Carpet calculator guide
The calculator above turns a room's dimensions into the numbers a carpet shopper actually needs — the running length of broadloom to buy, the square yards or square metres it works out to, and how many seams the job will have — plus carpet tiles, stair runner, pad and tack strip, for one room or a whole house, in metric or imperial, entirely in your browser.
Unlike a plain area calculator, it lays your carpet out in real full-width strips from a fixed roll, tries running them both ways, and recommends the direction with the shorter running length and fewer seams. The guide below explains roll width and seams, pattern repeat and waste, carpet tiles and stairs, with worked examples you can reproduce by hand.
How much carpet do I need?
Broadloom carpet is sold off a roll of fixed width — most often 12 ft (3.66 m) or 4 m — by the running length you cut from it, then billed by area in square yards or square metres. So the real question is not just "how many square metres is my floor" but "how long a piece of this roll-width carpet covers my room, and how many seams that takes".
Because a strip is always the full roll width and the pile all has to run the same way, you can't rotate alternate strips to save material — you buy whole strips. A room a little wider than the roll needs a second full strip and a seam, which is why an honest carpet estimate is often larger than a bare area figure. The calculator shows that extra so nothing surprises you at the counter.
In short: strips = ⌈ room dimension ÷ roll width ⌉, running length = strips × the room's other dimension, and carpet area = running length × roll width. The tool does every step as you type, in both run directions, and adds a waste buffer before rounding up to a buyable length.
Roll width, run direction and seams
Carpet comes on a roll of one fixed width. To cover a room wider than the roll you butt full-width strips side by side and join them with a seam. The number of strips — and therefore seams — depends on which way you run the carpet: across the width or along the length. Running it one way might fit in a single seamless strip; the other way might force a seam and a lot of offcut.
The calculator computes both orientations and recommends the one with the shorter running length (the cheaper buy), breaking ties toward fewer seams. You can override it to place a seam where it's least visible — away from the main window or out of the traffic path. Every strip's pile runs the same direction, shown by the arrows on the diagram, because mismatched pile catches the light and looks like two different carpets.
The broadloom numbers in the results come from these formulas.
Strips and seams
strips = ⌈ run-across ÷ roll width ⌉ · seams = strips − 1- Run-across is the room dimension the strips sit side by side across; the calculator tries both the width and the length.
- The pile all runs one way, so strips are never rotated — you always buy whole strips, which is why an extra strip means an extra seam.
Running length
running length = strips × strip length- Strip length is the room's other dimension, lengthened by any pattern repeat.
- The project total adds a waste buffer and rounds up to the cut-to-order purchase increment (1 ft / 0.1 m).
Carpet area purchased
carpet area = running length × roll width- This is what you're billed for. Divide ft² by 9 for square yards, or multiply m² by the roll width in metres.
- Obstructions lower the used area (and so the realised waste), but never the strips you buy.
| Roll width | Metric | When it applies |
|---|---|---|
| 12 ft | 3.66 m | The most common US broadloom width and the default here — a good fit for rooms up to 12 ft one way. |
| 13 ft 2 in | 4.0 m | The standard metric / European roll, usually sold as "4 m" — the default in metric mode. |
| 15 ft | 4.57 m | A wider US roll — fewer seams in large rooms, but more offcut in small ones. |
| 16 ft 5 in | 5.0 m | A wide metric roll for big, open-plan rooms where a 4 m roll would need a seam. |
The same top-down diagram the tool draws for your room, here for a fixed 5 m × 4 m room on a 4 m roll. Running the carpet one way fits in a single seamless strip; the other way forces a second strip and a seam. The arrows show the pile — all one direction.
Lengthwise — 2 strips, 1 seam
Widthwise — 1 strip, no seam
Pattern repeat and waste allowance
A patterned carpet has to line up across every seam, so each strip is cut to a whole number of pattern repeats — and a drop match adds half a repeat per strip. That lengthens every strip and is the carpet equivalent of pattern waste on wallpaper. Set the repeat and match and the calculator lengthens the strips for you; leave it plain and strips are cut to the bare room dimension.
