G-Max, explained for the person who has to answer for it.

What G-Max actually measures.

G-Max is a measure of surface hardness, or in the standards language, impact attenuation. The test is physical and simple: a technician drops an instrumented weight (the standard calls it a "missile") onto the field from a fixed height and records the deceleration when it hits. A soft surface absorbs the impact and produces a low number. A hard surface sends the impact back into whatever hit it and produces a high number.

That "whatever hit it" is the whole point. On a game field, the thing hitting the surface is an athlete, and often an athlete's head. A higher G-Max means more of the energy from a fall goes into the player instead of the field. That is why G-Max is the one number that shows up in bid specs, warranty language, insurance conversations, and lawsuits: it is the closest thing turf has to a single safety score.

The technical version of this page, with the ASTM chapter and verse, lives in our G-Max testing specification. This page is the version you can read before a board meeting.

Why a field that passed in year one can fail in year six.

A turf field does not have one G-Max score; it has a score that drifts upward over its life, and it drifts fastest where the field works hardest. Three forces do the drifting:

• Infill compaction. The rubber and sand between the fibers is the cushioning layer, and thousands of cleat impacts pack it down the way foot traffic packs a dirt path. Compacted infill absorbs less, so the number climbs.
• Infill migration. Play physically pushes infill out of high-traffic zones. Goal mouths, the area between the hashes, penalty spots, and lacrosse creases all lose depth first, which is why those zones harden years before the rest of the field.
• Freeze cycles. Even in North Texas, winter freezes stiffen the infill and the layers beneath it. A field tested in February can read meaningfully harder than the same field in May.

This is why a proper test is never one drop at midfield. Standard practice tests multiple points across the field, typically 4 to 12, deliberately including the high-wear zones. A field can average 120 and still have a goal mouth reading 175, and the goal mouth is where the goalkeeper's head hits the ground.

The drift is not hypothetical. In one North Texas district's public board records, a stadium field that measured 142 at its five-year test came back at 199 near the end of its life, one point under the ASTM ceiling, and the district replaced the turf that spring. That is the trend line doing its job: because the district was testing, the number forced the decision before an injury did.

The shock pad: the line item that moves the number most.

A shock pad is a resilient cushioning layer installed between the stone base and the turf carpet. It is the difference between a field that tests in the 140-160 range on day one and a field that tests at 90-110. It also protects the number over time: because the pad provides cushioning independent of the infill, the field is far less sensitive to the compaction and migration that push no-pad fields toward the ceiling.

On a full-size field the pad runs roughly $45,000 to $90,000, and it earns the money twice: it extends carpet life by absorbing the mechanical stress that would otherwise grind the turf backing against the stone, and it is effectively required by insurance underwriters for youth competitive play. Cutting the pad to trim a bid is how a district buys a field that starts 30 points from the ceiling with nowhere to go but up. The pad's role in total project cost is covered in our turf field cost guide.