The Short Answer at Three Common Bed Heights
A 4 foot by 8 foot raised bed filled 12 inches deep takes about 32 cubic feet of soil, which is roughly 1.2 cubic yards. That is the number most people are looking for, so here it is first.
The formula behind it is short. Length in feet times width in feet times depth in feet gives you cubic feet. Divide cubic feet by 27 and you have cubic yards. Depth is the part people fumble, because bed heights are given in inches, so divide the inches by 12 before multiplying.
- 10 inches deep. 10 divided by 12 is 0.833 feet. 4 times 8 times 0.833 is about 26.7 cubic feet, just under 1 cubic yard.
- 12 inches deep. 4 times 8 times 1 is 32 cubic feet, about 1.19 cubic yards.
- 18 inches deep. 4 times 8 times 1.5 is 48 cubic feet, about 1.78 cubic yards.
One caveat worth knowing before you order. When a kit or a plan says 4x8, that is almost always the outside dimensions of the frame. If the bed is built from 2x lumber, the actual inside footprint is an inch and a half narrower on each side, so the true volume is a little less than the calculated figure. That is fine. You want to be slightly over rather than slightly under, and the reason why comes up later in this post.
Why the 27 in the Math Trips People Up
A cubic yard is a cube 3 feet on every side. Three times three times three is 27, so a cubic yard is 27 cubic feet. It is not 3 cubic feet. That single mix up is the most common ordering mistake in the whole category, and it goes both directions. People order nine times too little soil, or they order nine times too much and end up with a mountain on the driveway.
The reason the conversion comes up at all is that the industry uses two units for the same product. Bags are printed in cubic feet. Bulk soil from a landscape supply yard is sold by the cubic yard. You are not doing hard geometry here, you are translating between two measuring systems that nobody bothered to reconcile.
Two shortcuts make this easier to hold in your head.
First, one cubic yard of soil spread 12 inches deep covers about 27 square feet. A 4x8 bed is 32 square feet of surface. So at a 12 inch depth, a 4x8 bed is one yard plus a little extra. That matches the 1.19 yard figure above and gives you a way to sanity check any number a calculator spits out.
Second, for a 4x8 footprint specifically, every single inch of depth is about 2.7 cubic feet. That is the number to keep if your bed is an odd height. A 14 inch bed is 14 times 2.7, or roughly 38 cubic feet. A 16 inch bed is about 43 cubic feet. You can run any height in your head without touching a calculator.
One practical note on ordering bulk. Most supply yards do not sell in thirds of a yard. Half yard increments are common, full yards are the norm, and some minimums start at a full yard. Round up to whatever increment your supplier sells and plan on using the extra. Soil is not hard to find a home for.
Bag Counts, Where Bulk Starts Winning
Bag Sizes You Will Actually See
Bagged soil comes in a handful of standard volumes. The 40 pound bag you see stacked on pallets in spring is usually 0.75 cubic feet. Above that you will find 1 cubic foot, 1.5 cubic feet, and 2 cubic foot bags depending on the product and the store.
Read the volume on the bag, not the weight. Weight tells you almost nothing useful here, because a bag of damp topsoil and a bag of light peat based mix can weigh wildly different amounts at the same volume. Two identical bags of the same product can differ by several pounds depending on whether the pallet sat out in the rain. Volume is the honest number.
In cubic yard terms, here is what those sizes work out to. One cubic yard equals roughly 36 bags at 0.75 cubic feet, 27 bags at 1 cubic foot, 18 bags at 1.5 cubic feet, or 14 bags at 2 cubic feet.
Bag Count for a 4x8 Bed
Run those bag sizes against the three bed heights and the picture gets clear fast.
- 10 inch bed, about 27 cubic feet. Roughly 36 of the 0.75 cubic foot bags, 18 bags at 1.5 cubic feet, or 14 bags at 2 cubic feet.
- 12 inch bed, 32 cubic feet. About 43 of the 0.75 cubic foot bags, 32 one cubic foot bags, 22 bags at 1.5 cubic feet, or 16 bags at 2 cubic feet.
- 18 inch bed, 48 cubic feet. About 64 of the 0.75 cubic foot bags, 32 bags at 1.5 cubic feet, or 24 bags at 2 cubic feet.
Look at the 18 inch row for a second. Sixty four bags of soil is a pallet's worth. That is not a garden center errand, that is a moving job, and you are doing it with a flatbed cart and a car trunk.
