Measuring a raised bed
Soil is sold by volume, so a bed is the area inside its frame times the height the soil will reach. Measure inside the boards, not round them. A round bed is its diameter, an L-shaped bed a main part and the arm that sticks out of it, and a tiered bed one line for each tier, each with the height of soil it holds over the tier below. Several beds of one size are one line with how many there are.
Worked example. One bed of 8 × 4 ft, filled 12 in deep.
- Soil: 8 ft × 4 ft × 12 in = 32 cu ft
- In cubic yards: 32 ÷ 27 = 1.19 cu yd
- In bags of 1.5 cu ft: 32 ÷ 1.5 = 21.33, rounded up to 22 bags
- In bags of 2 cu ft: 16 bags
Two such beds take 64 cu ft, which is 43 bags of 1.5 cu ft. The beds are added up before the bags are counted, so the last part-bag of one bed fills the other: counted bed by bed, the same soil would take 1 bag more.
One soil, or your own mix
Bagged raised bed soil comes in bags of 1.5 cu ft (Miracle-Gro) and of 2 cu ft (Kellogg); the calculator opens on the first and counts the other beside it. Type the size printed on yours.
To mix your own, Iowa State University Extension gives two recipes: about 70% topsoil and 30% compost, or equal parts of topsoil, organic matter (compost, well-rotted manure or peat) and coarse sand. Choose “A mix” and type the parts: 70 and 30, or 1, 1 and 1. For the example bed, 70 to 30 is 15 bags of topsoil and 7 of compost; bought loose, by the quarter yard, it is 1 cu yd of topsoil and 0.5 of compost. In equal parts it is 8 bags of each. Every part is counted in bags of the size you typed: change it to the size you buy.
The same source asks for topsoil free of soil-borne disease and contaminants, and for compost that is fully mature, and it adds the mix in layers of a few inches, each mixed into the soil below.
Pots and potting soil
A pot is measured across the top and by the depth of soil in it, and counted as a cylinder. A pot that narrows toward its base holds less than that, so the count leans to enough. Potting mix comes in bags of 8 to 50 qt (Miracle-Gro). The quart of such a bag is the US dry quart: the same mix sold by the litre comes in a bag of 8.8 L, which is 8 dry quarts. A cubic foot is 25.714 of them. Five pots of 12 in across, with 10 in of soil, take 84.1 qt: 6 bags of 16 qt.
Topping up a settled bed
Where the soil of a bed has sunk, choose “Top up” and type how far it is below where it should be. A drop of 2 in in the example bed is 5.33 cu ft: 4 bags of 1.5 cu ft.
Common mistakes
- Rounding bed by bed. Add the beds first: the calculator does, and rounds the bags once.
- Taking the height of the boards for the height of the soil. If the soil will stop below the top of the frame, type where it stops.
- Buying bags for a large bed without looking at bulk. A cubic yard is 18 bags of 1.5 cu ft: past a yard or two, ask a supplier the price of loose soil.
Questions about raised bed soil
How much soil do I need for a 4 × 8 raised bed?
Filled 12 in deep, 32 cu ft, or 1.19 cu yd: 22 bags of 1.5 cu ft, or 16 of 2 cu ft.
How many bags of soil does a raised bed take?
Its length times its width times the height of soil, divided by the size of a bag, rounded up. Several beds are added before the division.
What is a good soil mix for a raised bed?
Iowa State University Extension gives about 70% topsoil and 30% compost, or equal parts of topsoil, organic matter and coarse sand.
How many quarts of potting soil are in a cubic foot?
25.714 US dry quarts, the quart a bag of potting mix is counted in.
How much potting soil does a pot need?
Its area across the top times the depth of soil: 84.1 qt for five pots of 12 in with 10 in of soil.
Sources
- Scotts Miracle-Gro, Miracle-Gro Organic Raised Bed & Garden Soil. Sold in bags of 1.5 cu ft (40 qt). Read on .
- Kellogg Garden Organics, Raised Bed & Potting Mix. Sold in bags of 2.0 cu ft (56.6 L). One bag fills a raised bed of 4 sq ft about 6 in deep. Read on .
- Scotts Miracle-Gro, Miracle-Gro Potting Mix. Sold in bags of 8, 16, 25 and 50 qt, and of 1 and 2 cu ft. Read on .
- Terra Greenhouses, Miracle-Gro Potting Mix 0.21-0.11-0.16 8.8 L. Miracle-Gro Potting Mix in a bag of 8.8 L (0.31 cu ft), which fills about two containers of 20 cm (8 in). Read on .
- Iowa State University Extension and Outreach, What would be a good soil mix for a raised bed? (Yard and Garden, updated 20 April 2026). Raised bed soils should be light and well drained. Equal parts topsoil, organic matter (well-rotted manure, compost or peat) and coarse sand make an excellent mix; another option is about 70% topsoil and 30% compost. Check that imported topsoil is free of pathogens and contaminants, and that compost is mature. Add the mix in layers of a few inches, mixing each into the soil below. Read on .
- National Institute of Standards and Technology, Guide to the SI (Special Publication 811), Appendix B.9: conversion factors. A foot is 0.3048 m and an inch 2.54 cm, exactly. A cubic foot is 0.02831685 m³, a cubic yard 0.7645549 m³, a US gallon 3.785412 L, a square foot 0.09290304 m² and an acre 4,046.873 m². A US dry quart is 1.101221 L. Read on .
Updated . How we calculate. Found a mistake? Tell us.