The soil volume calculator above turns a bed, a border or a row of pots into a volume, a bag count and a delivered bulk price. It handles the four cases people actually buy soil for — a raised bed, a flat area being topdressed or made into a new border, a round container, and a rectangular planter — and it splits the answer into compost and topsoil so you can order the two products separately if you are blending them yourself.
Arb Digital publishes this as part of a free landscaping and construction estimating set. It exists because volume is the unit almost nobody has an intuition for. A bed that looks small swallows an alarming amount of mix, and the gap between what a bag holds and what a cubic yard holds is a factor of eighteen, which is exactly the sort of ratio that turns a Saturday project into three trips to the garden centre.
What This Soil Volume Calculator Does
It computes filled volume, adds a settlement allowance, converts that to bags and to bulk measure, splits it by your compost ratio, and then compares the price of buying bagged against buying bulk delivered. The four modes differ only in how the footprint is measured: three of them are length by width, and the round container mode uses the area of a circle from the diameter you enter.
Worth stating the boundary against the adjacent tools clearly, because these pages are easy to confuse. Our mulch calculator covers mulch volume over a flat area only — it has no raised bed mode and no container mode, and mulch is a surface layer rather than a growing medium. The compost calculator sizes compost as an amendment. The gravel calculator handles aggregate, where compaction rather than settlement is the correction that matters. This page is about the soil that plants actually root into, and about the bed and container geometry those other pages do not model.
How to Use It
- Choose units. Imperial reads feet for the footprint, inches for depth and cubic feet for bags. Metric reads metres, centimetres and litres.
- Pick what you are filling. The labels change with the mode, so a raised bed asks for bed length and width while a round pot asks for diameter.
- Enter the fill depth, not the frame height. These are different numbers and confusing them is the most common error on this calculation.
- Set the count. Four identical beds or twelve identical pots multiply straight through, so you only measure one.
- Set the settlement allowance and compost share. Then compare the bagged and bulk prices before deciding how to buy.
The Formula and How It's Calculated
Volume for a rectangular bed, planter or flat area is length × width × depth, with the depth converted into the same unit as the footprint. For a round container it is π × (diameter ÷ 2)² × depth. That figure is multiplied by the number of beds or pots, then by (1 + settlement ÷ 100). Bags to buy is ceil(volume ÷ bag size), and bulk measure is volume ÷ 27 for cubic yards or the volume itself for cubic metres.
A worked example that matches the published one: a bed 8 ft long, 4 ft wide and filled 10 in deep is 8 × 4 × (10 ÷ 12) = 26.67 cubic feet, which is close to one cubic yard. The University of Maryland Extension guide Soil to Fill Raised Beds and the University of Illinois Extension article Elevate your expectations with raised bed gardening both give this multiply-length-by-width-by-height method and both describe a 1:1 compost to topsoil or soilless mix blend, which is where the 50 percent default comes from. Add the 10 percent settlement allowance and the order becomes 29.3 cubic feet, or 20 bags at 1.5 cubic feet each. If you need the cubic yard figure on its own, the cubic yard calculator does that conversion directly.
Fill Depth Is Not Frame Height
This is the mistake worth its own section. A raised bed built from two courses of 6 inch boards is 12 inches tall. It does not need 12 inches of soil. You want an inch or two of freeboard below the rim so that watering and rain do not float mix over the edge, and if the bed sits on open ground rather than a hard surface, the plants root into the native soil below as well, so the fill depth is genuinely a fill depth rather than the whole root zone.
The arithmetic penalty is real. On an 8 by 4 ft bed, each inch of depth is 2.67 cubic feet — two full bags. Filling to 12 inches instead of 10 costs four more bags per bed, and across four beds that is sixteen bags of mix you did not need. Enter what you intend to fill, then look at the settled figure, which is what the bed will actually look like after a season. Illinois Extension notes that beds are commonly built 6 to 12 inches tall, and that the mix settles as organic matter decomposes and needs topping up annually.
Why Bulk Gets Cheaper So Quickly
The tool compares both prices because the crossover point surprises people. A cubic yard is 27 cubic feet, which is 18 bags at the common 1.5 cubic foot size. At $6 a bag that is $108 of bagged product against a bulk yard that might be $55 delivered. Bagged soil is roughly double the price per unit volume almost everywhere, and the premium buys convenience, a clean product and the ability to carry it through a house.
The honest counterweight is that bulk has a floor. Most yards will not deliver less than a cubic yard, many charge a flat delivery fee that makes a small order absurd, and the pile lands on your driveway and has to be barrowed and shifted the same day. The practical rule that falls out of the arithmetic: below about half a cubic yard, bags win on total cost once delivery is counted; above a full yard, bulk wins decisively; between the two it depends on the delivery charge and whether you have somewhere to put a pile.
