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🌾"Work in plenty of compost" is not a quantity.

✓ Tested & verified Updated: Aug 8, 2026 How we calculate this

Soil amendment

to make the top 6 in 25% compost:

0.9 cubic yards

= 25 cu ft · about 17 bags at 1.5 cu ft · spread 1.5 in deep before you dig it in

Bed area

sq ft

Working it in to

Compost by volume

Soil you are amending: 3.7 yd (100 cu ft)
Weight of the compost: 1,204 lb

Try also

How much compost do you need to amend a bed?

Enough to make up a quarter to a third of the soil you are actually mixing. The number depends on the depth you work to, which is the part every gardening book leaves out. Amending 200 square feet to six inches means you are working with 100 cubic feet of soil; making that 25% compost takes 25 cubic feet, which is 0.9 cubic yards or about 17 bags. Spread over the bed, that is an inch and a half of compost before you dig it in.

Working the same bed to twelve inches doubles it. Working to three inches halves it. “Plenty of compost” is unbuyable advice precisely because it hides that dependency, and it is why gardeners come back from the garden centre with either four bags or forty.

A quarter by volume is the standard starting point for a vegetable bed in reasonable soil, and a third for heavy clay or pure sand where structure is the problem. Above a third the mix stops behaving like amended soil: it holds water differently, settles several inches in the first season, and the bed you built in April is two inches lower by August.

What each working depth costs you

200 square feet at 25% compost by volume:

Worked in to	Soil volume	Compost needed	Bags at 1.5 cu ft	Spread depth
3 in	50 cu ft	0.46 yd	9	0.75 in
6 in	100 cu ft	0.93 yd	17	1.5 in
9 in	150 cu ft	1.39 yd	25	2.25 in
12 in	200 cu ft	1.85 yd	34	3 in

Percentages worth using

Situation	Compost by volume	Why
Established bed, annual feed	10–15%	Replacing what the soil consumed
New vegetable bed	25%	The usual extension recommendation
Heavy clay or pure sand	33%	Structure is the problem, not fertility
Filling a raised bed	30–40% with topsoil	It is a mix, not an amendment
No-dig surface layer	1–2 in on top	Worms do the incorporating

The neighbouring tools

To fill a new raised bed rather than amend an existing one, the raised bed soil calculator handles the mix. For compost on its own by area and depth, use the compost calculator; for the soil underneath it, topsoil.

Frequently asked questions

What percentage of compost should I add?

A quarter by volume for a normal vegetable bed, a third for clay or sand where you are fixing structure, and 10–15% for an established bed that just needs feeding. Container mixes go higher, but they are not soil and are not what this calculates.

How deep should I work it in?

Six inches for annual vegetables and most beds — it is the working depth of a fork and the root zone of nearly everything you plant from seed. Twelve inches for deep-rooted perennials or if you are building a bed from poor subsoil. Three inches for a light refresh between crops.

Can I just spread it on top?

Yes, and no-dig growers do exactly that — an inch or two on the surface each year, left for the worms to take down. It works, it is much less effort, and it is slower to change the structure of heavy clay. This calculator gives you the spread depth, so it answers both approaches.

Is more compost always better?

No. Past about a third by volume, beds settle dramatically, hold too much water in wet spells and can run high in phosphorus and salts after a few years of repeated heavy application. Compost is an amendment, not a growing medium.

How much does compost weigh?

About 1,300 lb per cubic yard when moist — roughly half the weight of topsoil, which is why a yard of compost is manageable in a half-ton pickup and a yard of topsoil is not.

Bags or bulk?

Anything over about a cubic yard is cheaper in bulk even after the delivery fee. The bulk vs bagged calculator works out where the line falls for your prices, and it moves a lot with the delivery charge.

How we calculate this

The soil volume being amended is area × working depth ÷ 12, in cubic feet. The compost needed is that volume × the percentage you choose — a volume-of-total measure, matching how extension services state amendment rates. The spread depth is that compost volume divided back over the area, which is what you actually lay down before digging. Bag counts assume 1.5 cubic feet a bag and round up; weight uses 1,300 lb per cubic yard for moist compost. Everything runs in your browser; nothing is stored or sent.

Worked example

With Bed area 200, Working it in to 6, Compost by volume 25, this page works out 0.9 cubic yards. = 25 cu ft · about 17 bags at 1.5 cu ft · spread 1.5 in deep before you dig it in

Soil you are amending3.7 yd ( 100 cu ft)
Weight of the compost1,204 lb

Sources:

Compost is a habit, not a project

Compost does two different jobs and they are usually confused. In sand it holds water and nutrients that would otherwise drain straight through; in clay it opens up the structure so water and roots can move. Both effects come from the organic matter fraction, and both fade — soil biology consumes it, typically losing a few percent of organic matter a year, which is why amendment is a habit rather than a project.

That also explains the diminishing returns of a single enormous application. A bed amended once to 40% compost is not set up for five years; it is a bed that will settle hard, then drift back toward its original organic matter level anyway. Adding a quarter of an inch to an inch a year, forever, does more than any single dig ever will.