Raised Bed Soil Calculator
How much soil your raised bed needs: in cubic yards and bags.
A raised bed is only as good as what fills it. And soil is sold by volume, not by bed. This calculator turns length, width, and depth into cubic feet and cubic yards, splits the mix into topsoil and compost, and compares bagged soil against bulk delivery so you can see which is cheaper before you order.
The usual starting mix is 60% topsoil and 40% compost, but the percentages are yours to change. Once the bed is filled, cover it with the Mulch Calculator and plan what goes in it with the Plant Spacing Calculator.
How this is worked out
Cubic feet = length (ft) × width (ft) × depth (in) ÷ 12. Cubic yards = cubic feet ÷ 27. Each mix component = total cubic feet × its percentage ÷ 100. Bags = cubic feet ÷ bag size, rounded up.
Assumptions:
- The bed is a rectangular box with straight sides. The calculator uses the inside measurements.
- Depth is the finished, settled depth. Fresh mix drops a little after the first watering, so expect to top up.
- One cubic yard equals 27 cubic feet. Bag sizes are as printed on the bag.
- Prices are entered by you. This tool never hardcodes store prices, because they go stale.
Three worked examples
1. A 4 × 8 ft bed, 12 in deep. 4 × 8 × 12 ÷ 12 = 32 cu ft, which is 32 ÷ 27 = 1.19 cubic yards. At 60/40 that is 19.2 cu ft of topsoil and 12.8 cu ft of compost. 16 bags of 2 cu ft.
2. A 4 × 4 ft bed, 6 in deep. 4 × 4 × 6 ÷ 12 = 8 cu ft, or 0.30 cubic yards. 4 bags of 2 cu ft. Small beds are almost always cheaper bagged.
3. Three 4 × 8 ft beds, 12 in deep. Three times 32 cu ft is 96 cu ft, or 3.56 cubic yards. At that volume, bulk delivery usually beats bags. Compare the two prices in the calculator before ordering.
Soil volume for common bed sizes
| Bed size | Depth | Cubic feet | Cubic yards | 2 cu ft bags |
|---|---|---|---|---|
| 4 × 4 ft | 6 in | 8 cu ft | 0.30 cu yd | 4 bags |
| 4 × 4 ft | 12 in | 16 cu ft | 0.59 cu yd | 8 bags |
| 4 × 8 ft | 6 in | 16 cu ft | 0.59 cu yd | 8 bags |
| 4 × 8 ft | 12 in | 32 cu ft | 1.19 cu yd | 16 bags |
| 3 × 6 ft | 12 in | 18 cu ft | 0.67 cu yd | 9 bags |
Common raised-bed mistakes
- Filling a bed with garden soil alone. Garden soil compacts in a bed and drains poorly. Mix it with compost and coarse material so water can move through.
- Forgetting that fresh mix settles. A bed filled to the brim drops an inch or two after the first watering. Fill it slightly proud, water it well, and top up with compost before planting.
- Buying by the bag for a big job. Bags are convenient for one small bed, but the price per cubic yard is usually much higher than bulk. Above about 2 cubic yards, get a bulk quote.
- Skipping the compost. Topsoil alone has little biology and little nutrient. Compost is what makes the mix alive. It feeds soil organisms, holds water, and releases nutrients slowly.
- Using treated timber without checking. Old treated timber can leach chemicals into bed soil. Use untreated wood, stone, or metal, or line the inside of treated boards.
What to do next
Order the soil, water the mix well, and let it settle for a few days before planting. Then mulch the surface with the Mulch Calculator and work out the planting layout with the Plant Spacing Calculator.
Putting the result to work
Order the soil a week before you need it, not the day you want to plant. Bulk soil arrives damp and heavy, and a fresh mix needs a few days to settle after watering. Fill the bed slightly proud of the top, water it thoroughly, and check the level again in three days before you sow anything into it. If you are filling several beds, order the total volume in one delivery: the delivery fee is usually per load, not per yard, so three beds in one order cost less than three separate orders.
After the first season, the bed will need topping up rather than refilling: an inch or two of compost each spring replaces what settled and what the plants used. That is the cheapest soil you will ever buy, and it is also the most useful. Compost on top feeds the soil life that does the work underground. Keep the recipe written down (your topsoil and compost percentages) so the next bed you build uses the same mix that worked.
Mix recipes for different beds
The 60/40 topsoil-compost mix is a starting point, not a law. Adjust it to what you are growing. Leafy greens are hungry and thirsty, so give them more compost. 50% topsoil and 50% compost holds more water and feeds faster growth. Root crops want a loose, stone-free mix that lets roots grow straight: keep the compost at 30 to 40% and make sure the topsoil is sieved, because a stone or a clod is what forks a carrot. Fruiting crops like tomatoes and peppers do well on the standard 60/40, with a handful of slow-release organic fertilizer mixed in at planting time.
| What you are growing | Topsoil | Compost | Note |
|---|---|---|---|
| Leafy greens | 50% | 50% | Holds more water; feed little and often. |
| Root crops | 60 to 70% | 30 to 40% | Sieve the topsoil; no fresh manure. |
| Fruiting crops | 60% | 40% | Add slow-release feed at planting. |
| Herbs (Mediterranean) | 70% | 30% | Add coarse grit for drainage. |
One thing never to use is fresh manure in a bed you are about to plant: it burns roots and can carry pathogens onto leafy crops. Composted manure is fine. And whatever mix you choose, write it down. The recipe that worked this year is the recipe to repeat next year.
How to fill a raised bed
- Measure length and width in feet, and the depth you want in inches.
- Volume in cubic feet = length x width x depth / 12. Divide by 27 for cubic yards.
- Split the mix: a common starting mix is 60% topsoil + 40% compost. Adjust the percentages to match your mix.
- Compare bagged vs bulk: bags are convenient for small beds; bulk delivery is usually cheaper per cubic yard for large beds.
- Water the mix well and let it settle a few days before planting. It will drop a little.
Can I fill a raised bed with garden soil alone?
Not recommended. Garden soil compacts in a bed and drains poorly, so roots sit wet and air cannot reach them. Mix garden soil with compost and, if needed, coarse material for drainage. A common starting mix is 60 percent topsoil and 40 percent compost, adjusted to what you can source locally.
How many bags do I need?
Divide the cubic feet by the bag size and round up. A 4 by 8 foot bed, 12 inches deep, needs 32 cubic feet, which is 16 bags of 2 cubic feet or 32 bags of 1 cubic foot. For more than about 2 cubic yards, compare the bulk price. Delivery is usually cheaper per yard.
Why does the soil level drop?
Fresh mix settles as it is watered and as organic matter breaks down. A bed filled to the brim can drop an inch or two in the first weeks. Fill it slightly proud of the top, water it well, and top it up with compost each season rather than with more topsoil.
What is the best soil mix for a raised bed?
A widely used starting mix is 60 percent topsoil and 40 percent finished compost. The topsoil gives body and minerals; the compost gives biology, water holding, and slow-release nutrients. Avoid mixes that are mostly peat or mostly bark. They dry out fast and hold little nutrient.
Is bagged or bulk soil cheaper?
It depends on volume. Bags win for one small bed because there is no delivery fee. Bulk wins once you pass about 2 cubic yards, because the price per yard is lower. Enter both prices in the calculator and it compares the two totals for your exact bed.
Formula: cubic feet = length (ft) x width (ft) x depth (in) / 12; 1 cubic yard = 27 cubic feet. Prices are entered by you. This tool never hardcodes store prices.
