Carpet pad thickness and density

Carpet pad thickness and density by room

Read this as a pair. The thickness is a ceiling, not a target, and the density is a floor: going thinner than the row says is usually fine, going softer than it says is not.

Density here is pounds per cubic foot, the residential convention. Fibre pads are often labelled in ounces per square yard instead — 32 oz is roughly a 7.5 lb pad, 40 oz roughly 10 lb.

What density measures, and why it outranks thickness

Density is weight per cubic foot of the pad material. An 8 lb pad has more material packed into the same space than a 6 lb pad of identical thickness, so it gives less under load and springs back further afterwards. Thickness only says how far the surface can travel; density says what happens on the way back up.

This is why a cheap half-inch pad feels wonderful in the store and flat within two winters. The carpet did not wear out — the cushion under it stopped pushing back, and every step after that ground the carpet backing against itself. Pad is the cheapest part of the floor and the part that decides how long the expensive part lasts.

The 7/16 inch warranty limit

Most residential carpet warranties name a cushion the carpet has to sit on, and the usual wording is no thicker than 7/16 in and no less dense than 6 lb. A half-inch pad, the one shoppers reach for because it feels the best underfoot, sits outside that on most cut-pile carpet sold in the US.

The reason is mechanical, not commercial. Carpet backing is held together by latex, and latex does not like being bent. On a pad that deflects too far, every footstep hinges the backing over the edge of whatever is beneath it — a seam, a tack strip, a furniture leg. Seams open, the backing delaminates, and the failure looks exactly like a carpet defect, which is precisely why the manufacturer excluded it.

Two side effects come with an over-thick pad even where nobody is checking a warranty: the tack strip loses its grip on carpet held that far above it, so the stretch relaxes and ripples appear, and the finished floor sits high enough that interior doors start catching. Check the warranty sheet for your carpet before ordering pad — the limit is per carpet line, and berber and commercial lines are usually stricter than the table above.

Carpet pad types, density and cost

Prices are the pad alone, per square foot, before installation. Pad is usually 10–20% of what the carpet costs, which is why upgrading it is the cheapest decision on the whole floor.

Moisture-barrier versions of rebond and memory foam carry a film on the top face and cost roughly $0.15 more per square foot. They earn it in a house with pets or small children, and on a slab where damp can come up from below. Felt made from natural fibre should not go on a slab at all — it holds what it absorbs.

Stairs need a different pad from the room they lead to

A stair tread bends the pad through ninety degrees at the nosing, and the pad is under the ball of a foot at the exact point of that bend. Soft, thick cushion compresses there first, the carpet above it loses support, and the nosing shows wear within a couple of years — the one place on a staircase everyone looks at.

Use 3/8 in at 8 lb or firmer, or a fibre pad at 1/4 in. Buy pad for stairs separately from the pad for the floors: the roll is the same width, but the specification is not, and a single roll bought for the bedrooms is the usual reason a staircase is cushioned wrong.

How much pad to buy

Pad comes in rolls, normally 6 ft wide and 45 ft long — around 270 sq ft, or 30 sq yd. It is laid seam to seam with the seams taped, and unlike the carpet above it the seams do not have to avoid anything, so pad wastes far less than the carpet does. Order it against the room area plus a small allowance, not against the carpet order.

The carpet calculator lays the broadloom out against the roll width, works out the seams and the offcut you pay for but never walk on, and counts the pad, tack strip and tape with it. For pad on its own — under laminate, vinyl or engineered board as well as carpet — use the underlayment calculator.