Understanding your result
The headline is the rolls of pad, with the square yards, the seam tape and the cost beside it. The table gives the floor area in square feet and yards, the pad type with its density and thickness, the pad needed with its allowance, the rolls, the seam length, the tape rolls and an approximate count of staples for a wood subfloor.
Rolls is the area plus the trimming allowance divided by the roll coverage, rounded up. Pad is butted at the seams, so 5% is enough in a simple room. Seams come from laying 6 ft strips across the room: the strips number the room’s width divided by six, rounded up, and the seams are one fewer, each as long as the room. The tape joins them so the strips cannot drift apart under the carpet.
The density and thickness row is the one to read against the carpet’s specification. The note warns when the pad you chose is thicker than the 7/16 in most makers allow for residential cut pile, or when 6 lb rebond is below what stairs and many warranties ask for. For the carpet that goes on top, the carpet calculator; for the strip that holds its edges, the tack strip calculator; for stairs, where the pad is cut tread by tread, the stair carpet calculator.
How we calculate this
Strip width is the standard 6 ft pad roll. When you enter an area rather than a room, the seams are estimated for a square room of that area. The density and thickness limits follow the common carpet manufacturer warranty requirements and the Carpet and Rug Institute’s guidance on cushion. The guide to carpet padding explains density, thickness and the types; the underlayment guide covers the equivalent layer under hard floors.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Pad | 8 lb rebond, 7/16 in | The most common residential pad specified by carpet warranties |
| Maximum thickness | 7/16 in for cut pile, 3/8 in for Berber | Common carpet manufacturer warranty requirements; Carpet and Rug Institute cushion guidance |
| Roll | 6 ft wide, 270 sq ft (30 sq yd) | Typical pad roll; 180 to 270 sq ft are common |
| Allowance | 5% | Pad is butted, not lapped |
| Seams | at right angles to the carpet, or 6 in away from its seams | CRI 105 installation standard and manufacturer installation guides |
| Staples | planning figure, every few inches along edges and seams | Installer practice |
Assumptions last reviewed October 8, 2026.
The calculator does not choose between pad brands or measure comfort; it holds the warranty limits that matter for wear. It does not count the moisture barrier for a slab separately, which the underlayment calculator does, or the stair pad, which is cut to the treads. It treats the room as a rectangle and assumes 6 ft strips; some premium pads come in other widths, in which case the seam count changes slightly.
Two worked examples
A 14 × 13 ft bedroom with 8 lb rebond
270 sq ft rolls, $5.50 per sq yd, 60 ft tape rolls.
- Pad: 182 × 1.05 = 191 sq ft = 21.2 sq yd; 1 roll
- Strips: 13 ÷ 6 → 3; seams 2 × 14 = 28 ft; one roll of tape
- Cost: 21.2 × $5.50 = $116.78
One roll with about 80 sq ft over, which will pad the closet. Pad is cheap next to the carpet, and the 8 lb grade costs very little more than 6 lb for a floor that lasts noticeably longer.
640 sq ft of slab rubber for loop-pile carpet
Area typed directly, 150 sq ft rolls.
- Pad: 640 × 1.05 = 672 sq ft = 74.7 sq yd; 672 ÷ 150 = 4.5, rounded up to 5 rolls
- Strips: a 25.3 ft square-equivalent room takes 5 strips; seams 4 × 25.3 = 101 ft; two rolls of tape
Loop-pile and Berber carpets need a thin, firm pad like this one, because a soft pad lets the loops flex and break. Slab rubber is heavier and pricier than rebond but suits the carpet and the warranty.
Where to find your inputs
Room. The same area as the carpet, including closets.
Pad type. From the carpet’s specification sheet, which names the acceptable density and maximum thickness.
Roll coverage. On the roll label or the dealer’s quote: width × length in feet.
Price. Per square yard from the dealer; often quoted with installation.
Common mistakes
- The thickest pad in the store. Over 7/16 in, the carpet ripples and the seams open. Thicker is not better.
- 6 lb pad on stairs. Stairs and halls need 8 lb or firmer.
- Pad seams under carpet seams. Lay pad at right angles or 6 in away.
- Overlapping pad. It makes a ridge under the carpet. Butt and tape.
- No barrier on concrete. Slab moisture soaks into pad. Use a filmed pad or poly.
- Soft pad under Berber. Loop pile needs a firm, thin pad.
Questions people ask
- How much carpet pad for a 12 × 15 room?
- The room is 180 sq ft, or 20 sq yd. With 5% for trimming that is 189 sq ft, so one 270 sq ft roll covers it with plenty over. Pad is butted, not overlapped, so it needs far less allowance than the carpet itself.
- What density of carpet pad should I buy?
- 6 lb per cubic foot is the common minimum for residential rebond pad; 8 lb is the usual choice and what many carpet warranties and most stair and hall installations call for. Berber and loop-pile carpets need a firmer, thinner pad, often 8 lb at 3/8 inch or a slab rubber.
- How thick should carpet pad be?
- Most carpet makers specify no more than 7/16 inch for residential cut pile and 3/8 inch or less for Berber. Thicker is not better; a pad that is too deep lets the carpet backing flex, which ripples the carpet and opens the seams, and can void the warranty.
- How should carpet pad seams be laid?
- Butted tightly and taped, running at right angles to the carpet seams, or at least 6 inches away from them, so the two seams never fall on top of each other. On a wood subfloor the pad is stapled every few inches along the seams and edges; on concrete it is glued.
- Does pad come in square yards or square feet?
- Both. US dealers usually price pad per square yard, often $3 to $8 per sq yd; rolls are commonly 6 ft wide and 30 to 45 ft long, so 180 to 270 sq ft. The calculator shows both units.
- Do I need a moisture barrier pad in a basement?
- Over concrete, a pad with a bonded moisture film, or a separate 6 mil poly sheet beneath the pad, keeps slab moisture out of the pad and carpet. Over a wood subfloor above a heated space, no barrier is needed.