Raised Bed Soil Calculator
Enter your bed in feet and inches and get cubic feet, cubic yards, and the exact number of bags to buy at every common bag size. Add as many beds as you like for a running project total.
Your beds
Fresh mixes settle in the first season, mostly as the compost and peat fraction breaks down and the whole thing gets rained on. Adding 10 percent now is cheaper than buying one more bag in June. The headline figure above stays the plain box volume so you can always see the raw number.
How this is calculated
Volume is length times width times depth, in feet. Inches are converted first, which is where most calculators quietly go wrong. A bed entered as 12 feet 6 inches must become 12.5 feet, never 126. This tool echoes back what it understood from every measurement, so a misread number cannot slip past you.
A 4 foot by 8 foot bed filled 12 inches deep is 4 x 8 x 1 = 32 cubic feet. A cubic yard is 3 by 3 by 3 feet, which is 27 cubic feet, so that same bed is 32 / 27 = 1.19 cubic yards.
Bags are always rounded up, and the leftover is shown separately. You cannot buy 21.3 bags. At 1.5 cubic feet per bag, 32 cubic feet needs 22 bags, which gives you 33 cubic feet and 1 cubic foot spare. Every bag size is shown at once rather than hidden behind a dropdown, because the cheapest option is usually whichever size your store actually has in stock.
Measure the inside of the bed, not the outside. Two inch boards on a 4 by 8 frame take roughly 4 inches off each dimension, which is about 2 cubic feet on a 12 inch deep bed. If your bed sits on open ground rather than a hard surface, subtract a little more, because the first inch or two settles down into the soil beneath.
The settling allowance and the mix ratio are both shown as their own lines rather than folded into the headline, so you can always see the plain box volume underneath.
Soil volume for common bed sizes
Cubic feet needed, by bed size and fill depth. This is the same arithmetic the calculator runs, so the two can never disagree.
| Bed size | 6 in deep | 8 in | 10 in | 12 in | 18 in | 24 in | Cu yd at 12 in |
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Keep planning
Once the bed is full, these decide what goes in it and when.
Common questions
How deep does a raised bed actually need to be?
Six inches over open ground grows most annual vegetables, because roots carry on down into the soil beneath. Eight to twelve inches is the comfortable standard and is what most kits are. Twelve inches becomes the minimum rather than a nicety if the bed sits on concrete, decking or heavy compacted clay that roots cannot penetrate, and deep-rooted crops such as carrots, parsnips and potatoes want that depth wherever the bed sits.
When is bulk delivery cheaper than bags?
Usually above about one cubic yard, which is 27 cubic feet, or roughly one 4 by 8 bed filled 10 inches deep. A cubic yard bought in 1.5 cubic foot bags is 18 bags, and bagged mix generally costs several times as much per cubic foot as the same material in bulk. Even after a delivery fee the bulk load tends to win at that point, and the gap widens fast as the project grows. Below a cubic yard the delivery fee usually swallows the saving, and bags are also easier if you have no driveway to dump on. Use the cost comparison in the result panel with your own local prices, because bulk pricing varies by region far too much for any table to be honest about.
What is the classic raised bed soil mix?
One third compost, one third peat moss or coconut coir, and one third coarse vermiculite, all measured by volume. It comes from the square foot gardening method and it is genuinely good: light, water retentive, and free of weed seed. It is also expensive at any real depth, and vermiculite in particular is not always easy to find in the coarse grade. For beds deeper than about 8 inches many people fill the lower portion with a cheaper topsoil and compost blend and reserve the classic mix for the top layer.
Peat or coir?
Both work. Peat is cheaper in most of the United States and slightly acidic, which suits most vegetables. Coir is a coconut processing byproduct, holds water at least as well, rewets far more easily once dried out, and avoids the peat bog harvesting question that some gardeners care about. The practical catch with both is that they are sold compressed, so the volume on the bag is not what you will end up with once it is broken up and moistened. Check the bag for an expanded volume figure rather than assuming a multiplier, since it varies a lot by product.
Can I just fill the bottom with logs, branches or cardboard?
You can, and the technique has a name, hugelkultur. It genuinely saves money and holds water well. Be honest about the trade: as that woody material breaks down the bed sinks, sometimes dramatically, and you will be topping it up every year for several years. It also ties up nitrogen at the wood and soil boundary while it decays. It works best in beds deeper than about 18 inches, where you can afford to lose the lower third to it. In a 10 inch bed it costs you most of your root zone.
Why does my new bed sink so much in the first year?
Because a fresh mix is full of air, and rain drives it out. Compost keeps decomposing and loses volume as it does. Peat and coir compress once wet. Losing an inch or two in the first season is completely normal and is not a sign you were sold bad soil. Plan for it by filling proportionally higher at the start, or budget one or two bags of compost as an annual top up, which the bed will want anyway for fertility.
Should I use bagged topsoil straight from the store?
Not on its own. Bagged topsoil varies enormously in quality between brands and can be heavy, poorly draining and close to sterile. It is fine as a bulking ingredient blended with compost, and cheap bagged topsoil is a common component of a deep-bed fill. It is a poor choice as the only thing in the bed, and it is a genuinely bad choice for containers.
How much compost should I add each year after that?
An inch or two spread over the surface annually is the usual advice, which is 3 to 7 cubic feet on a 4 by 8 bed. Set the bed depth in this calculator to 1 or 2 inches to get that figure for your own beds. You do not need to dig it in. Spread it on top and let the soil life pull it down, which is less work and better for the soil structure you have spent a season building.