On top of that, a fit/handling allowance covers trimming, doorways and the odd mistake. The calculator sets it from the room shape — a simple rectangle needs the least, an L-shape or a complex, many-cut room needs more — and you can override it or add a +5% nudge for an awkward room. The table below shows the default ladder.
| Room shape | Default waste | When it applies |
|---|---|---|
| Rectangular | 10% | A simple four-wall room — the lowest allowance. |
| L-shape | 15% | Model it as two rectangles; extra offcut where the strips meet the return. |
| Complex | 20% | Bays, angles or lots of doorways and cuts. |
| Complex-room bump | +5% | An optional nudge added on top of any shape for an especially awkward room. |
Carpet tiles: a different sum
Carpet tiles (modular squares, usually 50 cm or 24 in) skip the roll/seam model entirely. They're bought by the box, so the sum is the same as hard flooring: net floor area, plus a lay-pattern allowance, divided by the coverage per box, rounded up. There are no seams to plan and off-cut tiles at the wall often start the next row, so waste is low.
The lay pattern sets that allowance: laying every tile the same way (monolithic) wastes the least; quarter-turning alternate tiles or an ashlar brick-bond offset wastes a little more. Switch the calculator to Tiles mode, set the box coverage off the packaging, and it returns the boxes and tiles to buy.
Carpet tiles use the coverage / box model instead.
Boxes of carpet tiles
boxes = ⌈ net area × (1 + lay waste) ÷ box coverage ⌉- Net area is width × length minus obstructions; lay waste is 5% monolithic, 8% quarter-turn, 10% ashlar.
- Boxes round up, and off-cut tiles at the wall often start the next row, so realised waste is usually low.
The tile lattice the tool draws, here in a fixed 4 m × 4 m room with 50 cm tiles. Laying every tile the same way wastes the least; quarter-turn and ashlar add a little more.
Monolithic — 5% waste
Quarter-turn — 8% waste
Ashlar — 10% waste
Stairs, pad and tack strip
A staircase is measured as a runner, not an area: allow a length of carpet per step for the tread, riser and a tuck, add the depth of any landings, and apply a generous waste because stairs are cut individually. The default is 20 in / 0.508 m per step for a waterfall install. Switch to Stairs mode, enter the step count, per-step allowance and stair width, and add landings as needed.
For any mode you can also estimate the consumables. Separate pad (underlay) is bought by the roll from the floor area — unless your carpet has an attached cushion, in which case you buy none. Tackless strip (the gripper that holds the carpet at the walls) is estimated from the room perimeter, less the doorways where carpet isn't gripped.
Stairs, pad and tack strip use these formulas.
Stair runner length
running length = steps × per-step allowance + Σ landing depths- Default 20 in / 0.508 m per step covers the tread, riser and a tuck for a waterfall install.
- A 15% waste allowance is applied because stair pieces are cut individually.
Pad / underlay rolls
pad rolls = ⌈ net area ÷ roll coverage ⌉ (0 if the cushion is attached)- Estimated from the floor area and the roll's coverage; confirm an attached cushion and the count drops to zero.
Tack (tackless) strip
pieces = ⌈ (perimeter − doorways × doorway width) ÷ stock length ⌉- Perimeter is 2 × (width + length); doorways are subtracted because carpet isn't gripped across a threshold.
- Strip stock is 4 ft / 1.22 m by default — match it to what you're buying.
Waterfall stair runner
Worked examples
Each example reproduces what the calculator returns, so you can check the method by hand. Broadloom rounds up to a buyable running length after a waste buffer.
Each example reproduces the calculator's output for the given inputs.
A 12 × 15 ft bedroom — which way to run it
Imperial · broadloom- Given
- Room 12 ft × 15 ft
- Roll width 12 ft, plain carpet
- Rectangular room (10% waste)
- Formula
strips = ⌈ run-across ÷ roll width ⌉; running length = strips × room length- Steps
- Lengthwise: strips = ⌈12 ÷ 12⌉ = 1, no seam, running length = 1 × 15 = 15 ft
- Widthwise: strips = ⌈15 ÷ 12⌉ = 2, one seam, running length = 2 × 12 = 24 ft
- Cheaper run = lengthwise (15 ft, seamless)
- Buy: 15 × 1.10 = 16.5 → round up to 17 ft; area = 17 × 12 = 204 ft² ≈ 22.7 sq yd
- Result
- Run it lengthwise: one seamless strip, 17 ft of 12-ft carpet (≈ 22.7 sq yd). Widthwise would force a seam and 24 ft.
The same room in a patterned carpet
Imperial · drop match- Given
- Room 12 ft × 15 ft, roll width 12 ft, run lengthwise
- Pattern repeat 18 in, drop match
- Rectangular room (10% waste)
- Formula
strip length = (⌈ run ÷ repeat ⌉ + ½) × repeat (drop match)- Steps
- Run = 15 ft = 180 in; whole repeats = ⌈180 ÷ 18⌉ = 10
- Drop match adds half a repeat: (10 + 0.5) × 18 = 189 in = 15.75 ft per strip
- 1 strip → 15.75 ft; × 1.10 = 17.33 → round up to 18 ft; area = 18 × 12 = 216 ft² = 24 sq yd
- Result
- The drop-match repeat adds ¾ ft to the strip — buy 18 ft (24 sq yd) instead of 17 ft for plain.
