Compost Calculator in Yards, Bags and Truckloads

Calculate how much compost you need

The arithmetic is the easy part. Square feet × depth in inches ÷ 324 = cubic yards. That one number does both conversions for you, because a cubic yard spread one inch deep covers 324 square feet. This works for any material. But it matters more here, because compost depths are fractions, not whole inches.

Let’s say you’ve a 500 sq ft border and you want 2 inches on it. That’s 3.09 cubic yards. In garden compost that comes to about 1.5 tons, or 84 one-cubic-foot bags. Now topdress the same border at a quarter inch instead? You only need 0.39 yards. Same ground, eight times less compost, just from the depth.

Here’s the mistake worth avoiding though. Don’t measure the whole garden. Compost goes on beds and lawns. It doesn’t go on paths, drives or patios. At $40 a yard, measuring too much gets expensive fast. Measure the planted areas one by one and add them up.

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

You know, most materials on this site have a depth range of about two to one. Two inches here, four inches there. Compost runs twenty to one. A lawn takes a quarter inch. A raised bed takes six. And using the wrong one does real damage. Too much on grass smothers it. Too little in a bed does nothing at all.

Look down the yards column and you’ll see the point. It takes 0.8 yards to topdress a thousand square feet of lawn. The same thousand square feet as a raised bed takes 18.5 yards. It’s the rate, not the area, that decides whether you grab a few bags or book a delivery.

Topdressing a lawn without smothering it

A quarter to half an inch. Never more in one go. Grass has to grow up through whatever you put on it. Bury the crowns and you kill the lawn you were trying to feed. And if a lawn needs more than half an inch to level it out, that’s a soil job, not a compost job.

Spread it, then work it in with the back of a rake or a stiff brush. You want it falling between the blades, not sitting on top of them. Can you still see grass after spreading? Then the rate is about right. If it looks like a brown field, take some back off.

Fall is the usual timing in cool-season areas, late spring in warm-season ones. Either way, do it when the grass is growing hard enough to push through. And if you are overseeding at the same time, the compost doubles as seed-to-soil contact. The grass seed calculator handles that side of the job.

Amending garden beds, new and established

For a new bed, three inches dug into the top six to eight is the standard. That gives you about a third compost in the finished mix. For a bed that’s already healthy, an inch a year forked into the surface keeps it going. No need to dig it over, which only damages the structure you’ve built up.

Here’s the part people miss. The share matters more than the depth, and that’s why the calculator asks how deep you’re digging. Two inches mixed into six is a third compost. The same two inches mixed into twelve is a sixth. Same order, different job. Go above half and you’re not feeding the soil any more, you’re replacing it. Beds that rich also sink as the compost breaks down.

Raised beds are the exception. There’s no subsoil underneath to draw on, so half compost and half topsoil is normal. Just know it will need topping up every season as the compost rots down. Expect to lose an inch or two a year.

Compost coverage per yard, at every depth

One cubic yard covers 324 sq ft at 1 inch. It covers 162 sq ft at 2 inches, and 1,296 sq ft at the quarter inch a lawn wants. And this column never changes, whatever compost you buy. It’s volume, not weight, so the type makes no difference.

Here’s the trap on big lawns. Coverage halves every time the depth doubles. Going from a quarter inch to a half on 5,000 sq ft sounds like a small change. It’s not. That’s another 3.9 cubic yards, and probably another delivery. Bags tell the same story. At a quarter inch, one cubic foot bag does about 48 sq ft. So any real lawn is a bulk order, not a cart full of sacks.

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

Compost is light, and that changes what you can collect yourself. A cubic yard of screened garden compost runs about 1,000 lb. Moist topsoil is around 2,200. So a half-ton pickup that cannot legally carry a yard of soil will take a yard of compost no problem.

That column runs about 80% from lightest to heaviest, and it comes down to one thing: how much real soil is in the bag. A compost-topsoil blend is half soil by volume. So it weighs almost what soil weighs, and you should plan it like soil, not like compost.

Where weight really bites is the price. A supplier selling by the ton is giving you a lot of volume per ton, and that usually works in your favor. Selling by the yard doesn’t. So check which unit the quote uses before you compare two of them. The same job can look 40% cheaper just from the units.

