Sheet Mulch / Lawn-Conversion Calculator

How to Use the Sheet Mulch Calculator

• Measure the lawn or weedy area you want to convert, in square feet.
• Set the compost layer depth — 3 to 4 inches is the usual growing medium.
• Set the top mulch depth; 4 inches or more is what actually kills the grass below.
• Enter a bagged-material price if you want the cost estimate to fill in.
• Read the cardboard, compost and mulch quantities and buy or free-cycle them.
• Mow the grass scal-short, then build the layers, watering each one as you go.
• Overlapped cardboard seams are the whole trick, so plan for 25% more area than the bed.

The Lasagna-Bed Materials Math, Explained

Worked Example: Converting a 100 sq ft Patch

• You want to turn a 100 sq ft turf patch into a vegetable bed.
• Cardboard: 100 × 1.25 = 125 sq ft — about ceil(125 ÷ 32) = 4 flattened boxes.
• Compost at 3 inches: 100 × 3 ÷ 12 = 25 cu ft — ceil(25 ÷ 0.75) = 34 bags.
• Top mulch at 4 inches: 100 × 4 ÷ 12 = 33 cu ft — ceil(33 ÷ 0.75) = 45 bags.
• Establishment water: 100 × 0.6 = 60 gallons to wet the layers as you build.
• At $5 a bag the bagged materials run about $395 — the reason a bulk mulch delivery pays off at this scale.
• Mow the grass scal short, lay the overlapping cardboard, add compost, then mulch, watering each layer.
• Plant hungry transplants straight into the compost in spring, or let it cook a season for a clean root bed.

Standard Lasagna Layers

Key Terms

Sheet mulching

Building a bed by layering cardboard and organic matter over existing turf, killing it without digging or tilling.

Lasagna bed

A sheet-mulch built in distinct wetted layers — cardboard, compost, mulch — that cook down into soil.

Smothering

Blocking all light with overlapped cardboard so the grass and weeds beneath exhaust and die.

Growing medium

The compost layer you plant directly into the first year, before the sheet has fully broken down.

Cook-off

The settling and decomposition season a fresh sheet-mulch bed needs before it is prime for root crops.

What This Means for a No-Dig Build

• Use plain, tape-and-label-free corrugated cardboard — glossy or plastic-coated boxes shed water and put microplastics into a bed you grow food in for years.
• Build in fall if you can: a sheet-mulch laid in autumn has a freeze-thaw-cook season to kill the grass and start knitting into soil, so it is a ready bed by spring rather than a fight.
• The cardboard is a temporary weed barrier and food for soil life — it is gone within a season, so its only real job is complete light blockage on the day you lay it, which is why overlap beats thickness.
• For stubborn perennial grasses like quackgrass or Bermuda, double the cardboard and go heavier on mulch, or the rhizomes will find the one thin seam you left and stage a comeback.
• Plant the first crop as transplants of hungry annuals — squash, tomatoes, brassicas — straight into the compost; wait a season before sowing fine seed or root crops into a bed that is still actively decomposing.

Common Mistakes & Fixes

• Laying cardboard edge-to-edge with seams and gaps, leaving green highways for grass to march back up into the new bed.
• Using coated, printed or plastic-taped boxes that repel water and litter the bed with non-compostable debris.
• Skipping the watering of each layer, so the stack stays dry and air-pocked instead of knitting and cooking down into soil.
• Building too thin — an inch of compost and a sprinkle of mulch smothers nothing and gives roots nowhere to establish the first year.
• Sowing carrots or beets into a just-built bed and getting forking, unformed roots in media that was not finished decomposing.

When to Use This (and When Not To)

Sheet Mulch / Lawn-Conversion Calculator: Frequently Asked Questions