Two rooms on one 4 m roll
Metric · multi-room- Given
- Room A 5 × 4 m, Room B 3 × 4 m
- Roll width 4 m, plain carpet
- Rectangular rooms (10% waste)
- Formula
each room seamed separately; running lengths summed (never sum area then seam)- Steps
- Room A best run = widthwise: 1 strip × 5 m = 5 m
- Room B best run = widthwise: 1 strip × 3 m = 3 m
- Project running length = 5 + 3 = 8 m; × 1.10 = 8.8 m (already on the 0.1 m increment)
- Carpet area = 8.8 × 4 = 35.2 m²; used = 20 + 12 = 32 m²; realised waste ≈ 10%
- Result
- Buy 8.8 m of 4 m carpet (35.2 m²) — each room is seamed on its own because a strip can't span two rooms.
A 4 × 5 m room in carpet tiles
Metric · tiles- Given
- Room 4 m × 5 m = 20 m²
- 50 cm tiles, box covers 3 m²
- Ashlar lay pattern (10% waste)
- Formula
boxes = ⌈ net area × (1 + lay waste) ÷ box coverage ⌉- Steps
- Area with waste: 20 × 1.10 = 22 m²
- Boxes: 22 ÷ 3 = 7.33 → 8 boxes
- Coverage bought: 8 × 3 = 24 m² → about 4 m² spare
- Result
- Buy 8 boxes (24 m²) of carpet tiles — no seams to plan, spare tiles left for repairs.
A 13-step staircase runner
Imperial · stairs- Given
- 13 steps, waterfall install
- 20 in per step, stair width 3 ft
- No landings (15% waste)
- Formula
running length = steps × per-step allowance + Σ landing depths- Steps
- Runner length = 13 × 20 in = 260 in = 21.67 ft
- × 1.15 waste = 24.92 → round up to 25 ft
- Area = 25 × 3 = 75 ft² ≈ 8.3 sq yd
- Result
- A 13-step flight needs about 25 ft of runner (≈ 8.3 sq yd) at 20 in per step.
Common mistakes to avoid
Ordering by area alone. Carpet is cut off a fixed-width roll in whole strips, so a floor's square metres isn't the whole story — a room a little wider than the roll needs a second strip and a seam, and that's more carpet than the bare area suggests.
Ignoring the run direction. Running the carpet the wrong way can double the seams and the offcut. Let the calculator compare both ways, then override only to hide a seam where it won't be seen.
Confusing square yards and square feet. A square yard is nine square feet, not three — mixing them up over-buys or under-buys by a factor of nine. Enter the price in the same unit the calculator is showing.
Forgetting the pattern repeat. A patterned carpet is cut to whole repeats, and a drop match adds half a repeat per strip — skip it and every seam will be short.
Double-buying pad. Cushion-back carpet has its pad attached; adding a separate underlay wastes money and feels spongy. Confirm the attached-cushion toggle.
Frequently asked questions
Short answers to the questions people most often ask about estimating broadloom, tiles, stairs, pad and tack strip with this calculator.
How much carpet do I need?
Work out how many full-width strips cover the room (room dimension ÷ roll width, rounded up), multiply by the room's other dimension for the running length, then multiply by the roll width for the area. For a 12 × 15 ft room on a 12 ft roll run lengthwise, that's one strip 15 ft long — about 17 ft (22.7 sq yd) once a 10% allowance is added. The calculator does this both ways as you type and recommends the cheaper run.
What is a carpet seam and how many will I have?
A seam is where two full-width strips of broadloom are joined to cover a room wider than the roll. The number of seams is the number of strips minus one, so a room that fits in a single strip has none. Running the carpet the other way, or choosing a wider roll, can remove a seam — the calculator shows the seam count for both run directions.
Which way should the carpet run?
Two things matter: cost and appearance. The calculator recommends the direction with the shorter running length and fewer seams, which is usually cheapest. But the pile should run toward the main light or the room's entrance so it looks even, and any unavoidable seam should sit away from the window and the traffic path — override the run direction to honour that.
What's the difference between square yards and square feet of carpet?
