Raised Bed Soil Calculator

Why raised bed soil is sold by volume, not by weight

A bag of soil is a bag of volume. The number on the front, 1 cubic foot or 2 cubic feet, tells you how much space it fills, and that is the only figure you need to match against your bed. Weight is a different question and it changes with moisture: the same 2 cubic foot bag weighs about 40 pounds dry and 60 pounds after a rain.

So the job is to work out the volume of the box you are filling. A raised bed is a rectangular box, and the volume of a box is length times width times height. The catch is that beds are measured in feet and soil depth is almost always given in inches, so one unit has to be converted before anything else happens.

Divide the depth in inches by 12 and you have feet. Ten inches is 0.833 feet. Twelve inches is 1 foot. Six inches is 0.5 feet. Do that first, then multiply, and the answer comes out in cubic feet, which is the unit printed on every bag at the garden center.

One more unit shows up when you price bulk soil: the cubic yard. A cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet. Delivery is priced by the yard, so the last step for any big order is dividing cubic feet by 27.

The formula, and a bed worked through by hand

The whole calculation is length x width x (depth in inches / 12) x number of beds. Run it on the most common size sold, a 4 by 8 foot bed filled 10 inches deep.

• Area: 4 x 8 = 32 square feet
• Depth in feet: 10 / 12 = 0.833 feet
• Volume: 32 x 0.833 = 26.7 cubic feet
• With 10% for settling: 26.7 x 1.10 = 29.3 cubic feet
• Bags of 2 cubic feet: 29.3 / 2 = 14.7, rounded up to 15 bags
• Cost at $12.98 a bag: $194.70, or $6.64 per cubic foot

Depth is the input that moves the answer most. The same 4 by 8 frame at 6 inches needs 16 cubic feet, at 10 inches it needs 26.7, and at 12 inches it needs 32. Going from 6 to 12 inches doubles the soil, which is why the depth decision belongs before the lumber purchase, not after.

Those bag counts are before the settling allowance. Add 10% and a 4 by 8 bed goes from 14 bags to 15, and a 4 by 16 bed goes from 27 to 30. Rounding up is not waste, because the leftover bag gets used on the second season top dress.

How deep each crop needs to be

Depth is a root question, not a volume question. Shallow rooted crops are happy in 6 inches, and deep rooted ones need a foot or more before the soil stops constraining them. The numbers below are the ranges most extension services publish, applied to a 4 by 8 bed.

A bed shallower than the crop needs does not fail outright, it just caps the harvest. Carrots grown in 6 inches of soil come out short and fork at the tip where they hit the bottom of the frame. Tomatoes in 6 inches survive but need watering twice a day in July, because a shallow root zone holds little water.

The University of Minnesota Extension's guide to raised bed gardens puts the practical minimum at 6 inches for greens and 12 inches for anything with a taproot, and notes that deeper beds dry out slower and need less attention in August.

Bag sizes: how the count changes and what it costs

Bagged soil comes in 0.75, 1, 1.5, 2 and 3 cubic foot bags, and the price per bag does not scale with the size. The only fair comparison is cost per cubic foot, which is the price divided by the volume.

The spread here is 11%, so on a single bed the bag size is mostly a lifting question rather than a money question. A 3 cubic foot bag weighs 100 to 130 pounds and is awkward to carry up steps. A 1 cubic foot bag weighs 35 to 45 pounds and one person can move 30 of them in a few trips.

Where bag size does matter is the leftover. Buy 15 bags of 2 cubic feet and you use 14.7 of them, leaving a third of a bag. Buy 30 bags of 1 cubic foot and you use 29.3, leaving most of a bag for the top dress. Either way the extra is not lost, but a partly used 3 cubic foot bag is a lot of soil to store.

Watch the quart labeling as well. A 20 quart bag is 0.75 cubic feet, not 1 cubic foot, and the two sit next to each other on the same shelf at similar prices.

