How to calculate volume in square
and rectangular raised beds
GOOD SOIL is the single most important ingredient for a good garden. Raised beds give you an immediate advantage over a regular garden, because when you fill your raised bed, you can fill it with a blend of soil that's superior to the native soil in your yard. Soil that's loose and rich with nutrients and organic matter will allow the roots of your plants to grow freely, and ensure that they have access to the water and nutrients they need to sustain healthy growth.
Example: How Much Soil Do I Need for a 3x6 Bed?
To fill a 3x6 bed with 10" sides, you will need 15 cubic feet of blended soil. To create the blend, use the following quantities:
- 9 cubic feet of topsoil (9 20-quart bags)
- 4.5 cubic feet of compost (4.5 20-quart bags)
- 1.5 cubic feet of soilless growing mix
- 1.5 cups Gardener's Supply All-Purpose Fertilizer
Before placing your raised beds in their permanent location, be sure to remove any grass or perennial weeds from the area. Use a garden fork or shovel to loosen the native soil to a depth of 6-10″. This will improve drainage and moisture retention in the raised beds. It also means that even with a 5″-high raised bed, your plants will think they're growing in a bed that's 12-18″ deep — plenty of room for carrots, potatoes, full-size tomato plants and most any other vegetable you'd ever want to grow.
If you'll be filling more than one raised bed, you may want to buy your soil in bulk — by the cubic foot or cubic yard. Use the Soil Calculator to figure out the total amount of soil you'll need for each bed. For most situations, we recommend these proportions:
- 60 percent topsoil
- 30 percent compost
- 10 percent soilless growing mix that contains peat moss, perlite and/or vermiculite, such as Transplant Mix or Container Mix
Where to Buy
In our Raised Beds department, you'll find DIY raised beds, raised bed corners, elevated raised beds, watering systems, pest protection, season-extending tools and accessories. For the widest selection of raised beds, visit our partners at raisedbeds.com.
Keep in mind that proportions are approximate because soil volume varies from source to source. For instance, if the calculator specifies .444 cubic yards of soil for your bed, go ahead and round it up to a half yard.
If you do not have access to quality topsoil, an acceptable substitute would be a 50-50 blend of soilless growing medium and compost. If you want to add peat moss to the bed, it should not be more than 20 percent of the total mix. Peat moss is naturally acidic and is not a good medium for growing vegetables.
Recharge an Existing Bed
- Pull any weeds that have overwintered.
- Use a hoe to loosen and fluff the soil because winter snow and rain can compact it.
- If the soil has settled, top it off with an inch or two of fresh compost and mix it into the existing soil.
- Replenish nutrients and re-energize the soil with the Raised Bed Booster Kit.
Gardener's tip: If you are not planting within a week, cover the bed with a layer of straw to protect the soil. This will, however, prevent the soil from warming up and drying out. So you'll need to uncover the bed at least a week before planting warm-season crops.
Recharge a Pot or Planter
For small pots, it's best to start with fresh planting mix, tossing the old soil into the compost. With larger planters, you can recharge the existing soil with Container Booster Mix. Just mix in 4-8 tablespoons per quart of existing soil.