A 4 ft by 8 ft raised bed filled 12 inches deep needs about 32 cubic feet of soil, which is roughly 1.2 cubic yards or 16 bags at 2 cubic feet each. For any other bed, multiply the footprint in square feet by the depth in feet, then add about 10 percent for settling.
That is the whole trick. The annoying part is the unit juggling: beds are measured in feet, depth in inches, bags in cubic feet, and bulk soil by the cubic yard. Punch your measurements into our free volume calculator and it handles every conversion at once. This guide covers what a calculator cannot guess, like bag counts and why your soil level will quietly sink in June.
Start with the box formula
Working out how much soil for a raised garden bed starts with one fact: every bed is a box, even the curvy metal kind. Box volume is length × width × depth, all in feet. Depth is nearly always quoted in inches, so divide by 12 first. A 10 inch fill is 0.83 feet.
Measure to where the soil will sit, not the top of the frame. Most gardeners leave a 1 to 2 inch lip so watering does not wash soil out. For an L shape or a circle, split the bed into rectangles and add the results; our guide to volume formulas covers cylinders and odd shapes.
Soil for a 4×8 raised bed and other common sizes
The 4×8 is the default bed for a reason: you can reach the middle from either side without stepping on the soil. Here is the math for common sizes, using the 2 cubic foot bags most US stores stock.
| Bed size | Fill depth | Cubic feet | Cubic yards | Bags (2 cu ft) |
|---|---|---|---|---|
| 2 × 4 ft | 12 in | 8 | 0.30 | 4 |
| 4 × 4 ft | 12 in | 16 | 0.59 | 8 |
| 3 × 6 ft | 12 in | 18 | 0.67 | 9 |
| 4 × 8 ft | 6 in | 16 | 0.59 | 8 |
| 4 × 8 ft | 12 in | 32 | 1.19 | 16 |
| 4 × 8 ft | 18 in | 48 | 1.78 | 24 |
These are exact box volumes with bags rounded up; no settling buffer is built in. Wondering how many bags of soil you need for a size not listed? Divide your cubic feet by the volume printed on the bag and round up. Once the total passes about a cubic yard, price bulk delivery instead. Sixteen bags is a lot of plastic and a lot of trunk trips.
A worked example, start to finish
Say your bed is 3 ft by 6 ft and you plan to fill it 10 inches deep.
- Convert the depth: 10 ÷ 12 = 0.83 ft.
- Find the footprint: 3 × 6 = 18 square feet.
- Multiply: 18 × 0.83 = 15 cubic feet (rounded up from 14.94).
- Add a 10 percent settling buffer: 15 × 1.1 = 16.5 cubic feet.
- Convert to bags: 16.5 ÷ 1.5 = 11 bags at 1.5 cubic feet, or 9 bags at 2 cubic feet.
Now sanity check bulk: 16.5 ÷ 27 = 0.61 cubic yards. If a 1.5 cubic foot bag runs $9, eleven bags is $99. A supplier charging $45 a yard plus delivery lands in the same neighborhood here, and wins by a mile on anything larger. (Delivery fees are why tiny orders still favor bags.)
Cubic feet to cubic yards: divide by 27
One cubic yard is 27 cubic feet. A yard is 3 feet, and a cube 3 feet on each side works out to 27. So the cubic feet to cubic yards soil conversion is a single step. Divide by 27. That is it.
Bulk suppliers quote in yards; your tape measure and the bags speak feet. A full cubic yard of moist soil weighs somewhere near a ton, so it arrives by dump truck, not in your hatchback. Most suppliers sell half yard increments, meaning the 0.61 yards above becomes a one yard order with leftovers for pots.
The same box math works for liquids too; our water tank capacity guide runs it in gallons and liters, handy if you are sizing a rain barrel for these beds.
Filling deep beds for less
Mix first. A popular raised bed soil mix ratio pairs 60 percent topsoil with 30 percent compost. The last 10 percent is an aeration amendment such as perlite or coarse bark fines. Straight topsoil compacts hard inside a frame and straight compost slumps, so the blend behaves better than either.
Now the money saver. Beds 18 inches or deeper do not need premium mix in the bottom third. An old raised-bed gardening trick, often called hugelkultur, buries logs and branches in the base and caps them with proper soil. The rotting wood holds moisture like a sponge and slowly feeds the bed. Use untreated wood only, and skip black walnut, which releases a chemical that stunts vegetables. Grass clippings and fallen leaves make a decent middle layer.
Gravel never belongs in the bottom. It does not improve drainage; it creates a perched water table that keeps roots wetter, and it steals depth your carrots wanted.
Why the soil level drops, and how to plan for it
Fill a bed in April and by June the surface sits an inch or three lower. Nothing went wrong. Fresh mix is full of air, and the first real waterings collapse those pockets, which typically costs 10 to 20 percent of the height. Wood-based beds sink further as the base breaks down.
Water each layer as you fill and you will lose less later. Keep a couple of spare bags for topping up next season. Honestly, for a wood-bottom bed I would just buy 20 percent extra up front. It is easier math, and leftover soil never goes to waste.
Frequently asked questions
How much soil do I need for a 4×8 raised bed?
A 4×8 raised bed needs 32 cubic feet of soil for every 12 inches of depth, about 1.2 cubic yards. That equals 16 bags at 2 cubic feet or 22 bags at 1.5 cubic feet. Buy roughly 10 percent extra, since fresh mix settles once you start watering.
How many bags of soil does it take to fill a raised bed?
Divide the bed’s volume in cubic feet by the volume printed on the bag, then round up. A 4×8 bed filled 12 inches deep takes 16 bags at 2 cubic feet or 22 bags at 1.5 cubic feet. Bag sizes vary by brand, so check the label before counting.
How deep should a raised garden bed be?
Twelve inches of soil depth suits most vegetables. Shallow rooted crops like lettuce and herbs manage with 6 to 8 inches, while tomatoes and root crops prefer 12 to 18 inches. If the bed sits on open ground, roots can push into the native soil below, so shallower frames still work.
Can I fill the bottom of a raised bed with something cheaper than soil?
Yes, beds 18 inches or deeper can have their bottom third filled with logs and branches, a method borrowed from hugelkultur. Cap the wood with at least 12 inches of proper soil mix. Expect extra settling as the wood decomposes, and avoid gravel, which worsens drainage rather than helping it.
How many cubic feet are in a cubic yard of soil?
One cubic yard contains exactly 27 cubic feet. A yard measures 3 feet, and a cube 3 feet on each side works out to 27. To convert a soil estimate, divide cubic feet by 27, so a 32 cubic foot bed fill is about 1.2 cubic yards.
Why did the soil level in my raised bed drop after a few weeks?
Fresh soil mix settles by roughly 10 to 20 percent as watering collapses its air pockets. That drop is normal, not a sign of bad soil. Beds filled over logs or leaves sink further while the material decomposes. Top the bed up with compost before the next planting round.
Measure your bed tonight and run the numbers through our volume calculator, which doubles as a raised bed soil calculator. The whole thing takes about a minute. It beats a second garden center run with the back seat folded down.