The rough crossover point for most homeowners sits around two cubic yards. Under that, bags can still make sense. Past it, bulk delivery is usually the cheaper route, and the gap keeps widening as the order grows, because with bags you are paying for plastic, filling, palletizing and shelf space on every single unit. A single 12 inch 4x8 bed sits right on the line at about 1.2 yards. Two or three beds clear it without any argument.
Price is not the only reason to go bulk either. There is no plastic to break down and haul off, no shuttling three carloads across town, and no discovering that the store sold out of your mix when you are two thirds of the way through the fill. Matching soil later is harder than it sounds, because blends vary between batches and between suppliers.
Bags still win in specific situations. One shallow bed, no truck access down a narrow driveway, a freshly sealed driveway you do not want a pile sitting on, or a spring top off where you only need a few cubic feet. In those cases the convenience is worth the premium and nobody should feel bad about it.
Add a Settling Allowance Before You Order
Fresh raised bed mixes are fluffy. They are loaded with compost and peat or coconut coir, all of which trap air and hold their shape until water moves through them. The first deep watering closes a lot of that air space, and working the bed with a shovel or a rake closes more. Beds look noticeably lower a few weeks in and lower again by the end of the first season.
Compaction is only half of it. Organic matter in the mix is actively breaking down, and as it does, volume literally leaves the bed. That is not a defect. It is the same process that makes the soil good, and it is why gardeners add compost every year rather than filling once and walking away.
The fix at ordering time is simple. Buy a bit more than the calculated volume, not exactly the calculated volume. Rounding up to the next half yard covers the settling on a single bed in most cases.
At fill time, go all the way to the top rim rather than stopping an inch or two low. Water the bed in thoroughly, wait a day, then top it off before you plant anything. Doing that once at the start saves you from working around seedlings later.
After that, plan on adding an inch or two of compost across the surface each spring. That is standard maintenance, not evidence that you under filled the first time.
If you end up with leftover soil, it is easy to place. Fill containers, build up a low spot in the lawn, run it along a fence line, or bank it beside the bed for next year's top off. Being short is the worse problem, because you will be standing at the store trying to match a blend you cannot name.
How Deep the Bed Actually Needs to Be
Most vegetables do well with 12 to 18 inches of soil, which is exactly why those two heights dominate the math above. Inside that range you are giving nearly everything in a home garden room to work.
Shallow rooted crops are more forgiving. Lettuce and other greens are content in about 6 inches of soil, which is why low profile beds and even deep containers grow salad perfectly well. Carrots and other root crops want the deeper end so they are not hitting a hard stop and forking. Tomatoes also appreciate the depth, since they root aggressively and a cramped bed shows up as stress in midsummer heat.
What sits under the bed changes the answer, though. If the frame is set on open ground, roots can move down past the bottom board once they get there. A 10 inch frame over native soil that you loosened with a fork before building behaves like a deeper bed than the frame height suggests.
The opposite case is the one to watch. If your bed sits on a patio, a driveway, or landscape fabric over hard compacted ground, the frame height is the entire root zone. There is nowhere else for roots to go. In that situation, build toward 18 inches and do not talk yourself into 10.
Just know that height costs volume quickly. At roughly 2.7 cubic feet per inch on a 4x8 footprint, going from 12 inches to 18 inches adds about 16 cubic feet, which is most of another half yard per bed. Decide the height before you order, not after.
What to Put in the Bed, Not Just How Much
Volume is the question people search, but the blend is the question that determines whether the bed grows anything. A workable starting point is a majority of screened topsoil or a bulk garden blend, mixed with a solid share of compost, plus something coarse for drainage if the base material runs heavy.
Gardeners commonly land somewhere near two parts topsoil, two parts compost, and a smaller part coarse sand. Treat that as a direction rather than a recipe. If your topsoil is dense clay, shift toward more compost and more coarse material. If it is already light and sandy, you need less sand and more organic matter to hold moisture.
A few things to avoid.
- All compost. It sounds generous and it works badly. Straight compost holds too much water, settles hard into a dense mat, and can run too rich for young seedlings. Use it as a component and as an annual top dressing.
- Bagged potting mix as the whole fill. It is expensive at this volume and it dries out fast in a large open bed, since it was formulated for pots with limited surface area. Save it for containers and seed starting.
- Wood chips or branches in the bottom third. Filling the lower portion with woody material to save on soil is tempting, but it does not count as soil volume. It breaks down and sinks, and you spend the next two seasons topping off a large gap.
- Fill dirt. Fill dirt is subsoil sold for grading and leveling. It is not topsoil, it has little organic matter, and it does not belong in a growing bed.
