Soil Calculator

The arithmetic

A rectangular bed is length × width × depth. A round one is π × diameter² ÷ 4 × depth. Multiply by the number of identical beds, and divide cubic feet by 27 for cubic yards.

The order is that volume × (1 + settling allowance ÷ 100). The bags are the order ÷ the bag’s volume in cubic feet, rounded up to a whole bag. A division that comes out exact is not rounded up.

Depth is entered in inches and converted, so 12 in is 1 ft in the working. Multiplying feet by inches is the mistake this catches.

Depth: what extension services say

The calculator takes the depth you enter and recommends none. Enter the depth of soil you want, not the height of the frame. Iowa State University Extension and Outreach, on filling a raised bed: “Allow space for 1 to 2 inches of mulch on top of the soil.”

Extension services give depths for particular situations. Each is quoted below with the situation it covers. None of them is built into the calculator.

• University of Maryland Extension, for beds on a hard surface such as a driveway: “a minimum of 8 inches deep for leafy greens, beans, and cucumbers, and 12-24 inches deep for pepper, tomato, and squash”.
• Washington State University Extension (FS075E), for beds built because there is no soil, with the fill laid directly on the existing surface: “you will want beds at least 12 to 18 inches deep, because the entire root zone of the plants will be in the fill material”.
• Utah State University Extension, on the height of the bed box rather than the depth of soil: “The raised bed box should be at least 6 to 12 inches high to accommodate the rooting depth of most vegetables.” It adds: “Raised bed gardens shallower than 12 inches deep should be constructed with no bottom to allow plant roots access to existing soil below the box.” The calculator shows both sentences when the soil depth entered is under 6 in.
• Rutgers NJAES (FS1328), for imported soil layered over contaminated on-site soil: “a minimum 12" depth of imported topsoil is recommended”.
• Iowa State University Extension and Outreach, for elevated and tabletop gardens (its suggested height is 30 to 33 inches to the top of the bed), not for beds on the ground: “Most plants need at least 6 to 12 inches of soil for good rooting and growth.”

Settling: why the allowance is yours

The extension publications below say freshly filled beds settle, and none of them gives a percentage. University of Illinois Extension quotes horticulture educator Chris Enroth: “Settling soil is simply a fact of raised bed gardening.” Washington State University’s FS075E: “The settling of fill material and decay of organic matter will reduce the depth of soil in raised beds.” Iowa State: “Often, the soil will settle after adding it to the bed”. Oregon State University’s EM 9308: “Expect your newly filled beds to sink over time as the compost in the mix further decomposes.” University of Minnesota Extension says it of raised ground beds (mounds, not framed beds): “As the season progresses, the soil will settle, but the mound will remain.”

In the extension sources, two figures turned up, both answers on Ask Extension, the Extension Foundation’s question service. UNH Cooperative Extension’s Yard and Garden Infoline, answering one reader about new 18 in raised beds (April 28, 2025): “I would anticipate a couple of inches of settling per year.” That is a depth per year, not a percentage. A volunteer Master Gardener in Hennepin County, Minnesota, answering a question about filling a 20 in bed, asked June 25, 2022: “As a guideline, your box will settle 25%.” He gives no source or method, and describes himself as not a degreed or certified horticulturist. It is the only percentage found in the extension sources reviewed, and it is not used as a default. Two garden retailers’ soil calculator pages also print a percentage, and neither gives a source for it. Frame It All: “Quantities include a 10% settling allowance to account for natural soil compaction over time.” Eartheasy: “fresh soil and compost settles 10–15% after the first few waterings.”

So the settling allowance starts empty, and the figure you enter is yours. It is measured on the finished volume: the order is the volume × (1 + allowance). If you expect to lose h inches from a finished depth of D inches, the allowance is 100 × h ÷ D percent; to show the arithmetic only, 1 in over 12 in is 8.3%. A figure given as a share of the depth first filled converts differently: losing p percent of the filled depth needs an allowance of 100 × p ÷ (100 − p) percent.

Bags: how they are labelled

Enter the volume printed on the bag, in cubic feet, dry quarts or litres. The calculator converts it to cubic feet and divides the order, settling allowance included, by it. No bag size is assumed.

A quart on a bag of soil should be a dry quart. NIST Handbook 130 (2026) contains the model Uniform Packaging and Labeling Regulation (IV.A), written for states to adopt. It says the quantity of a dry commodity is declared in dry measure, unless a firmly established trade custom uses other terms (section 6.4 and Table 6.4). It puts dry measure in terms of the “U.S. bushel of 2150.42 in3, or peck, dry quart, and dry pint subdivisions of the bushel” (Table 6.7). And it requires: “Whenever the declaration of quantity is in terms of the dry pint or dry quart, the declaration shall include the word ‘dry.’” (6.7.2).