The 10% settling allowance, and why beds sink

Freshly filled soil is full of air. Rain packs it, watering packs it, and the organic matter in the mix breaks down and loses volume over the first season. The result is a bed that was level with the top of the frame in April sitting 2 or 3 inches low by October. The 10% allowance covers that drop.

Ten percent of 26.7 cubic feet is 2.7 cubic feet, which is one and a third bags. That is cheap insurance against a bed that needs a second trip to the store in the middle of the growing season. Beds filled with a fluffy compost heavy mix settle more, up to 15%, while a mineral topsoil blend settles closer to 5%.

Two things make the settling worse. Filling the bed with dry mix and never watering it in leaves the air pockets in place, so the first hard rain drops the level all at once. And mixing in uncomposted material, like fresh wood chips or leaves, causes a second drop later as it decomposes underground.

The fix is to fill in two stages. Add soil to about 2 inches above the final level, water it slowly until it stops draining fast, then top up to the level you want. Leave 1 to 2 inches of freeboard below the top of the frame so mulch and water stay in the bed instead of running off.

Mixing your own blend instead of buying a bag

Bagged raised bed mix is convenient and consistent. Mixing your own is cheaper by the cubic foot and lets you tune drainage, which matters for beds on a patio or over clay. The common starting point is 60% topsoil, 30% compost and 10% coarse material such as pine bark fines or coarse sand.

Do not fill a bed with compost alone. It holds water like a sponge, shrinks by a third in a season, and packs down into a dense layer that roots struggle to push through. A bed that is all compost also dries into a hard crust on the surface, which sheds water instead of absorbing it.

The EPA's page on composting at home covers what finished compost should look and smell like before it goes into a bed, and Michigan State Extension has a raised bed resource page with soil mix ratios for different situations. Both are worth ten minutes before you buy a truckload of anything.

Bulk by the cubic yard, and when it beats bags

Bulk soil is sold by the cubic yard, and 1 cubic yard is 27 cubic feet. Our 29.3 cubic foot bed is 1.09 cubic yards, so ordering 1.25 yards covers it with a little spare. The arithmetic is the same as bags, only the divisor changes.

Price is where bulk wins. Bagged soil at $6.49 per cubic foot is $175 per cubic yard. Bulk garden mix runs $30 to $50 per cubic yard, plus a delivery fee of $50 to $120 depending on distance. The break even point is around 1 cubic yard: at that volume bulk plus delivery already beats bagged soil, and above it the gap widens fast.

Three practical notes on a bulk order. Soil is heavy, roughly 1.1 to 1.3 tons per cubic yard, so a 3 yard delivery is over 3 tons and needs a firm driveway, not a lawn. A dump truck cannot always place the pile where you want it, so plan for wheelbarrow trips. And bulk soil is not sterilized the way some bagged mixes are, so weed seeds are part of the deal.

For one or two beds, bags win on convenience. For a full garden build, four or more beds, or a bed deeper than 12 inches, price out bulk before you load a cart.

Filling the bed without wrecking your back

Fifteen bags at 80 to 100 pounds each is 1,200 to 1,500 pounds of material, and it all has to go over the side of a frame. A few habits make the job easier and safer.

• Stage the bags next to the bed before you open any of them, so you are not carrying weight across the yard.
• Cut the bag flat and slide the soil out rather than scooping. A 2 cubic foot bag emptied this way takes about 20 seconds.
• Fill in thirds and water between layers. Water settles each layer so the next one sits on a firm base.
• Keep the wheelbarrow inside the path, not on the bed edges, because a loaded barrow concentrates weight on one point of the frame.
• Leave 1 to 2 inches of freeboard so mulch and rain stay in the bed.

After the last bag, water slowly until the soil stops pulling away from the sides of the frame. Then wait a week and check the level again before planting. If it has dropped more than an inch, top it up with the spare bag and water again.

Soil volumes here are geometric estimates. Bagged products settle at different rates and some mixes are sold by weight rather than volume, so check the bag before you buy a large order.