Compost Calculator

Calculate how much compost you need

Square feet × depth in inches ÷ 324 = cubic yards. That single divisor folds in both conversions, because a cubic yard spread one inch deep covers 324 square feet. It works for any material, but it matters more here because compost depths are fractions rather than whole numbers.

A 500 sq ft border at 2 inches is 3.09 cubic yards, which in garden compost comes to about 1.5 tons or 84 one-cubic-foot bags. The same border topdressed at a quarter inch would take 0.39 yards. Same ground, eight times the difference, entirely from the rate.

The mistake worth avoiding is measuring the whole garden. Compost goes on beds and lawns, not paths, drives or patios, and at $40 a yard an over-measured area is an expensive rounding error. Measure the planted areas separately and add them.

Application rate by use, and why one depth will not do

Every other bulk material on this site has a depth range of maybe two to one across its uses. Compost runs twenty to one. A lawn takes a quarter inch and a raised bed takes six, and applying either figure to the other job does real damage: too much on turf smothers it, too little in a bed does nothing measurable.

Read down the yards column and the spread is the whole point: 0.8 yards to topdress a thousand square feet of lawn against 18.5 to fill the same footprint of raised bed. The rate, not the area, is what decides whether you are collecting a couple of bags or booking a delivery.

Topdressing a lawn without smothering it

Quarter to half an inch, and never more in one pass. Grass has to grow through whatever you put on it, and compost deep enough to bury the crowns kills the turf it was meant to feed. If a lawn needs more than half an inch of levelling, that is a soil job rather than a compost one.

Spread it, then work it in with the back of a rake or a stiff brush so it falls between the blades rather than sitting on them. On a lawn you can still see grass through after spreading, you have the rate about right. If it looks like a brown field, take some back off.

Autumn is the usual timing in cool-season regions and late spring in warm-season ones, in both cases when the grass is growing hard enough to push through. Pair it with overseeding and the compost doubles as seed-to-soil contact — the grass seed calculator sizes that side of the job.

Amending garden beds, new and established

Three inches tilled into the top six to eight is the standard for a new bed, which gives you roughly a third compost by volume in the finished mix. For a bed already in good heart, an inch a year forked into the surface maintains it without the disturbance that digging causes.

The share matters more than the depth, which is why the calculator asks how deep you are digging. Two inches worked into six is a third; the same two inches worked into twelve is a sixth, and the same order does a different job. Above about half, you are no longer amending soil but replacing it, and beds that high in organic matter slump as it breaks down.

Raised beds are the exception. With no subsoil beneath to draw on, a mix nearer half compost to half topsoil is normal, and it will need topping up each season as the organic fraction decomposes. Budget for losing an inch or two a year.

Compost coverage per yard, at every depth

One cubic yard covers 324 sq ft at 1 inch, 162 at 2 inches and 1,296 sq ft at the quarter inch a lawn wants. That column is pure geometry and never changes with the type of compost, because it is volume rather than weight.

Coverage halves every time the depth doubles, which is the trap on large lawns. Going from a quarter inch to a half on 5,000 sq ft is not a small adjustment, it is another 3.9 cubic yards and probably another delivery. Bags are the other side of the same table: at a quarter inch a single cubic foot bag does about 48 sq ft, so a lawn of any size is a bulk order rather than a trolley full of sacks.

Compost weight and delivery: a yard that fits in a pickup

Compost is light. A cubic yard of screened garden compost runs about 1,000 lb against roughly 2,200 for moist topsoil, which changes what you can collect yourself. A half-ton pickup that cannot legally take a yard of soil will take a yard of compost without complaint.

The spread across the column is about 80% from the lightest to the heaviest, and it tracks how much actual soil is in the product. A compost-topsoil blend is half soil by volume, so it weighs nearly what soil weighs and should be planned like soil rather than like compost.

Where weight bites is delivery pricing. Suppliers who sell compost by the ton are selling you a lot of volume per ton, which usually works in your favour; those who sell by the yard are not. Confirm which unit the quote is in before comparing two of them, because the same job can look 40% cheaper purely on units.

Mushroom compost: what it is and where it belongs

Mushroom compost is the spent growing medium from commercial mushroom farms, usually a straw, manure and gypsum mix that has already been pasteurised and cropped. It is sold as spent mushroom substrate or mushroom soil as well, and all three names describe the same by-product.