NIST Handbook 44 (2026), Appendix C, gives the bushel as “2 150.42 cubic inches (exactly)” and 32 quarts. A dry quart is therefore 2,150.42 ÷ 32 = 67.200625 cubic inches, which its table prints as “67.200 625”; the same table gives 1 cubic foot as “25.714 05” dry quarts. The calculator uses that definition. A liquid quart is 57.75 cubic inches, so reading a dry-quart bag as liquid quarts makes the bag count 16.4% too high.

NIST Handbook 133 (2026), the procedures for testing packages, prints the same equivalence rounded, “28.3 L (1 cu ft or 25.7 dry qt)” (Table 3-4). For mulch and soil labelled by volume it applies a maximum allowable variation of “5 % of the declared quantity” to individual packages (3.10.3), and the sample as a whole must still meet the handbook’s average requirement. The bag count here assumes each bag holds the volume on its label.

Mix: not split here

The calculator returns one volume and does not divide it into topsoil, compost and other parts, because the extension sources reviewed give different mixes. University of Minnesota Extension: “around 2/3 to 1/2 topsoil and 1/2 to 1/3 plant-based compost”. Iowa State: “equal parts topsoil, organic matter (well-rotted manure, compost, or peat), and coarse sand”, or “~70% topsoil and ~30% compost”. Clemson’s HGIC 1257 puts compost “in a range of 10-20% of the total volume”. For your own recipe, multiply the volume by each share.

Why tons need a density from you

Bagged and bulk blends differ, and moisture moves the weight. The density field stays empty and takes the supplier figure for the exact mix.

Sources quoted on this page

Each was read on 2026-09-29. Quotations are exact, and each is given with the situation it applies to.

• University of Maryland Extension, “Soil to Fill Raised Beds”, by Jon Traunfeld, Spring 2019, updated November 6, 2024 (extension.umd.edu).
• Washington State University Extension, FS075E, “Raised Beds: Deciding If They Benefit Your Vegetable Garden”, by Craig Cogger, May 2012 (PDF, s3.wp.wsu.edu).
• Utah State University Extension, “Raised Bed Gardening”, February 2012 (extension.usu.edu).
• Rutgers NJAES Cooperative Extension, FS1328, “Soil for Raised Beds”, by Stephanie Murphy, September 2025 (njaes.rutgers.edu).
• Iowa State University Extension and Outreach, “Creating Raised Bed Planters”, by Richard Jauron and Aaron Steil, last reviewed March 2024 (yardandgarden.extension.iastate.edu).
• University of Illinois Extension, “Refreshing raised bed soil generates exceptional results”, news release, April 5, 2021 (extension.illinois.edu).
• Oregon State University Extension Service, EM 9308, “How to use compost in gardens and landscapes”, February 2021, reviewed 2025 (extension.oregonstate.edu).
• University of Minnesota Extension, “Raised bed gardens”, by Natalie Hoidal, Carl Rosen, Troy Salzer and Freya Thurmer, reviewed in 2026 (extension.umn.edu).
• Clemson Cooperative Extension, HGIC 1257, “Raised Beds”, revised August 1, 2017 (hgic.clemson.edu).
• Ask Extension #898824, “filling raised beds”, answered by the UNH Cooperative Extension Yard and Garden Infoline, April 28, 2025 (ask.extension.org).
• Ask Extension #797788, “Filling Organic Raised Garden Bed”, answered by a volunteer Master Gardener, Hennepin County, Minnesota; question asked June 25, 2022 (ask.extension.org).
• Frame It All, “Raised Garden Bed Soil Calculator”, a garden retailer’s calculator page, quoted only for its settling figure (frameitall.com).
• Eartheasy, “Soil Calculator: How Much Soil for a Raised Bed?”, a garden retailer’s calculator page, quoted only for its settling figure (eartheasy.com).
• NIST Handbook 44 (2026), Appendix C, “General Tables of Units of Measurement” (nist.gov).
• NIST Handbook 130 (2026), IV.A, “Uniform Packaging and Labeling Regulation”, sections 6.4, 6.7 and 6.7.2 (nist.gov).
• NIST Handbook 133 (2026), Chapter 3, section 3.10, “Mulch and Soils Labeled by Volume”, Table 3-4 and 3.10.3 (nist.gov).