Mulch calculator / Cardboard sheet coverage
Sheet Mulch Cardboard Calculator
Collect enough usable sheets after the seams overlap. Compare two directions for one measured, flat rectangular bed. Have different box sizes? Check your stock.
Use the sheets you already have
Different sizes need a placement plan, not an average sheet. Enter each usable size and how many whole sheets you have. This separate mode does not change your equal-size grid above.
What does “no plan found” mean?
The planner tries three row-layout strategies, plus repeated stock-reserved kits for multiple layers. Each row credits its shortest sheet height. A successful plan is inspectable; an unsuccessful search does not prove that every arrangement fails.
If the raw cardboard area is smaller than the bed area across your chosen layers, there is a definite material shortfall even before seams. Otherwise, try a different grouping or inspect the available dimensions. Do not buy more solely because these bounded strategies failed.
Why check real stock? UC ANR's 2022 project worked with donated delivery boxes of different sizes and ran short. A UC Davis garden account from 2014 used large bicycle boxes. These examples explain the task, not a universal overlap or layer count.
Enough area is not a finished layout
Count the seams first. A 10 × 6 ft bed is 60 ft², and a 48 × 36 in sheet is 12 ft². Dividing those areas gives 5 sheets, but it ignores every overlap.
What happens with a chosen 6 in seam? One direction needs 3 columns and 3 rows, or 9 sheets. Turn every sheet: 4 columns and 2 rows take 8 sheets. That is the smaller of the two grids, not a minimum over every possible arrangement.
The 8 sheets contain 96 ft² of material per layer. Of that, 60 ft² is the bed, 17.25 ft² is repeated coverage at seams and 18.75 ft² is the unique footprint beyond the bed. The last figure is not a reusable scrap allowance.
The overlap formula
The first sheet covers its full dimension. Each next sheet adds only its dimension minus the overlap. Apply that rule along both bed axes, then compare a 90-degree rotation.
Otherwise: 1 + round up ((bed span − sheet span) ÷ (sheet span − overlap))
Sheets per layer = columns × rows
Collection count = sheets per layer × chosen layers
Would a different box size fit better? Try its usable dimensions in the equal-size grid. For a limited supply of different sizes, use the separate stock check above. Neither mode reuses trimmed scraps.
Choose the method, then count the material
UC ANR's February 2022 garden account describes running short of cardboard and seeing weeds emerge at gaps. The project used 6 to 8 in overlaps. That practical failure is why this planner includes seams instead of adding a guessed percentage.
Oregon State Extension's guide, revised July 2025, describes plain brown corrugated cardboard with tape and staples removed. It also notes that thick cardboard can restrict air and water, and that conditions differ by site. Choose a suitable method and layer count before using this quantity tool.
Measure usable rectangles after unfolding. Holes, missing flaps and damaged edges cannot contribute their full nominal size. Top-layer mulch is a separate volume calculation based on the bed's ground area, not the larger cardboard footprint.
Cardboard coverage questions
Can I divide the bed area by the cardboard sheet area?
That gives an area-only lower bound, not a complete overlapping layout. Each seam uses material twice, and whole sheets can extend beyond the bed. Enter usable sheet dimensions and overlap to count a grid.
How much should the cardboard overlap?
Choose the overlap for your method and site. UC ANR describes 6 to 8 inches in one garden project; Oregon State's lasagna-compost method specifies at least 8 inches. Those examples do not set a universal value, and this tool does not choose it for you.
How many layers of cardboard should I use?
This calculator does not prescribe layers. Enter the count you have chosen for an appropriate method. Oregon State notes that thick cardboard can restrict air and water; a sheet-mulch quantity result is not a soil or plant-health recommendation.
Can I use different box sizes or reuse trimmed pieces?
The equal-size grid uses one common usable sheet size. Use the separate stock check on this page for different rectangular sizes and available counts. It shows actual whole-sheet placements, not a global minimum; cutting, offcut reuse, holes and damaged sheets are not modelled.
Does this include the mulch placed on top of the cardboard?
No. The output counts cardboard sheets only. Use the separate mulch calculator with the measured bed area and your chosen top-layer depth. Cardboard overlap does not increase the ground area to be mulched.
What should I put down before laying mulch?
There is no single underlayer for every bed. For sheet mulching, Oregon State Extension describes cardboard, newspaper, or heavy paper beneath mulch and cautions that thick cardboard can restrict air and water around existing roots. If you are smothering turf with a temporary cover, University of Maryland Extension says to remove it once the grass has died, then add compost; that cover is not a permanent buried layer. Iowa State Extension also describes covering paper or cardboard with organic mulch and warns that overly thick layers can limit water and air movement. This calculator counts user-entered cardboard sheets and overlap only; it does not choose a method or calculate top mulch or compost volume.