Mushroom compost: what it is and where it belongs

Mushroom compost is what’s left over after commercial mushroom farms finish a crop. It’s usually straw, manure and gypsum that has already been heat-treated and used once. You’ll also see it sold as spent mushroom substrate or mushroom soil. Same stuff, three names.

Why use it? Two reasons: structure and price. It’s bulky and it opens up heavy clay really well. And because it’s a waste product, it’s usually the cheapest bulk compost a yard sells. It also holds water without turning sour, which makes it a solid mulch around shrubs that are already established.

But here’s the catch. Salt. Fresh spent substrate holds enough salt to stop seeds germinating and burn young plants. Leave it out over a winter, or at least give it a good soaking, before it goes anywhere near a seedbed. It’s also on the alkaline side from the gypsum. That suits cabbages and most vegetables, but it’s wrong for blueberries, rhododendrons and anything else that wants acid soil. And at 1,200 lb a yard it’s heavier than garden compost, so watch the weight when you order.

How long composting takes, and how to speed it up

Anywhere from three months to two years. And honestly, that gap is almost all down to how much work you put in. A heap that’s the right size, well balanced and turned regularly finishes in three to six months. The same pile dumped in a corner and forgotten takes one to two years.

Four things control the speed. Size first: a heap under about a cubic yard cannot hold heat, and heat is what does the work. Then surface area, because chopped or shredded material breaks down several times faster than whole stems. Then moisture . It should feel like a wrung-out sponge, and both dry and soggy will stall it. And last, air. Turning is the biggest lever you’ve. Turn it weekly and it runs hot enough to finish in one season.

How do you know it’s done? It smells like a forest floor and nothing else. It has cooled down to air temperature and stays cool after you turn it. And you cannot recognize what went into it any more, apart from the odd woody stem. If it still smells sour or of ammonia, it’s not ready. Dig it in at that stage and it will steal nitrogen from your plants instead of feeding them.

Compost browns and greens ratio

About 25 to 30 parts carbon to 1 part nitrogen by weight. In practice that just means two or three parts browns to one part greens by volume. Browns are the dry woody stuff: dead leaves, straw, cardboard, sawdust, paper. Greens are the wet stuff: grass clippings, kitchen scraps, coffee grounds, fresh prunings.

But don’t treat the ratio as a recipe. It’s really a way of reading your heap. Is it slimy, sour, smelling of ammonia? Too much green. Add browns and turn it. Is it dry and cool and just sitting there doing nothing? Too much brown. Add greens and water. You fix it by what the pile is doing, not by weighing anything.

And here’s the real problem. Browns show up once a year in fall. Greens show up all summer. Bag your leaves in October and keep them next to the bin as a browns bank. Then every bucket of kitchen waste or grass has something dry to go in with. That one habit fixes more struggling heaps than anything else.

How to apply compost, layer by layer

On a lawn, spread it thin and rake it in. On a bed that’s already established, put an inch on the surface in fall and let the worms pull it down. Digging a mature bed over does more damage to the structure than the compost does good. On a brand new bed, spread three inches and fork it into the top six to eight.

Turning lawn into a bed? Try sheet composting. Cardboard straight onto the grass, then two or three inches of compost, then mulch on top. Wet each layer as you go. Leave it over winter and the grass underneath dies off. Come spring you’ve a bed you can plant into without ever picking up a spade.

One last thing. Keep compost off stems and trunks. A ring of damp compost against bark traps moisture exactly where the plant doesn’t want it, and that’s how rot starts. Leave a couple of inches clear at the base and spread the layer outward toward the drip line instead. That’s where the feeding roots are anyway.

What compost costs

Bulk garden compost runs $30 to $60 a cubic yard if you collect it, and $40 to $90 delivered. Mushroom compost sits at the bottom of that range, screened organic blends at the top. Bagged compost is $5 to $9 for a one cubic foot bag. Work that out per yard and it’s $135 to $240.

Bags cost three to five times more. That’s the widest gap of any material on this site, and it means the crossover comes early. There are 27 bags in a yard, so bulk wins from about half a yard up, even once you pay for delivery. Bags still make sense for one raised bed, for anywhere a truck cannot tip, and for carrying material through the house.

And here’s one worth knowing about. Municipal compost. A lot of councils and transfer stations sell finished green-waste compost for a fraction of commercial prices, and some hand it to residents free. The quality moves around and it’s rarely screened as fine. But for bulk bed work where looks don’t matter, nothing beats it 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.