EstimatorKitFree compost calculator
Compost as a share of the soil, not a depth
Tell this how deep you are working and what proportion of that layer you want to be compost, and it returns the depth to spread, whole 1 cubic foot bags, and the same volume in cubic yards. Compost is an amendment, so the number that decides whether a bed works is the ratio — a quarter to a third of the worked layer — and two inches means nothing until you say what it is turned into. There is a reverse mode for compost you have already bought.
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- Ratio, not just depth
- Whole 1 ft³ bags
- Works backwards too
30% of the top 6 in, which is where the familiar 2 in of compost comes from.
Share is the compost depth divided by the depth you turn it into, which is how every published rate is written. extension amendment rates: 1–3 in of compost turned into the top 6–8 in of soil — it moves with how poor the soil is to start with, and whether it is being planted this season.
Compost to buy
12 bags
- Blend by volume
- 30% compost
- Compost depth
- 1.8 in
- In bulk
- 0.44 cu yd
- Volume
- 12 cu ft
Spreading 1.8 in and turning it into the top 6 in leaves a rooting zone that is 30% compost and 70% the soil you already had. That ratio is the number worth writing down — the inches are just how it gets delivered.
Nothing above is padded, and it deliberately is not. The 1.8 in of compost going in is the part of this bed that will not still be there in a few years: organic matter is consumed rather than stored, so the level drops as it goes and the answer is to reapply on a schedule rather than to over-apply once.
Per 1,000 sq ft that is 150 cu ft, which is the figure to compare against a bulk price. 27 of these one cubic foot bags make a cubic yard, so anything past a couple of yards is a delivery rather than a car load.
That volume weighs about 444 lb at 1,000 lb a cubic yard — loose finished compost, varying with moisture and how finished it is. Finished compost is roughly half the weight of the same volume of screened soil, which is the one respect in which it is the easier material to move.
How to work out a compost rate for a bed or a lawn
Blend first, depth second — the pair is what a published rate actually means.
Decide whether you are blending or covering
Compost worked into soil is measured as a share of the layer it goes into; compost left on the surface is measured as a depth. Those are different sums with different failure modes, so the tool asks first. A new bed or a clay soil takes the blend route. A lawn top-dressing or a yearly feed on an established bed takes the depth route, since nothing there is going to be turned over.
Give the bed its size, then the ratio or the depth
Measure length and width, or the diameter of a round bed. For a blend, enter how deep you intend to work — six inches for a fork or a tiller pass, eight where clay needs the deeper structure change — and the share of that layer you want to be compost. Thirty percent of six inches is 1.8 inches spread on top, which is where the familiar advice about two inches comes from.
Read the bags, or run it backwards from what you own
The result gives whole one cubic foot bags, the same volume in cubic feet and cubic yards, and what it weighs. The third mode reverses everything: type in the six bags already in the car and it returns the depth they spread to over your bed and the blend that makes, which is the version of this question people ask standing in a parking lot rather than at a keyboard.
Technical specifications
| Blend definition | Compost depth divided by the depth it is turned into: 2 in into 6 in is 33% |
|---|---|
| Default rate | 30% of the top 6 in for a new bed, 33% of 8 in on heavy clay |
| Surface rates | Lawn top-dressing 0.25 in, established bed 1 in a year |
| Two ceilings flagged | 50% blend, past which it is a growing medium rather than amended ground; 1 in of surface compost, past which it is a mulch and an excellent weed seedbed |
| Retail bag | 1 ft³, so 27 bags to a cubic yard |
| Weight | About 1,000 lb per cubic yard finished — half what the same volume of screened topsoil weighs |
| Reverse mode | Bags, cubic feet or yards you already have, turned into the depth and blend they make |
| Kept on file | Nothing — bed sizes and rates are worked out in your browser and never transmitted |
Frequently asked questions
How much compost should I mix into a new bed?
Enough to make the worked layer a quarter to a third compost by volume, which for a six inch pass is one and a half to two inches spread over the surface first. Stating it as a ratio rather than a depth is not pedantry: two inches turned into four inches of soil is a fifty percent blend and too rich, while the same two inches turned into a foot is sixteen percent and barely registers. The published rates all say a depth and an incorporation depth together, and it is the pair that carries the meaning.
Can I plant straight into pure compost?
You can, and most things will grow for a season before the problems arrive. Pure compost has no mineral fraction, so it holds water like a sponge and then dries to a crust that sheds it, has almost no structural stability, and settles hard as its carbon is consumed. Nutrient levels that suit an amendment at a third of the mix are excessive as the entire root zone, particularly the soluble salts in composts made from manures or municipal feedstock. Compost is what you add to soil, or one ingredient in a mix — not a substitute for either.
How much compost does a lawn top-dressing take?
A quarter of an inch, which is 20.8 cubic feet — about 21 bags, or three quarters of a cubic yard — per 1,000 square feet. The point of top-dressing is to feed the soil biology and slowly build organic matter in the top inch where the roots are, not to add height, so the compost has to be screened fine enough to be brushed down into the canopy with the back of a rake. Doing it right after core aeration puts a good part of it straight down the holes, which is worth far more than doubling the rate.
Is more compost better?
No, and the way it goes wrong is invisible for a year or two. Every application adds phosphorus, and unlike nitrogen, phosphorus is not used up or leached away in most soils — it accumulates. Beds composted heavily every season eventually test high enough that phosphorus starts running off into the nearest storm drain while tying up iron and zinc in the soil it came from. Composts from manure and biosolids also carry soluble salts, which are harmless at amendment rates and damaging at bed-filling ones. A soil test costs about the same as three bags and tells you whether this bed wants any compost at all.
Why does a bed filled with compost sink?
Because the compost is being eaten, and what leaves is mostly carbon going off as carbon dioxide. That is a different mechanism from the settling a mineral soil does, which is air voids closing under water and gravity and finishes in a few weeks. Organic matter loss is mass leaving the bed and it carries on for years: the higher the compost fraction, the further the surface drops and the longer it keeps dropping. A bed at thirty percent barely shows it. A bed filled with pure compost can lose a visible several inches in a season, which is why the fix is topping up rather than tamping down.
I have six bags — is that enough for a 4 by 10 bed?
Yes, almost exactly: six cubic feet over 40 square feet spreads 1.8 inches, and turned into the top six inches that is a 30 percent blend, which sits in the middle of the amendment range. What makes this worth checking rather than guessing is how fast it falls apart when the area changes. The same six bags over a 20 by 20 lawn is 0.18 of an inch, roughly three quarters of the standard quarter-inch top-dressing rate and nothing like an amendment. The volume did not move, the area did, and rates are per square foot — which is what the reverse mode on this page is for.
About amendment rates, and the ratio nobody quotes
Compost is the only material in this kit that is not really measured in inches. Every other page here fills a defined space: a bed, a trench, a slab, a ring. Compost goes into soil that is already there, and what determines the result is the proportion of the finished root zone it makes up. This is why the standard advice — “add two inches of compost” — is incomplete to the point of being misleading on its own. Two inches tilled into four inches of soil is a fifty percent blend, which drains oddly, slumps through the season and reads high on every nutrient in a soil test. The same two inches turned into a foot of soil is sixteen percent, which is a light improvement you will struggle to detect. The published rates have always specified both halves, one to three inches worked into the top six to eight, and this page asks for both because the ratio is what you are actually choosing.
The second thing worth knowing is that compost has an upper bound and it is not a matter of taste. Phosphorus is the reason. Nitrogen is used, leached and lost, so it needs replacing; phosphorus mostly stays where it is put, and repeated heavy composting builds it up until a soil test comes back high and stays high for years. High phosphorus soils run off into waterways and lock up iron and zinc in place. Salts are the same story on a shorter timescale, particularly in manure-based and municipal composts, which is why the calculator raises a flag when the blend passes half. That flag is not about waste. It is the boundary between improving ground and building a growing medium, and if the plan really is the second one, the honest way to do it is in a container with a mix designed for the job — the raised bed soil calculator splits that into ingredients and bag counts.
Finally, compost sinks, and it sinks for a reason that is worth separating from the settling every gardener has seen in a load of soil. Soil consolidates: water closes the air pockets left by the bucket and the surface drops once, over a few weeks — the process the topsoil calculator allows for. Compost does something else. It is being consumed, and most of what leaves departs as carbon dioxide, so the volume loss is mass loss and it continues for as long as there is organic matter to mineralize. A bed amended at thirty percent barely shows it. A bed filled with the stuff drops for years, which is the correct argument for using it as an ingredient rather than a filling. Left on the surface it behaves differently again: fine, dark and moist is a perfect seedbed, so compost makes a poor top layer and a good one underneath something else — mulch is what goes over it, and the bag-against-bulk price comparison lives on that page. Where the compost is going onto grass rather than into a bed, the grass seed calculator covers the overseeding that usually follows the same weekend.
What the rates you enter do next
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — measurements, quantities, the prices your supplier quoted you — is uploaded, logged or kept, which is also why the calculators carry on working on a site with no signal.
Soil test results and bed layouts say a lot about a property. Nothing typed here is one of the things this site could send anywhere even if it wanted to: the ratio arithmetic runs in the page and stops there.