Raised Bed Soil Calculator

How much soil do I need for a raised bed?

Measure the inside of the bed rather than the outside of its lumber or metal frame. Multiply the planting area by the depth you still need to fill. A rectangular 4-by-8-foot bed with a 12-inch soil depth holds 32 cubic feet before any settling allowance. If it already has 4 inches of usable, evenly spread soil, only the remaining 8 inches need new material: about 21.33 cubic feet. The calculator separates that physical space from the larger order you may choose to allow for settling.

For round beds, enter the inside diameter in the length field. The area is a circle, not a square with the same diameter. A 4-foot-diameter bed has about 12.57 square feet of area, compared with 16 square feet for a 4-foot square. Count multiple beds together only when they have the same dimensions and existing fill. For different beds, calculate each size separately, add their unrounded order volumes, and then round the combined purchase to your supplier's bag or delivery increments.

Formulas, conversions, and rounding

• Rectangular area: inside length multiplied by inside width.
• Round area: pi multiplied by half the inside diameter squared.
• Fill volume: area multiplied by (target depth minus existing depth), multiplied by number of beds. Convert all dimensions to matching length units first.
• Order volume: fill volume divided by (1 minus expected settling percentage / 100).
• Bag count: order volume divided by bag volume, rounded up to a whole bag. Bag cost is whole bags multiplied by price per bag.
• Bulk cost: order volume in the supplier's units multiplied by unit price, plus delivery. A zero-volume order has zero delivery cost.
• Mix shares: order volume multiplied by compost percentage; the remainder is the other growing medium. These are volume shares, not mass shares.

One cubic yard is 27 cubic feet. One cubic foot is exactly 28.316846592 liters, and one cubic meter is 1,000 liters. In US mode, bed length and width are feet, depth is inches, bags are cubic feet, and bulk price is per cubic yard. Metric mode uses meters, centimeters, liters, and price per cubic meter. Switching units preserves the physical measurements and converts the bulk price; all monetary entries remain US dollars.

A 10% settling assumption means the delivered material retains 90% of its loose volume. Dividing by 0.90 therefore adds about 11.11%, not exactly 10%. This is a planning model, not a prediction of your soil's behavior. With material that settles substantially, fill gradually and keep extra mix for later top-ups rather than piling everything above the bed's edge at once.

Worked example: a 4-by-8 bed, bags versus bulk

The default bed is 8 feet long, 4 feet wide, and 12 inches deep with no existing fill. Its geometric volume is 32 cubic feet. Allowing for 10% settling raises the order to 35.56 cubic feet, or 1.317 cubic yards. With 2-cubic-foot bags, buy 18 bags; they provide 36 cubic feet, leaving about 0.44 cubic feet beyond the adjusted order. At $6 per bag, the total is $108.

At $45 per cubic yard plus $60 delivery, the exact-volume bulk quote is approximately $119.26. Bags are about $11.26 cheaper in this example. At a one-third compost share, the adjusted order contains about 11.85 cubic feet of compost and 23.70 cubic feet of other growing medium. The price comparison assumes you are buying a complete blend; it does not price those ingredients separately. Change both quotes to comparable products before making a purchasing decision.

Choosing a mix without confusing volume and soil quality

The University of Maryland Extension discusses compost-and-topsoil combinations such as 1:2 or 1:1 for certain ground-based raised beds, and different approaches for beds on hard surfaces. The one-third compost example here is an editable quantity illustration, not a recipe for every crop or location. Existing soil, drainage, rooting depth, compost maturity, and the contents of a purchased growing mix all matter. Do not assume that more compost always produces better results.

For a bed with a manufactured blend that already contains compost, avoid counting its compost twice. Enter zero in the optional breakdown if you only need bags of a ready-to-use product. If buying ingredients separately, use the calculated volumes to request individual prices and round each ingredient to its own package size. A soil test and local extension guidance can help with fertility and pH; this calculator does not recommend fertilizer rates or evaluate contamination.

Buying checklist and limitations

Read the bag's volume label rather than comparing weight alone: damp material can weigh more without filling more space. Bulk suppliers may sell only in half-yard or full-yard increments, impose a minimum order, or charge extra for distance and unloading access. Our bulk estimate deliberately uses the exact adjusted volume. Ask for an actual quote before assuming that a displayed cost is what you will pay. Bag pickup, sales tax, labor, bed construction, and disposal of old soil are not included.

This tool assumes straight-sided beds and uniform existing soil depth. Tapered containers, sloped bottoms, irregular shapes, roots, and retained objects change the available volume. It does not calculate loads on decks, balconies, roofs, or bed walls. Wet growing medium can be heavy; obtain appropriate structural guidance before placing a filled bed on an elevated surface. The settling allowance is limited to 0-40% to keep this a moderate top-up model rather than a large decomposition simulation.

Frequently asked questions

• How many 2-cubic-foot bags fill a 4-by-8 raised bed? At 12 inches deep, 16 bags fill its 32-cubic-foot geometric volume. With the example's 10% settling assumption, the calculator recommends 18 bags instead.
• Can I calculate a top-up instead of a new bed? Yes. Enter the current uniform soil depth and your target depth. If they match, new fill, bags, and delivery are zero; a current depth above the target is an input error rather than a negative order.
• Does it include logs or gravel at the bottom? No separate layer is modeled. Only subtract a known, uniform retained depth that really displaces new mix. Decomposing material, irregular voids, and drainage layers need a more specific plan.
• Is bulk always cheaper? No. Delivery and order minimums can outweigh a lower unit price on a small project. Compare the same blend, obtain a local quote, and consider how you will move and store it.
• Why do mix shares not add up to whole bags? Shares use the adjusted order volume. The complete-blend bag count rounds separately. Buying individual ingredients requires rounding each ingredient to its own package size.
• Are measurements stored? The calculator runs in your browser and does not save or send these measurements to a calculation service. The site's standard advertising and analytics still follow the privacy policy.

Sources and further reading

• University of Maryland Extension: Soil to Fill Raised Beds explains site-dependent fill options, compost ratios, soil testing, and buying topsoil. Its guidance informs the discussion, not a guaranteed settling rate or local price.

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Last updated: 2026-10-07. Educational quantity and cost estimate; not a soil prescription, supplier quote, or structural assessment.