Its strengths are structure and price. It is bulky, opens up heavy clay well, and because it is a waste stream it is usually the cheapest bulk compost a yard stocks. It also holds moisture without going anaerobic, which makes it a reliable mulch layer around established shrubs.

The caution is salts. Fresh spent substrate carries enough soluble salt to check germination and scorch seedlings, so it wants weathering over a winter, or at least a good soaking, before it goes near a seedbed. It also trends alkaline from the gypsum and chalk, which suits brassicas and most vegetables but is wrong for blueberries, rhododendrons and anything else that wants acid ground. At 1,200 lb a yard it is heavier than garden compost, so order by weight carefully.

How long composting takes, and how to speed it up

Three months to two years, and the range is almost entirely down to how much work you put in. A turned, balanced, well-sized heap finishes in three to six months. The same material piled in a corner and left alone takes twelve to twenty-four.

Four things decide the speed. Size: a heap under about a cubic yard cannot hold heat, and heat is what does the work. Surface area: shredded or chopped material composts several times faster than whole stems. Moisture: the heap should feel like a wrung-out sponge, and both dry and waterlogged stall it. Air: turning is the single biggest lever, and a heap turned weekly runs hot enough to finish in a season.

You will know it is done when it smells of woodland floor rather than anything else, has cooled to air temperature and no longer heats up after turning, and none of the original ingredients are recognisable except the odd woody stem. Compost that still smells sour or of ammonia is not finished, and digging it in at that stage will rob nitrogen from your plants rather than feed them.

Compost browns and greens ratio

Roughly 25 or 30 parts carbon to 1 part nitrogen by weight, which in practice means two or three parts browns to one part greens by volume. Browns are the dry, woody, carbon-rich things: dead leaves, straw, cardboard, sawdust, paper. Greens are the wet, nitrogen-rich ones: grass clippings, kitchen scraps, coffee grounds, fresh prunings.

The ratio is really a diagnostic tool rather than a recipe. A heap that is slimy, sour or smells of ammonia has too much green and needs browns and air. A heap that is dry, cool and simply sitting there has too much brown and needs greens and water. Adjust by what the pile is doing rather than by weighing anything.

The practical difficulty is that browns arrive once a year in autumn and greens arrive all summer. The fix is to bag leaves in October and keep them beside the bin as a browns bank, so each bucket of kitchen waste or grass has something dry to go in with. That single habit fixes more struggling heaps than any other change.

How to apply compost, layer by layer

On a lawn, spread thin and rake it in. On an established bed, lay an inch on the surface in autumn and let the worms take it down — digging a mature bed does more harm to its structure than the compost does good. On a new bed, spread three inches and fork it into the top six to eight.

For sheet composting on ground you are converting from lawn, the order runs cardboard on the grass, then two or three inches of compost, then mulch on top. Wet each layer as it goes down. Left over a winter that kills the turf beneath and leaves a bed you can plant into in spring without lifting a spade.

Around existing plants, keep compost off stems and trunks. A collar of damp organic matter against bark holds moisture where the plant least wants it and invites rot. Leave a couple of inches clear at the base and taper the layer outward toward the drip line, which is where the feeding roots actually are.

What compost costs

Between $30 and $60 a cubic yard collected for bulk garden compost, and $40 to $90 delivered, with mushroom compost usually at the bottom of that range and screened organic blends at the top. Bagged compost runs $5 to $9 for a one cubic foot bag, which is $135 to $240 a cubic yard equivalent.

That is a three- to fivefold premium for bags, wider than any other material on this site, and it makes the crossover point unusually early. At 27 bags to the yard, a bulk order wins from about half a yard upward even after a delivery charge. Bags still make sense for a single raised bed, for anywhere a truck cannot tip, and for carrying material through a house.

Municipal compost is the outlier worth knowing about. Many councils and transfer stations sell finished green-waste compost at a fraction of commercial prices, and some give it away to residents. Quality is variable and it is rarely screened as fine, but for bulk bed amendment where appearance does not matter it is hard to beat on price.

Related calculators

• Topsoil calculator for the soil that goes under or alongside the compost.
• Lawn square feet calculator to get the area figure before you start.
• Grass seed calculator for overseeding at the same time as topdressing.
• Fill dirt calculator where the job is raising a level rather than feeding soil.
• All soil and dirt calculators.