Settlement, Compaction and What They Are Not
Settlement is not the same correction as compaction, and the two get muddled between garden and construction calculators. Bagged growing mixes are compressed in the bag, so they expand on opening and then subside again over weeks as the peat or coir rehydrates and the structure knits. Separately, organic matter decomposes, so the bed loses volume every season for real biological reasons. Those two effects are what the settlement allowance covers.
Aggregate behaves in the opposite direction: gravel and road base are ordered loose and compacted down to a smaller finished depth, which is why our aggregate pages apply a compaction factor rather than a settlement allowance. If you are building a bed on a base layer or laying a path alongside it, run the aggregate through the gravel calculator rather than this one, because using the wrong correction leaves you short in one case and over-ordered in the other.
Containers Need More Volume Than They Look Like They Do
A pot is a cylinder, and cylinder volume scales with the square of the diameter, so container maths runs away from intuition fast. A 24 inch pot filled 20 inches deep holds about 5.2 cubic feet — nearly four bags for one pot. A 12 inch pot at the same depth holds 1.3 cubic feet. Halving the diameter cut the volume to a quarter, not a half. If you are filling a mixed collection, run each size separately with its own count and add the results, or use the cylinder volume calculator for any awkward vessel that is not a simple pot.
Two container-specific adjustments are worth knowing. Tapered pots hold less than the straight-sided calculation suggests, so treating them as a cylinder of the top diameter over-orders by roughly 15 to 25 percent depending on the taper; use a diameter partway down the pot if you want a closer figure. And drainage material, broken crocks or a layer of coarse bark at the base displaces mix, which reduces the order but also reduces the root volume.
Blending Your Own Versus Buying Ready Mixed
The compost split exists because many gardeners buy two products and mix them rather than buying one pre-blended bagged mix. The extension guidance behind the 1:1 default is specific about context: Maryland's guide recommends compost with a soilless growing mix in a 1:1 ratio for beds on hard surfaces, adds that topsoil can make up to 20 percent by volume in beds at least 16 inches deep, and gives a target organic matter content of 25 to 50 percent by volume.
Set the compost share to whatever your own plan calls for and the tool splits the total for you, so you can place two separate orders in the right proportions. Set it to zero when you are buying a single ready-mixed product and the whole volume reports as one line. If you are also turfing part of the site, the sod calculator handles the turf order and the square footage calculator gives you the underlying area figures.
Arb Digital builds free calculators that earn search traffic for landscaping, garden centre and construction businesses. Browse the library, or tell us what your customers keep asking you to work out.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Using frame height as fill depth — you want freeboard below the rim, and every extra inch on a standard bed is two more bags.
- Mixing units inside one sum — feet for the footprint and inches for depth must be reconciled before multiplying, which is what the tool does for you.
- Ignoring settlement — a bed filled to the brim in spring will be visibly low by the following season.
- Treating a tapered pot as a straight cylinder — it over-orders by roughly a fifth on a strongly tapered container.
- Comparing bag price to bulk price directly — compare price per cubic foot including delivery, because eighteen bags make one cubic yard.
Related Free Tools From Arb Digital
Pair this with the mulch calculator for the surface layer, the compost calculator for amendments, the cubic yard calculator for bulk conversions, the sod calculator for turf and the cylinder volume calculator for unusual containers. The full free online tools hub lists every calculator we publish.
Frequently Asked Questions
Filled 10 inches deep it needs 26.67 cubic feet, which is just under one cubic yard. Adding a 10 percent settlement allowance takes it to about 29.3 cubic feet, or 20 bags at 1.5 cubic feet each.
A cubic yard is 27 cubic feet, so it is 18 bags at 1.5 cubic feet, 13.5 bags at 2 cubic feet, or 27 bags at 1 cubic foot. Bag sizes vary by brand, so check the label rather than assuming.
No. Leave an inch or two of freeboard below the rim so watering does not wash mix over the side. Enter the fill depth you intend rather than the height of the timber, because the difference is two bags per inch on a standard bed.
University of Maryland Extension describes filling a bed with compost and a soilless growing mix in a 1:1 ratio, with topsoil making up to 20 percent by volume in beds at least 16 inches deep. Local soil, climate and what you are growing all affect the right answer.
Bagged mixes are compressed and expand on opening, then subside as they rehydrate, and organic matter decomposes over the season. Both effects reduce the finished level, so ordering the raw geometric volume leaves the bed low.
Bulk is usually far cheaper per unit volume once you need a full cubic yard or more, but most suppliers have a minimum order and a delivery charge. Below about half a yard, bags typically win on total cost. The tool compares both prices for your actual volume.
Yes. Choose the round container mode for pots and the rectangular planter mode for troughs, enter one container's dimensions and set the count. Tapered pots hold less than the straight-sided figure, so expect to be over-ordered by roughly a fifth on those.
This tool produces material volume estimates only. Growing medium composition, drainage and suitability for particular plants should be confirmed with your supplier or a local extension service.
