For most garden beds, you need enough compost to spread a layer 1 to 2 inches deep. That works out to about one cubic yard for every 160 to 320 square feet of bed. Lawns need far less, because a top dressing is only ¼ to ½ inch thick.
The exact amount depends on three things: the area you are covering, the depth you want and whether you are spreading compost on top of the soil or mixing it in. If you already have your measurements, the compost calculator works out cubic yards, cubic feet or litres, bags and cost for you. This guide shows how to do the same sum by hand, with worked examples for beds, raised beds, lawns and containers.
What You Need Before You Calculate
- The area you want to cover, in square feet or square metres.
- The depth of compost, in inches or centimetres. This depends on the job, as shown below.
- How you will use it: spread as a layer on top, or mixed into soil or potting mix at a set ratio.
- How it is sold near you: by the bag (check the volume on the label) or in bulk by the cubic yard or cubic metre.
How to Measure Your Garden Area
Measure only the beds, borders or lawn you plan to treat, not the whole yard. For a rectangle, multiply the length by the width. A bed 12 ft long and 4 ft wide covers 48 square feet.
Irregular and kidney-shaped beds
Split the bed into simple shapes, such as rectangles and half circles, work out each one and add them together. For a long curved bed, measure the length along the middle, take the average of several widths and multiply the two. Another quick method is to lay a grid of 1 ft squares over a sketch of the bed and count the squares, counting half-filled squares as half.
For a round bed, the area is 3.14 × radius × radius, where the radius is half the distance across. To switch between units, one square metre is about 10.76 square feet.
The Formula for Compost Volume
Compost volume is the area multiplied by the depth. Because area is usually in square feet and depth in inches, divide the depth by 12 to turn it into feet.
- Cubic feet = area in square feet × depth in inches ÷ 12
- Cubic yards = cubic feet ÷ 27 (a cubic yard is 3 ft × 3 ft × 3 ft)
- Shortcut: cubic yards = area in square feet × depth in inches ÷ 324
- Metric: cubic metres = area in m² × depth in cm ÷ 100, and one cubic metre is 1,000 litres
Step by step
- Measure the length and width of each area in feet.
- Multiply them to get square feet, and add up all the areas.
- Choose a depth in inches for the job.
- Multiply the area by the depth and divide by 12 to get cubic feet.
- Divide by 27 for cubic yards, or by the bag size for the number of bags, and round up.
Worked example: a 12 × 4 ft bed at 3 inches
Area: 12 × 4 = 48 square feet. Volume: 48 × 3 ÷ 12 = 12 cubic feet. In cubic yards: 12 ÷ 27 = 0.44 cubic yards. In bags: 12 ÷ 1.5 = 8 bags of 1.5 cu ft, or 12 ÷ 2 = 6 bags of 2 cu ft.
Worked example in metric: a 3 × 1.2 m bed at 5 cm
Area: 3 × 1.2 = 3.6 m². Volume: 3.6 × 5 ÷ 100 = 0.18 m³, which is 180 litres. That is 4 bags of 50 litres, or 5 bags of 40 litres.
To check your own numbers, enter them in the compost calculator, which handles feet, inches, metres and centimetres and rounds bags up for you.
How Deep Should Compost Be?
There is no single correct depth. It depends on what you are doing, the state of your soil and how rich the compost is. These are common starting points:
| Job | Typical depth | How it is used |
|---|---|---|
| Topping up garden or raised beds | About 1 in (2.5 cm) | Spread on the surface each year or season |
| Vegetable gardens and new or poor beds | 1 to 2 in (2.5 to 5 cm) | Spread on top or forked into the top layer |
| New lawn before seeding or turf | 1 to 2 in (2.5 to 5 cm) | Worked into the top 6 to 8 in (15 to 20 cm) |
| Established lawn | ¼ to ½ in (0.6 to 1.3 cm) | Top dressing, ideally after aerating |
| No-dig beds | Several inches to start a new bed, then about 1 in a year | Laid on the surface, not dug in |
Richer composts, such as those made mainly from manure, are usually used more lightly. In both raised and in-ground beds, each inch of compost needs about 0.08 cubic feet (roughly 2.4 litres) per square foot.
Will compost stop weeds?
Not on its own. Weed seeds can germinate in compost just as they do in soil. If weed control is the goal, spread a mulch layer over the compost. Our guide to how much mulch you need covers mulch depths.
How Much Compost for Common Projects
A 4 × 8 ft raised bed
The bed covers 32 square feet. A 1 inch top-up takes 2.7 cubic feet, which is 2 bags of 1.5 or 2 cu ft. A 2 inch layer takes 5.3 cubic feet (0.2 cubic yards), which is 4 bags of 1.5 cu ft or 3 bags of 2 cu ft.
Filling a new bed is different, because you need a full growing mix rather than a layer. A 4 × 8 ft bed 12 inches deep holds 32 cubic feet. At a common mix of one part compost to three parts soil, that is 8 cubic feet of compost (6 bags of 1.5 cu ft) and 24 cubic feet of topsoil or garden soil.
A vegetable garden or community plot
A 10 × 20 ft plot covers 200 square feet. At 2 inches deep, that is 33.3 cubic feet, or 1.23 cubic yards. In bags, it is 23 bags of 1.5 cu ft, so for a plot this size a bulk delivery is often easier to handle.
