How the raised bed planner works
Filling raised beds usually means buying several products at once, in bags of different sizes, for beds that may not all be the same. This planner groups identical beds together, works out the volume to fill, splits it into soil, compost and an aeration material by the mix you choose, and rounds each one up to whole bags.
Measuring your beds
- Measure inside the frame, not outside, because the boards take up space.
- Group identical beds. Add one bed type and set “Number of identical beds” instead of entering each bed.
- Use the diameter for round beds, such as stock tanks or metal rings.
- Split L-shaped beds into two rectangles and add both.
Fill height
The mix only goes up to a little below the rim, so water and mulch stay in the bed. The fill height is the inside height of the bed, minus the gap you leave below the rim, minus any bottom fill layer. With the defaults, an 11 in bed with a 2 in gap is filled to 9 in.
Choosing a mix
The planner offers three starting points: 60% soil, 30% compost and 10% aeration; equal thirds; or a single product at 100%. You can also type your own percentages, which must add up to 100%. These are planning presets, not growing advice. Soil mixes vary by product. Check the bag label for volume and recommended use.
Worked example
Two 4 × 8 ft beds, 11 in tall inside, with a 2 in gap below the rim, 10% settling and a 60/30/10 mix in 1.5 ft³ bags. Load it with the “Try the worked example” button.
- Fill height: 11 − 2 = 9 in, or 0.75 ft.
- Each bed: 4 × 8 × 0.75 = 24 ft³, so two beds need 48 ft³.
- With settling: 48 × 1.10 = 52.8 ft³, or 1.96 yd³.
- Soil (60%): 31.68 ft³ = 22 bags.
- Compost (30%): 15.84 ft³ = 11 bags.
- Aeration (10%): 5.28 ft³ = 4 bags.
Bags or bulk
A few small beds are easy to fill from bags. For many beds, a bulk delivery of a premixed raised-bed blend can be simpler. The planner shows a bulk alternative for the total mix, rounded up to your supplier’s step, and you can switch the shopping list between bags and bulk.
Important assumptions
- Inside height starts at 11 in (280 mm) and the rim gap at 2 in (50 mm).
- Settling of 10% is added to the total before the mix is split.
- Bag sizes start at 1.5 ft³ (50 L) for each component and can be set separately.
- Bags and bulk are always rounded up, one component at a time.
What this tool does not calculate
- Which soil, compost or aeration product suits your plants, or nutrient and pH needs.
- Frame lumber, screws, liners, hardware cloth or irrigation.
- Drainage under the bed or what to use as a bottom layer.
Raised bed soil FAQ
Why does the planner add a settling allowance?
Bagged soil and compost are loose. After the first few waterings the mix packs down and the level drops. The 10% default adds a little extra so the bed ends up closer to the height you wanted. Raise or lower it to match your product and experience.
What counts as an aeration material?
It is any coarse, light material added to keep a mix open and draining, such as perlite or pumice. The planner does not choose one for you; it only works out the volume and bags for the share you enter.
Can I put something else in the bottom of a tall bed?
You can enter a bottom fill layer and its depth. The planner then fills only the height above it with your mix and lists the bottom layer’s volume separately. It does not advise which material to use.
How do I calculate a top-up for an existing bed?
Set the inside height to the depth you want to add, set the gap below the rim to 0 and leave the bottom layer at 0. The planner then works out only the extra mix for that top layer.
Related planners
Soil quantities are estimates. Bag volumes and how much a mix settles vary by product, so check the labels before ordering. See how we calculate for the full method.