One square yard is nine square feet, because a yard is three feet and area goes up with the square. US carpet is often priced per square yard, so a price that looks low per yard is nine times the per-square-foot figure. The calculator shows the area in square yards, square feet or square metres and keeps the price in the same unit so the ×9 trap can't bite.
What widths does broadloom carpet come in?
The two common widths are 12 ft (3.66 m) in the US and 4 m (about 13 ft 2 in) in metric markets, with 15 ft (4.57 m) and 5 m rolls also sold for larger rooms. A wider roll can cover a room in one seamless strip but leaves more offcut in a small room. Set the roll width to what your supplier stocks — it drives the whole seam plan.
How does a pattern repeat change how much carpet I need?
A patterned carpet must line up across every seam, so each strip is cut to a whole number of pattern repeats — and a drop match adds half a repeat per strip. That lengthens every strip, much like pattern waste on wallpaper. Enter the repeat and match type and the calculator lengthens the strips automatically; leave it plain and strips are cut to the bare room length.
How many carpet tiles do I need?
Carpet tiles are bought by the box like hard flooring: net floor area × (1 + a small lay-pattern allowance) ÷ the coverage per box, rounded up. There are no seams to plan and off-cut tiles at the wall often start the next row. Switch to Tiles mode, set the box coverage off the packaging and pick the lay pattern, and the calculator returns the boxes and tiles to buy.
How much carpet do I need for stairs?
Stairs are measured as a runner, not an area: allow a length per step — about 20 in / 0.508 m for a waterfall install covering the tread, riser and tuck — times the number of steps, plus the depth of any landings, with a generous waste because each piece is cut on its own. Switch to Stairs mode and enter the step count, per-step allowance and stair width.
How much carpet pad (underlay) do I need?
Separate pad is bought by the roll: divide the floor area by the roll's coverage and round up. Set the roll coverage to match what you're buying. If your carpet has an attached cushion you don't need any separate pad — confirm the attached-cushion toggle and the count drops to zero.
Do I need pad if my carpet has an attached cushion?
No — cushion-back carpet has its pad bonded to the back, so a second underlay wastes money and can feel spongy or void the warranty. Confirm the attached-cushion toggle and the calculator sets the pad roll count to zero. Whether a carpet has an attached cushion is printed on the product, not decided by its style.
How much tack strip do I need?
Tackless strip runs around the room's perimeter to grip the carpet, except across doorways. The calculator estimates it as the perimeter (2 × width + 2 × length) minus the doorway widths, divided by the strip stock length (4 ft / 1.22 m by default) and rounded up. Set the doorway count and stock length to match your job.
Can I calculate a whole house at once?
Yes. Add a room for each area and the calculator seams each one independently — a broadloom strip can't span two rooms — then sums the running lengths, pad and tack strip into one shopping list. It sums the running lengths rather than the areas, so the seam plan stays honest across the whole job.
Does it work in metric and imperial?
Yes. Enter rooms in metres or feet, roll widths in metres or feet, and carpet area in square metres, square yards or square feet. Switch the unit toggle at any time and every input and result converts, with sensible default roll widths for each region.
Can I export or print the result?
Yes. Export CSV downloads the breakdown for Excel, Google Sheets or Numbers, and Print / PDF produces a clean bill of materials listing the carpet, pad and tack strip (or boxes, or stair runner) with quantities and cost — handy to take to the carpet store.
Is the calculator private?
Yes. Every calculation runs in your browser — nothing you enter is uploaded to a server. Your project is saved only in your own browser's local storage, so you can close the tab and return to it later.
Estimate your carpet in eight steps
Follow these steps to go from a room to a buyable carpet order.
Estimate your carpet in eight steps
- Choose unitsPick metric (m, m²) or imperial (ft, sq yd / sq ft); every value converts when you switch.
- Pick a modeChoose Broadloom for roll carpet, Tiles for modular squares, or Stairs for a runner — each has its own inputs.
- Add a roomAdd a room for each area you're covering — the calculator seams each one and consolidates them into one shopping list.
- Enter the room dimensionsType the room's width and length; add obstructions like a hearth or closet to lower the used area.
- Set the roll widthEnter the roll width your supplier stocks (12 ft, 4 m …); it drives the strips, seams and run-direction recommendation. Leave it blank for an area-only estimate.
- Set pattern and run directionFor a patterned carpet enter the repeat and match; check the recommended run direction and override it to hide a seam if needed.
- Check pad and tack stripSet the pad roll coverage (or confirm an attached cushion) and the doorway count to complete the materials list.
- Read the results, export or printCheck the running length, carpet area, seams, pad and tack strip, then export to CSV or print the bill of materials.