Top dressing a lawn
For every 1,000 square feet, a ¼ inch top dressing takes 20.8 cubic feet (0.77 cubic yards) and a ½ inch layer takes 41.7 cubic feet (1.54 cubic yards). Spread it after aerating so it settles into the holes, and rake it in so the grass is not smothered.
Larger areas and small farms
A quarter acre is 10,890 square feet. At ¼ inch that needs about 8.4 cubic yards, and at 1 inch about 33.6 cubic yards. A full acre (43,560 square feet) needs about 33.6 cubic yards at ¼ inch and 134 cubic yards at 1 inch. At this scale compost is usually bought by the truckload, and some suppliers sell it by weight.
Containers and balcony gardens
In pots you mix compost into potting mix by volume rather than spreading a layer. A common mix is one part compost to three parts potting mix. A 5 gallon container holds about 19 litres (0.67 cubic feet), so it needs about 4.7 litres, roughly 1.25 gallons, of compost. For a balcony full of pots, add up the volume of each container. The potting mix calculator works out pot volumes for round, square and tapered pots.
Planting fruit trees
Many extension services advise backfilling a planting hole with the soil you dug out rather than a rich compost mix, so the roots spread into the surrounding ground. Use little or no compost in the hole, and spread a thin layer on the surface around the tree instead.
Converting Volume, Bags and Weight
| Unit | Equals |
|---|---|
| 1 cubic yard | 27 cubic feet, about 765 litres |
| 1 cubic metre | 1,000 litres, 35.3 cubic feet, 1.31 cubic yards |
| 1 cubic foot | about 28.3 litres |
| 1 cubic yard in bags | 27 bags of 1 cu ft, 18 bags of 1.5 cu ft or 13.5 bags of 2 cu ft |
| 1 cubic metre in bags | 20 bags of 50 L or 25 bags of 40 L |
How many 40 lb bags are in a cubic yard?
Bags sold by weight vary in volume, because damp compost is heavier than dry compost. Our compost calculator estimates about 1,020 lb per cubic yard, which works out to roughly 25 to 26 bags of 40 lb. If the bag lists a volume, use that instead, as it is more reliable for working out coverage.
How much does a cubic yard of compost weigh?
A typical estimate is around 1,000 lb, roughly half a US ton, but real weight depends on the material and especially on moisture. Wet compost can be much heavier, so ask your supplier for the weight per yard before loading a pickup or trailer, and check your vehicle’s payload limit.
Buying Compost: Bulk or Bags
Bags are easy to carry and suit small beds, top-ups and containers. Bulk compost, sold by the cubic yard or cubic metre, usually suits larger jobs. To compare fairly, multiply the bag price by the number of bags in a cubic yard (18 bags of 1.5 cu ft, for example) and compare that with the bulk price plus delivery. Delivery charges and minimum orders vary by supplier, so get a quote before deciding.
Making your own compost costs little, but a home bin produces a modest amount and takes months, so most people buy compost for big projects and use homemade compost for top-ups.
Spreading and storing compost
For a bulk delivery, have it tipped onto a tarp close to the beds and move it with a wheelbarrow, a shovel or fork and a rake. Several cubic yards is a lot of loads, so plan for help or spread the work over a few days. If you have compost left over, keep it covered with a tarp or in sealed bags so it does not dry out or wash away, and use it for your next top-up.
Settling and Ordering Extra
Loose compost settles once it is watered and walked on, and organic matter keeps breaking down over the following months. How much it shrinks depends on the material, so there is no fixed figure. Our compost calculator gives the exact volume for an even layer and rounds bags up, but it does not add extra. Round up to the next whole bag, or the next half yard for bulk, and check the depth once the compost has settled to guide next year’s order.
Common Mistakes to Avoid
- Measuring the whole yard instead of just the beds or lawn you are treating.
- Mixing units, such as multiplying square feet by inches without dividing by 12.
- Confusing area with volume: a cubic yard covers very different areas at different depths.
- Piling compost against plant stems or tree trunks.
- Using a lot every year without checking. Heavy, repeated applications can build up nutrients such as phosphorus and salts, so a soil test every few years shows whether you can cut back.
Frequently Asked Questions
Is compost the same as topsoil?
No. Topsoil is the bulk material for filling beds and levelling ground, while compost is a soil improver mixed into it or spread on top. If you are filling a bed, work out the topsoil first, then add compost as a layer or as part of the mix.
How do compost and mulch amounts differ?
Compost is usually applied thinly, around 1 to 2 inches in beds, to feed the soil. Mulch sits on the surface to hold moisture and block weeds, so it goes on thicker, often 2 to 3 inches. Many gardeners use both, with compost first and mulch on top, including for winter protection.
When is the best time to add compost?
Garden beds are commonly topped up in spring before planting or in autumn after clearing them, which gives the compost time to settle in. Lawns are usually top-dressed during the growing season, right after aerating.
Work Out Your Own Compost Order
Measure each area, choose a depth for each job and decide whether you are spreading or mixing. Then calculate how much compost you need in cubic yards, cubic feet or litres, with bags and cost, before you place your order.
About the author
Haali is the creator of Place Calculator. With a BSc in Chemistry, Physics and Mathematics, an MBA in Marketing and a master’s degree in International Business and Sales Management, Haali combines science training with hands-on purchasing experience to build calculators that help people order the right amount of material. Haali’s articles on agriculture have been published by AG Morning Clips and Wikifarmer.