Bags Needed
Recommended Soil Mix (40/40/20)
Buying Tips
- Under 27 cu ft: Buy bags - easier to handle
- 27+ cu ft (1 yard): Consider bulk delivery — usually cheaper per yard
- Pre-mixed "raised bed soil" is convenient but pricier
- 14 bags (2 cu ft) = 1 cubic yard
- Soil settles after watering, so beds usually need topping up in the first season — we do not publish a percentage because we could not source one. - plan accordingly
Ready to buy? Search for garden soil:
🌻 How This Raised Bed Soil Calculator Works
A raised bed is a box, and the volume of a box is the easiest sum in landscaping — which is precisely why people get it wrong. The mistake is almost never the arithmetic. It is forgetting that a bed 4 ft × 8 ft × 12 in deep swallows 32 cubic feet of soil, and that 32 cubic feet is sixteen of the big bags at the garden centre. Nearly everyone underbuys the first time.
Enter each bed's length and width in feet and its depth in inches. The calculator multiplies the three together (converting inches to feet), reports cubic feet and cubic yards, splits the total into bag counts at three common bag sizes, and breaks the volume down by the mix ratio.
Formula Reference
Cubic yards = cubic feet ÷ 27
Bags = ⌈ cubic feet ÷ bag size ⌉
Mix (40/40/20 by volume) = cubic feet × 0.4 / 0.4 / 0.2
Units: length and width in feet, depth in inches. The 40/40/20 split is applied to volume in cubic feet — it is not a weight ratio, and mixing it by weight would give you something quite different, because compost is far lighter than topsoil for the same bucket.
No waste factor is applied. Unlike the mulch and gravel calculators on this site, this one adds nothing for settling. Bag counts are the exact volume rounded up to the next whole bag. Buy a little over the figure it gives you — see the settling note below.
📏 How Deep Should the Bed Be?
University of Maryland Extension's Soil to Fill Raised Beds (updated November 2024) gives the clearest numbers we could find for beds sitting on a hard surface, where roots cannot escape downward: a minimum of 8 inches deep for leafy greens, beans and cucumbers, and 12–24 inches for peppers, tomatoes and squash.
That is why 12 inches is the sensible default for a general vegetable bed — it clears the shallow-rooted crops comfortably and reaches the bottom of the range for the deep-rooted ones. A bed sitting on open ground is more forgiving, because roots can push into the native soil beneath, but the fill you buy still has to fill the box.
🧮 A Worked Example
One 4 ft × 8 ft bed, filled 12 inches deep:
- Volume: 4 × 8 × (12 ÷ 12) = 32 cubic feet
- Cubic yards: 32 ÷ 27 = 1.19
- Bags: 16 at 2 cu ft, 22 at 1.5 cu ft, or 32 at 1 cu ft
- Mix at 40/40/20: 12.8 cu ft topsoil, 12.8 cu ft compost, 6.4 cu ft coarse sand or perlite
A shallower 8 ft × 3 ft herb bed at 8 inches is 16 cubic feet — eight 2-cubic-foot bags, and a job you can do out of a car boot.
🪱 About the 40/40/20 Mix
The 40% topsoil / 40% compost / 20% coarse sand or perlite split shown in the results is this tool's own default, not an extension recommendation, and the honest position is that published guidance varies.
University of Minnesota Extension's Raised bed gardens (Natalie Hoidal, Carl Rosen, Troy Salzer and Freya Thurmer) recommends "a mixture of around 2/3 to 1/2 topsoil and 1/2 to 1/3 plant-based compost," adding sand only where the topsoil carries substantial clay. University of Maryland Extension suggests compost and purchased topsoil in a 1:2 or 1:1 ratio, or compost and a soilless growing mix at 1:1.
So the 40:40 topsoil-to-compost part of this tool's default is a 1:1 ratio, which sits at the compost-heavy end of both recommendations. The 20% coarse fraction is the part that goes beyond them — Minnesota would add it only for clay-heavy topsoil. If your topsoil is already loose and friable, you can reasonably drop the sand and split the volume 50/50, and the calculator's total is unaffected either way. Minnesota also warns against using potting soil in raised beds because it dries out too quickly, and against overdoing the compost, which causes water-retention problems.
⚠️ Common Mistakes
Not allowing for settling. A freshly filled bed drops noticeably after the first few waterings as air pockets close and organic matter breaks down. This calculator does not add anything for it, so buy a little over its figure or expect to top up in a few weeks.
Measuring the outside of the frame. Use the internal dimensions. On a bed built from dimensional lumber the frame eats a few inches in each direction — small, but often the difference between one bag over and one bag short.
Buying by weight. Soil is sold by volume for a reason: a wet bag and a dry bag of the same cubic footage weigh very different amounts. Match the bag's labelled cubic feet to the number on this page and ignore the pounds.
Ignoring the bags-versus-bulk crossover. The calculator flips its recommendation at 27 cubic feet, which is exactly one cubic yard, because that is the point at which fourteen 2-cubic-foot bags start to look silly next to a single delivered yard.
❓ Questions People Actually Ask
How do I calculate how much topsoil I will need?
Multiply length × width × depth with everything in feet. Since depth is normally quoted in inches, divide it by 12 first. Divide the cubic feet by 27 for cubic yards. A 4 ft × 8 ft bed filled 12 inches deep is 32 cubic feet, or 1.19 cubic yards.
How much does a 40 lb bag of topsoil cover?
Weight is a poor guide, because moisture changes it. Using the EPA's published figure of 2,392 lb per cubic yard for loose soil and sandy loam (about 88.6 lb per cubic foot), 40 lb is roughly 0.45 cubic feet — about 5.4 sq ft one inch deep, or 2.7 sq ft two inches deep. Buy by the labelled cubic-foot volume instead.
Is 2 inches of topsoil enough to grow grass?
It is thin. Mississippi State University Extension advises that soil amendments "need only be incorporated in the top 6 to 10 inches where turf roots grow," and describes amending the top 3 inches with 1 to 2 inches of compost to give a 4-inch root zone. Two inches laid on top of untouched subsoil gives roots very little; working it into what is already there does far more than leaving it as a layer.
How many wheelbarrows is 1 yd of topsoil?
It depends on your barrow — the rated capacity is usually stamped on the tray. A cubic yard is 27 cubic feet, so it is 9 loads of a 3-cubic-foot barrow, 5 loads of a 6-cubic-foot contractor barrow, or 14 loads of a small 2-cubic-foot one. That assumes level fills, which nobody manages.
This calculates volume for ordering. It is not horticultural advice: it does not know your crops, your climate, your drainage or what is in your existing soil, and it makes no allowance for settling. If you are growing food, a soil test through your county extension office will tell you far more about what to put in the bed than any ratio on this page.
Bed-depth figures come from University of Maryland Extension's Soil to Fill Raised Beds (updated 6 November 2024), the source for the 8-inch minimum and the 12–24-inch range. Mix guidance is from University of Minnesota Extension's Raised bed gardens (Hoidal, Rosen, Salzer and Thurmer) and the same Maryland page; where they differ from this tool's 40/40/20 default, the difference is stated above rather than smoothed over. The turf root-zone depths quoted in the FAQ are from Mississippi State University Extension publication P2951, Sod Installation and Maintenance (Jay McCurdy and Barry Stewart). The loose-soil weight of 2,392 lb per cubic yard is from the US EPA's Standard Volume-to-Weight Conversion Factors, which credits it to the Tellus Institute. The four FAQ questions were captured verbatim from Google's People Also Ask panel for "topsoil calculator" on 31 July 2026.