Grass Underlay Calculator

Artificial grass underlay, also called shockpad, is a foam layer laid beneath artificial grass turf. It cushions the surface underfoot, helps smooth out an uneven base such as an old patio or concrete slab, and, at the right thickness, provides a meaningful level of fall-height protection for play areas.

Measure your lawn's length and width in metres and multiply them to get the area, add a wastage allowance (typically 5-10%) to cover trimming and offcuts, then divide that figure by your chosen roll's width and round up to the next whole metre — that's the length of roll to order. Enter your figures into the calculator above to do this automatically.

Underlay rolls are cut to length from a fixed-width roll, and suppliers generally sell by the whole linear metre rather than part-metres. Rounding up guarantees you have enough material to fully cover the calculated area plus your wastage allowance, rather than coming up just short.

Real lawns are rarely a perfectly efficient rectangle to cut from a roll — trimming around edges, borders, drainage points and any curves all use up a bit of extra material. UK trade guidance commonly suggests adding 5-10% on top of the raw area before ordering, which the calculator applies by default (adjustable if you already have a precise cutting plan).

There's no single correct answer — all three are genuine UK shockpad roll widths, and which wastes the least material depends on your lawn's dimensions. The calculator's scenario table shows the linear metres, area covered and surplus for all three widths side by side using your own measurements, so you can pick whichever rounds up to the smallest surplus, or simply match whatever width your chosen supplier stocks.

For most domestic lawns and pet areas, 10mm foam underlay gives a soft feel underfoot and is also commonly used to smooth an uneven base such as an old patio. 20mm is specifically recommended where there's a genuine risk of falling from height, such as under play equipment, since meaningful fall-height protection under the Head Injury Criterion generally needs the thicker foam rather than the standard domestic option. Select your intended use above for a tailored recommendation.

It's optional but common — a foam underlay adds cushioning, drainage and a softer feel underfoot, and it can also disguise an uneven or damaged base that would otherwise show through the turf. Whether it's worth the extra cost depends on your budget, the condition of your existing base, and how the area will be used (a play area benefits far more than a purely decorative side return, for example).

Yes, but not in one go. Split the lawn into rectangular sections (for example, an L-shape splits into two rectangles), measure and calculate the area of each section separately, add the areas together, and use that combined total with your chosen roll width — the same splitting approach UK measuring guides recommend for the turf itself.

That's implausible for a domestic or light-commercial lawn, so the calculator shows "Check Your Measurements" rather than a fabricated result. It's almost always a unit mix-up (feet or inches entered instead of metres) or a misplaced decimal point — re-measure and double-check the unit.

Yes, the calculator will still give you a linear-metres figure, but many suppliers apply their own minimum order length regardless of how small the calculated figure is. Check the specific retailer's minimum order quantity before finalising a very small order.

Underlay (shockpad) is the foam cushioning layer that sits underneath; artificial grass turf is the green, grass-like top layer people actually see and walk on. They're bought separately, are typically sold in different roll widths (underlay commonly 1m-1.7m; turf commonly 2m or 4m), and this calculator covers underlay only — though the same area/wastage/roll-width method applies to turf if you use its own roll width instead.

Yes — underlay is an optional upgrade, not a structural requirement. Artificial grass can be laid directly onto a well-prepared, well-draining sub-base (such as compacted MOT type 1 and sharp sand). Underlay is chosen for extra comfort, drainage, or to disguise an uneven existing surface, rather than because the turf itself needs it to function.

Not necessarily for every use — thicker foam gives a softer, more bouncy feel, but on some surfaces (particularly a firm base) it can feel less stable underfoot for general walking. That's why 10mm, not 20mm, is the standard recommendation for most domestic lawns and pet areas, with 20mm reserved for genuine fall-height requirements like play equipment rather than being an automatic upgrade.

Measure and lay along your lawn's longer dimension in the same direction as the roll — this minimises the number of joins and reduces offcut waste compared with laying across the shorter dimension. The calculator's roll-width comparison table can also help you pick whichever width rounds up to the smallest surplus for your specific measurements.

This calculator is built around artificial grass shockpad — a foam product with its own specific roll widths and thickness options. Real (natural) turf does not use a foam underlay in the same way, so this tool isn't the right fit for a natural lawn project.