigation bring them back down, but a drought or a heavy tournament weekend can undo a season of work.
Building and maintaining a field to stay under the limit
- Design the base for uniformityA stone base compacted in lifts to a consistent density, with grades checked by laser, gives the same support everywhere.
- Specify a shock padA pad takes the cushioning burden off the infill, so the field stays in range even as infill compacts.
- Install infill to the specified depthInfill goes in over multiple passes and is checked with a depth gauge. Short-filling is the most common construction cause of early high readings.
- Test at completionAn independent Gmax test at the ten standard points before acceptance confirms the installation and sets the baseline.
- Groom and decompact regularlyBrushing redistributes infill; periodic decompaction with a tine implement loosens settled infill, especially in heavy-use zones.
- Top up infill where depth dropsLow infill is the leading cause of a failed annual test.
- Test annuallyMany warranties and school policies call for yearly testing before the fall season. Points approaching 165 get targeted remediation before they reach 200.
These principles apply to every full-size field layout. Whether you are planning to the standard soccer field dimensions or working out how many acres a football field needs for a new site, the base design and infill specification are what determine the Gmax numbers you will live with for the next decade.
What to ask a field builder about Gmax
Passing on day one is the minimum; the useful questions are about year five. Ask what Gmax the builder targets and whether they include a shock pad, how infill depth is verified and documented, what the warranty says about Gmax, and who performs the testing. Experienced crews answer without hesitation. For a field designed to stay safe and playable season after season, get matched with an installer who builds to a Gmax target rather than just to a finished look.
Gmax testing FAQ
What is a good Gmax score for a synthetic turf field?
New synthetic fields with a shock pad commonly test between about 90 and 130. The working guideline is to keep every point under 165, and 200 is the ASTM F1936 ceiling.
How often should Gmax testing be done?
At installation and then at least annually, typically before the main playing season. Fields with high use or readings approaching 165 are often tested more frequently.
What happens if a field fails a Gmax test?
Points above 200 should be remediated before play, usually by decompacting and adding infill at the failing locations, then retesting. Persistent failures may indicate base or pad problems.
Is Gmax testing used on natural grass fields?
Yes, often with a Clegg Impact Tester rather than the F355 missile. Grass hardness varies with moisture and compaction, so readings change through the season.
The complete turf installation guide
What a quality install looks like, the eight installation steps, spec sheets, and how to choose an installer.