If you are ordering bulk, ask the yard what their raised bed blend actually contains. Terms like garden soil, planting mix, raised bed mix and topsoil are not standardized between suppliers, and two yards ten miles apart can sell very different products under the same name. A supplier that can tell you the components is a supplier worth buying from. When you are ready to price it out, bulk garden soil is the category to compare against your bag total.
A Worked Example, Two Beds at 12 Inches
Say you built two 4x8 beds, both 12 inches tall. Two times 32 cubic feet is 64 cubic feet, which divided by 27 is about 2.37 cubic yards. Order 2.5 yards so you have settling allowance built in. You have also cleared the two yard crossover, so bulk is the clear call here.
For comparison, the same 64 cubic feet in bags is roughly 43 bags at 1.5 cubic feet. That is several vehicle loads, a full afternoon of loading and unloading, and a contractor bag of empty plastic when you are done.
A sensible way to split the bulk order is to buy the base and the amendment separately. Something like 1.5 yards of screened topsoil and 1 yard of compost gets you close to a strong blend. Mix it on a tarp in batches, or layer and turn it with a shovel as you fill the beds. Mixing in the bed is slower but it saves handling the material twice.
Think about where the pile goes before the truck arrives. Pick a spot on the driveway or on a tarp as close to the beds as the truck can reach. Moving two and a half yards by wheelbarrow is an afternoon if the pile is twenty feet away, and it is a genuinely miserable weekend if it is a hundred feet away and uphill. Distance matters more than total volume.
Fill in layers of six to eight inches rather than dumping the whole load in at once. Water each layer as you go, which settles it and shows you where the low spots are. When both beds are level with the rim, water hard, leave it a day, and top off whatever dropped. Then plant.
How We Started
We started Mulch Mound because we got tired of the hassle that came with buying landscaping materials. The options were either loading bags into your car at a garden center or calling around to local suppliers, trying to figure out pricing, minimums, and delivery schedules. Neither option felt convenient or transparent.
Three of us – Alec, Mo, and Tyler – decided there had to be a better way. Alec and Tyler got their start back in 2013 running a landscaping business during college, moving mulch and mowing lawns to pay tuition. That experience taught them how frustrating it was to source materials, and years later, that frustration turned into Mulch Mound.
We focus on making it simple to get mulch, stone, and soil delivered directly to your home. Order online, pick your delivery date, and we handle the rest. No loading bags. No calling multiple suppliers. No wondering if you bought enough or paid a fair price.
We work with quality local suppliers in the areas we serve and aim to be straightforward about what we offer and what it costs. Landscaping is hard work. Buying the materials for it shouldn't be.
Frequently asked questions
Can I mix bagged and bulk soil in the same bed?
Yes, and it is a reasonable hedge. Order bulk for the main fill and keep two or three bags in the garage. Those bags cover the top off after settling, fill in seed starting rows where you want finer material, and save you a trip when you realize the last bed came up short. Nothing bad happens where a bagged mix meets a bulk mix as long as both are actual growing media and not one of the products in the avoid list above.
Does soil weight matter for hauling it myself?
It matters more than most people expect, and it often decides the bag versus bulk question before price does. A cubic yard of moist soil is heavy enough that most half ton pickups take one yard per trip, and a car trunk realistically holds a handful of bags before the suspension complains. If you need 2.5 yards and you own a sedan, delivery is not a luxury, it is the only practical option.
Do I need to remove the sod under the bed first?
Not necessarily. For a deep bed, laying plain cardboard over the grass and filling on top of it works fine. The grass dies under the light block and the cardboard breaks down within a season. For a shallow bed, do take the time to cut the sod or at least loosen the ground underneath with a garden fork. That gives roots somewhere to go and keeps water from perching on a compacted layer right below your new soil.
How much soil do I need to top off in year two?
Usually an inch or two of compost spread across the surface. On a 4x8 bed, one inch is a little under 3 cubic feet, so two bags at 1.5 cubic feet covers an inch and four bags covers two inches. That is the one job where bagged material is genuinely the right answer, since ordering a bulk delivery for 3 cubic feet makes no sense.
Can I fill a raised bed with only compost?
You can physically do it, but it is not a good plan. A bed of pure compost stays wet, packs down into a dense layer within a season, and can carry nutrient levels high enough to burn tender seedlings. It also loses volume faster than a blended mix, so you will be refilling sooner. Compost belongs in a raised bed as a large component of the blend and as the annual top dressing, not as the entire fill.