The seams are the calculation
Bed area divided by roll area is the answer everyone gives, and it is
short on every job that needs more than one strip. Fabric strips have to
overlap or weeds come up the join, and the overlapped width is covered
twice — so every strip after the first gains you its width
minus the overlap, not its width.
effective width = roll width − overlap
strips = ⌈(bed width − roll width) ÷ effective width⌉ + 1
linear ft = strips × bed length
30 × 10 ft bed, 3 ft roll, 6 in overlap
effective width 2.5 ft, not 3
strips 4
linear ft 120
rolls of 100 ft 2
Area ÷ roll area says 1.
A wider roll can use less fabric
Because a wider roll has fewer seams, and each seam costs a fixed strip
of width regardless of how wide the roll is. The same
300 square foot bed takes 120 linear
feet in 3 foot fabric — 20% of it seam — and
60 linear feet in 6 foot fabric, at
20%. On anything wider than a narrow border, the
wide roll usually wins per square foot covered, even though it looks
like the expensive option on the shelf.
Strips and seam waste by roll width
From this calculator at a 6 in overlap, the minimum Dewitt publish. The seam overhead column is fabric bought over and above the bed area, and it is not recoverable.
Six inches is a floor, not a target
Dewitt publish the overlap as 6 to 12 inches and explain why the band
exists: a small overlap looks fine on the day, and soil movement and root
pressure widen it over the seasons until there is a gap. Six inches is
the working minimum. Where weeds are aggressive, the top of the band is
cheap insurance — the fabric is doing its job everywhere except at the
joins, so the joins are the only place worth spending on.
Turning the edges down
Fabric cut flush to the bed edge lifts, and then it shows. Turning six
inches down into a shallow trench at the perimeter fixes it, and it costs
fabric in both directions — twice the burial on the width and twice on
the length. The tool has a field for it and adds it to both, because the
ends need the same treatment as the sides and that is the half people
forget.
Whether to use it at all
Worth saying plainly, on a page selling the idea of buying it: under
organic mulch, fabric is a short-term measure. Bark breaks down into
compost that sits on top of the barrier, and within a few seasons
that layer is a seed bed where the fabric cannot help. Under gravel,
stone and permanent plantings — where nothing decomposes on top and
nothing will be dug — it earns its keep for years.
The rest of the bed
What goes over the fabric is usually the bigger order:
mulch by the yard or the bag,
or gravel and stone
where the fabric is doing separation rather than weed control. For a
planting bed being built rather than topped up, the
raised bed soil calculator
uses published extension-service mixes, and where the fabric is wrapping a
trench instead of covering a bed, the
french drain calculator
already sizes it to wrap rather than just to line. To measure an awkward
bed in the first place, the
square footage calculator
adds up rectangles and circles.
Common questions
How much landscape fabric do I need?
More than the area, because the seams are not free. Every strip after the first covers its own width minus the overlap, so a 3 foot roll lapped 6 inches only gains you 2.5 feet of new ground each pass. A 10 by 30 foot bed takes 4 strips and 120 linear feet — 20 percent more fabric than the bed area, and 2 rolls where area alone would have said 1.
How much should landscape fabric overlap?
Dewitt publish 6 to 12 inches, and say a small overlap widens over time as soil moves and roots push. Six inches is the working minimum that closes the gap; go to the top of the band where weed pressure is high, because seams are where weeds come through — the fabric itself is doing its job everywhere else. This calculator defaults to six and lets you raise it, and it re-counts the rolls when you do.
Does a wider roll use less fabric?
Often, yes, and it is the useful thing this page tells you. A wider roll means fewer seams, and each seam costs you a fixed strip of width. The same 30 by 10 foot bed takes 120 linear feet of 3 foot fabric — 20 percent seam — but only 60 linear feet of 6 foot fabric at 20 percent. On anything wider than a narrow border, the wide roll is usually the cheaper buy per square foot covered.
Do I need extra to bury the edges?
If you are turning the fabric down into a trench at the bed edge — which is what stops it lifting and shows a white line after a windy week — then yes, and it adds to every dimension, twice. Six inches turned down at both edges of a 10 foot bed means cutting for 11 feet. The tool has a field for it and adds it to the length as well as the width, because the ends need the same treatment as the sides.
What roll sizes does landscape fabric come in?
Dewitt list their woven barrier in 3, 4, 5, 6, 8 and 12 foot widths, all in 250 foot rolls for the trade. Consumer packs of the same fabric are usually 3 or 4 feet wide in 50 or 100 foot lengths. Roll length is an input here rather than a preset, because it is the number that varies most between who is selling it — check the roll rather than assuming.
Should I use landscape fabric under mulch at all?
Honestly, often not. Organic mulch breaks down into compost that sits on top of the fabric, and within two or three seasons that layer is a perfectly good seed bed with weeds rooting in it — above the barrier, where the barrier cannot help. Fabric earns its keep under gravel and stone, which do not decompose, and under it in permanent plantings where nothing will be dug. Under bark that gets topped up annually, it mostly makes future digging unpleasant.
How many pins or staples will I need?
This calculator does not count them, because no manufacturer publishes a spacing that applies generally — Dewitt say to secure the seams and edges and use their anchor pins, without a number. Work on pinning the seams and the perimeter, more often on a slope and in wind, and buy by the box rather than by the calculation. An overlap that is not pinned separates the first time the mulch on top of it settles.
Which side goes up?
On a woven barrier with a printed stripe, the printed face goes up — Dewitt stripe theirs every twelve inches specifically to help you line plants up, which only works face up. On a non-woven fabric with a fuzzy side and a smooth side, the fuzzy side goes down against the soil. Neither affects the quantity, but both affect whether it works, and both are easy to get wrong on a roll you unrolled the far way.
Sources
Overlap is stored as the 6 to 12 inch band Dewitt publish, with no typical value, because they publish no single figure. The calculator defaults to the low end and lets it be raised: six inches is what the manufacturer calls the minimum that eliminates gaps, and more is a judgement about weed pressure that no calculator can make. Roll widths are the ones Dewitt list for PRO-5; length is left as an input because the same fabric is sold in 50, 100 and 250 ft rolls depending on who is selling it.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures
