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View all 5 tools →Compost Calculator
Work out how much compost you need by area and job -- lawn topdressing, a new lawn, an existing bed or a brand-new one -- each with its own sourced depth, plus weight, bag count and cost.
Project Details
1. Area to cover
2. What are you applying it to?
Worked into the top 8 to 12 inches each year to maintain organic matter, applied thinner than a first-year bed since the soil is already amended.
A typical finished compost with no further detail on source material -- the figure to use when you are not sure which of the below you are buying.
Compost does not have one recommended depth the way mulch does. A thin topdressing worked into a lawn that already exists is a very different job from tilling compost into ground that has never been planted, and mixing up the two either wastes most of a truckload or barely improves the soil at all. This calculator asks what you are applying it to first, pre-fills the sourced depth for that job, and works out the volume, weight, bag count and cost from there.
Reviewed by Usama, Civil EngineerHow we source our numbers
How it works
- Measure the area as a simple shape, or enter the square footage directly if you already know it.
- Pick what you are applying compost to: an established lawn, a new lawn before seed or sod, an existing garden bed you amend yearly, or a brand-new bed. Each one loads a different sourced depth, which stays editable.
- Pick the type of compost, too -- mushroom compost, composted manure, leaf mold, a screened topsoil blend, or a generic bulk figure if you are not sure. Bulk density differs enough between them to change the weight estimate, even at the same depth and area.
- Read the results: the volume you need, its weight, and the bag count or bulk cubic yardage with an editable cost.
Compost application depth by job, all from the same source
| Applying to | Depth | Worked into soil | Notes |
|---|---|---|---|
| Existing lawn (topdressing) | 0.25 in | Not tilled | A thin layer worked into the surface with a core aerator, in spring while the grass is actively growing -- not tilled, so the lawn is never buried. |
| New lawn (before seed or sod) | 1–2 in | Not tilled | Raked or rototilled into bare soil before seeding or laying sod, to give new roots a richer layer to establish in. |
| Existing garden bed (yearly) | 0.25–1 in | 8–12 in deep | Worked into the top 8 to 12 inches each year to maintain organic matter, applied thinner than a first-year bed since the soil is already amended. |
| New garden or landscape bed | 3–4 in | 8–12 in deep | A full amendment pass for ground that has never been planted, tilled deep before the first planting. |
Why compost depth depends entirely on the job
Every figure on this page comes from one Oregon State University Extension Service publication, EM 9308, "How to Use Compost in Gardens and Landscapes" (February 2021), because it happens to cover all four jobs this calculator handles in a single guide. Topdressing an established lawn calls for about a quarter inch worked in with a core aerator -- any more than that and the compost sits on top of the grass rather than reaching the roots. A brand-new garden bed, by contrast, gets a full 3 to 4 inch layer tilled into the top 8 to 12 inches of soil, since there is no existing turf to protect and the whole point is to change the soil structure before the first planting.
Topdressing versus tilling in
Topdressing means spreading a thin layer over ground that is already planted and leaving it to work in on its own, or with light aeration -- this is what an existing lawn or a maintained garden bed calls for, at a quarter inch and up to an inch respectively. Tilling in means physically incorporating the compost into the soil with a fork, spade or rototiller before anything is planted, which is what a new lawn or a new bed calls for, at a much deeper 1 to 4 inches depending on which. Using a tilled-in depth as a topdressing would smother an existing lawn; using a topdressing depth on a new bed would barely change the soil at all.
How much compost is too much
More is not automatically better. The same OSU guide warns that adding too much compost, especially in vegetable gardens, can stunt plant growth and, over several applications, contribute to nutrient runoff. That is exactly why this calculator ties the depth to the specific job rather than offering one flat "compost depth" the way a simpler tool might -- a quarter inch a year on an established bed keeps organic matter topped up without overdoing it, while a one-time 3 to 4 inch pass is reserved for ground that has never been amended before.
Compost is not one material
The depth guidance above applies to compost generally, but a cubic yard of one compost is not the same weight as a cubic yard of another. Mushroom compost -- spent mushroom growing substrate, composted and screened -- and leaf mold, made from fully decomposed leaves, are both noticeably fluffier and lighter than manure-based compost, which holds more moisture and mineral content. A screened topsoil blend sits at the heavy end, since it carries real mineral soil weight on top of the organic matter, closer to plain garden soil than to a light compost. None of these have a dedicated government bulk-density dataset the way loam soil does, so the ranges here are working Cooperative Extension bulk-material figures rather than a single cited number, and the generic option is the safe default when the source is not specified.
Buying compost in bags or bulk
Bagged compost commonly comes in 1 or 2 cubic foot bags, per the same OSU guide, which is the bag size this calculator uses to work out how many bags a job takes. Past roughly 3 cubic yards, bulk delivery from a landscape supplier is almost always cheaper and less effort than carrying dozens of bags, the same threshold this project uses for mulch and soil. Compost quality varies by source, so if buying in bulk, ask the supplier about moisture content and whether the compost has been screened -- a wet or chunky batch is harder to spread evenly at the thin depths a lawn topdressing calls for.
Further reading
Frequently asked questions
How much compost do I need?
It depends on the job. This calculator asks what you're applying compost to first -- an existing lawn, a new lawn, an existing garden bed or a brand-new one -- since each has a genuinely different recommended depth, then multiplies that depth by your area to get a volume in cubic yards, plus a bag count and cost.
How much compost do I need for a garden bed?
For an existing bed you're maintaining, about 1/4 to 1 inch of compost worked into the top 8 to 12 inches each year, per Oregon State University Extension. For a brand-new bed that has never been planted, a deeper 3 to 4 inch layer tilled into the top 8 to 12 inches gives the soil a full amendment before the first planting.
How much compost do I need for my lawn?
About 1/4 inch worked into the surface with a core aerator for an established lawn, per Oregon State University Extension -- any deeper and it sits on top of the grass instead of reaching the roots. A brand-new lawn, before seed or sod goes down, gets a thicker 1 to 2 inch layer rototilled into bare soil.
How much is a yard of compost?
This calculator won't give one fixed number, deliberately, since compost price varies by supplier and region. It pre-fills a starting price per bag and per cubic yard, both fully editable, so you can enter your own supplier's price and get an exact total rather than a market guess.
How much does a cubic yard of compost weigh?
No government dataset covers finished compost the way it covers native soil, so this calculator anchors the weight to the same USDA NRCS bulk density reference for loam used elsewhere on BunnyCorner: roughly 1,000 to 1,500 pounds per cubic yard, heavier when wet.
What is the difference between compost topdressing and tilling compost in?
Topdressing spreads a thin layer, about 1/4 to 1 inch, over ground that is already planted and leaves it to work in on its own or with light aeration. Tilling in physically incorporates a much deeper 1 to 4 inch layer into bare soil with a fork, spade or rototiller before anything is planted. Using the wrong one either smothers an established lawn or barely changes a new bed's soil at all.
Can you add too much compost?
Yes. Oregon State University Extension warns that adding too much compost, especially in vegetable gardens, can stunt plant growth and, applied repeatedly, contribute to nutrient runoff. That is why this calculator ties depth to the specific job -- a thin yearly topdressing on an established bed, a deeper one-time pass only for new ground.
How much compost do I need per square foot?
It depends on depth, which depends on the job. At a 1/4 inch lawn topdressing, 100 square feet takes about 0.08 cubic yards; at a 3 to 4 inch new-bed till-in, the same 100 square feet takes roughly ten times that. Enter your area and pick the application type above for an exact figure.
How many bags of compost are in a cubic yard?
27 bags at a 1 cubic foot bag, or 14 bags at 2 cubic feet, rounding up so you never come up half a bag short. Bagged compost commonly comes in 1 or 2 cubic foot bags, per Oregon State University Extension, which is the bag size this calculator's purchase section uses.
Should I buy compost in bags or in bulk?
Bulk, past about 3 cubic yards, the same threshold this project uses for mulch and soil -- at that point bulk delivery is almost always cheaper and easier than carrying dozens of bags. Below that, bags are usually more practical even where bulk works out slightly cheaper per yard.
How much compost for a new garden bed versus an existing one?
A new bed that has never been planted needs a full 3 to 4 inch layer tilled into the top 8 to 12 inches of soil. An existing bed you are maintaining needs far less, just 1/4 to 1 inch worked in each year to keep organic matter topped up, per Oregon State University Extension guidance.
What is the difference between compost and topsoil?
Compost is partially broken-down organic material that feeds soil life and adds nutrients; it is an amendment, not a standalone growing medium. Topsoil is closer to complete soil and can be used to fill or build up ground on its own. BunnyCorner's Topsoil Calculator covers topsoil specifically; this page covers compost as an amendment applied at a job-specific depth.