GMax is the number that answers the question a visual inspection can't: is this field too hard to play on? A synthetic field can look immaculate — fibers upright, lines crisp, no visible wear — and still have infill compacted hard enough that a head or hip hitting it takes an impact closer to concrete than to turf. Fibers are what you see. Impact attenuation is what actually protects the athlete, and it degrades quietly.
This guide covers what GMax measures, what score you should be holding your field to, how often to test, what it costs, and what to do when the number comes back high. It's written for the athletic directors, facilities managers, and public works departments running fields across Essex County.
GMax measures impact attenuation — the share of impact energy the surface absorbs instead of returning to whatever hit it. The test is mechanical and straightforward: a weighted missile is dropped onto the surface from a specified height, and an accelerometer inside it records peak deceleration, expressed in g's (multiples of gravity).
Drop it on a mattress and deceleration is gradual — a low number. Drop it on a sidewalk and it stops instantly — a very high number. Your field sits somewhere between, and the whole point of the program is knowing where, and watching that number drift over time.
| GMax Range | What It Means | Action |
|---|---|---|
| Under 120 | Well-cushioned, typical of a newer or recently serviced field | Maintain normal schedule |
| 120 – 165 | Acceptable range for most facilities; monitor the trend | Track against prior tests |
| 165 – 200 | Common conservative action level — investigate and remediate | Decompaction grooming, infill assessment, retest |
| Over 200 | Widely cited industry maximum exceeded | Generally considered unsafe for play; remediate before use |
The critical caveat: these are industry conventions, not a single universal regulation. The specific threshold your facility is held to can come from your league or governing body, your turf manufacturer's warranty terms, your insurer, or your own district policy — and they don't always agree. Find out which number actually applies to your field and write it into your specifications rather than assuming 200 is the only line that matters. Most well-run programs treat 165 as the trigger to act, precisely so they never have to find out what 200 feels like.
GMax almost always climbs for the same underlying reason: the infill layer compacts. Foot traffic packs the crumb rubber and sand tighter and tighter, and packed infill can't compress on impact — so the surface stops absorbing energy and starts returning it.
The encouraging part: the leading cause is also the most reversible one. Compaction responds to grooming. A field on a real decompaction schedule holds its GMax far longer than one that only gets cleaned when it looks dirty — which is the core argument for a documented maintenance program in the first place.
| Facility Type | Recommended GMax Testing |
|---|---|
| Practice-only field, light use | Annual, or bi-annual as it ages |
| Standard school athletic field | Annual, plus pre-season spot checks |
| Multi-team municipal complex | Semi-annual to quarterly |
| Tournament / event host site | Semi-annual, plus pre-event verification |
| Field over 8 years old | Increase frequency — compaction accelerates late in service life |
Also test after any major infill work (to confirm you got the result you paid for), before a primary season opens, and whenever coaches or athletes report the field "playing hard." That last trigger is underrated — the people on the field every day often notice a change before the calendar says it's time to test.
DreamFields provides GMax impact-attenuation testing, decompaction grooming, infill restoration, and written field inspection reports for schools and municipalities across New Jersey, New York, and Connecticut.
Get a QuoteProfessional GMax testing and certification for a single field generally runs $800 to $1,500, depending on field size, the number of test locations, and whether it's bundled with other inspection work. Facilities that own a Clegg hammer can spot check in-house for less, but in-house readings are a monitoring tool, not a substitute — if the number ever matters legally, it needs to be third-party and documented.
Testing is markedly cheaper per visit inside an ongoing maintenance contract than as a standalone call-out, since the crew is already on site. The same is true for bacteria testing and drainage checks — most facilities bundle all three into scheduled visits. Our guide to turf maintenance contracts for schools and municipalities covers how to structure that scope, and the turf cleaning cost guide covers the cleaning side of the budget.
Don't panic — a high score usually isn't an end-of-life verdict. It's most often compacted infill, and compaction is fixable. The standard remediation sequence:
While a field is over threshold, restricting use is the conservative call — and if you're restricting it anyway, document that decision along with the test result and the remediation plan.
GMax testing measures impact attenuation — how much shock the surface absorbs. A weighted missile is dropped from a set height and an accelerometer records peak deceleration in g's. Lower means softer and more forgiving; higher means harder. Testing follows ASTM F355 methodology, using a Clegg hammer for spot checks or a Triax-type device for certification.
200 is the widely cited industry maximum above which a field is generally considered unsafe for play, and many specifications set a conservative action level around 165. Requirements vary by league, governing body, and manufacturer warranty — confirm the threshold your facility is actually held to rather than assuming one universal number.
Annually is the common baseline for a standard school field, with high-use municipal complexes and tournament sites testing semi-annually or quarterly. Increase frequency as a field ages, and test after major infill work, before a primary season, and whenever the field is reported to be playing hard.
Roughly $800 to $1,500 per field for professional testing and certification, depending on size, test locations, and bundling. It's considerably cheaper per visit within an ongoing maintenance contract than as a standalone call-out. Request a quote here.
Usually it's compaction, not end of life. The sequence is deep decompaction grooming, an infill top-off where depth is short, then a retest to confirm the field is back in range. If the score won't come down, the shock pad or base layer may be the issue — a capital-level repair.
DreamFields handles decompaction grooming, infill restoration, and GMax retesting for school and municipal fields across New Jersey, New York, and Connecticut — with the documentation your district needs on file.
Get a QuoteSee also: Field Inspections · Bacteria Testing on Sports Fields · Turf Maintenance Contracts · How to Maintain a Turf Sports Field · Fix Flattened & Matted Turf