Free Online Calculators
Fill a raised bed: layers, soil volume & bags
This calculator figures out how much soil you need to fill your raised bed, based on its length, width and height plus the different layers like drainage material, compost and topsoil. It converts the total volume into the number of bags you'll need to buy.
It depends on the fill depth and the bag size: multiply 4 ft × 8 ft × your depth in feet to get the cubic feet needed, then divide by the cubic feet printed on the bag. Deeper beds and smaller bags push the bag count up quickly.
Weight doesn't translate directly into volume, because potting mixes differ in density and moisture content. Check the cubic-foot or quart figure printed on the bag instead of estimating from the pounds.
A common mix is roughly equal parts topsoil and compost, plus a smaller share of a coarse material such as coir, bark fines or perlite for drainage and aeration. The compost supplies nutrients, the coarse part keeps the bed from compacting.
A 50 quart bag holds slightly less than 2 cubic feet of dry material. For planning purposes you can treat it as just under 2 cubic feet.
Multiply the bed's length, width and fill depth in the same unit to get the volume. Fill depth is usually a bit below the top edge, so measure to the soil line you actually want, not the full frame height.
No, 25 dry quarts is a little less than 1 cubic foot. The difference is small, so 25 quarts is often used as a rough stand-in for a cubic foot.
1 cubic foot is considerably bigger than 8 quarts — you'd need several 8 quart bags to make up a single cubic foot.
Not exactly — 50 dry quarts is a bit less than 2 cubic feet. It's close enough for rough garden planning, but you'll come up slightly short if you count it as a full 2 cubic feet.
A raised bed soil calculator works out exactly how much filling material you need before you start building layers in your new bed. Instead of guessing and either running short halfway through or ending up with three leftover bags of compost, you enter the inside dimensions of your bed and the tool tells you the total volume in litres, plus how that volume should be split across the classic layered "hugelkultur-style" filling method that experienced gardeners use for raised beds. It also tells you how many bags of potting soil you actually need to buy, since bags are only required for the top layer, not for the whole structure.
This is genuinely useful because filling a raised bed correctly is not just about pouring in soil until it looks full. The layering approach mimics what happens in a natural forest floor: coarse woody material breaks down slowly at the bottom releasing warmth and nutrients over years, while finer, richer material sits near the surface where young roots need it most. Getting the proportions right saves money — coarse compost and chippings are far cheaper than bagged potting soil — and it gives your plants a self-fertilising, well-draining foundation for several growing seasons.
The math itself is simple. The calculator multiplies the internal length, width and height of the bed, all measured in centimetres:
Volume (litres) = (Length cm × Width cm × Height cm) ÷ 1000
Dividing by 1000 converts cubic centimetres into litres, since 1000 cm³ equals exactly 1 litre. Once the total litre figure is known, the calculator applies the standard layer percentages, measured from the bottom of the bed upward:
Because bagged potting soil is the only layer you typically need to buy in pre-packaged form (the rest often comes from your own garden, a local supplier, or a compost heap), the calculator converts only that 10% share into a number of bags, based on the bag size you select. It also adds a note about first-year settling: freshly filled beds compact by roughly 25% of their total volume during the first year as organic material breaks down and air pockets collapse, so you should plan to top up the bed with fresh soil from the top once settling has occurred.
There is also an alternative, simpler mode for gardeners who don't want a multi-layer setup and just want to fill the entire bed with potting soil. In that mode the calculator skips the percentages entirely and converts 100% of the total volume directly into bags.
Each layer in a raised bed has a specific job, and skipping one or getting the ratios badly wrong can leave you with waterlogged roots, poor nutrition, or a bed that sinks dramatically within months.
The bottom layer of coarse material — thick branches, twigs, and wood chippings — creates air pockets that let excess water drain away quickly, preventing root rot. As this wood slowly decomposes over several years, it also generates a small amount of heat, which can gently warm the root zone from below, extending the growing season slightly in cooler climates.
Above that, turf and leaves (grass clumps turned upside down, autumn leaves, or leafy garden waste) form a slow-release nutrient layer. Because these materials break down gradually, they feed the bed steadily over a long period rather than delivering a quick burst that gets used up in one season.
The coarse compost layer is the main nutrient engine of the whole bed. This is where the bulk of the fertility for the first couple of years comes from, so it's worth using good quality, well-rotted compost here rather than skimping.
Garden soil or fine compost above that acts as a buffer layer, holding moisture and nutrients close enough to the surface roots while still connecting down into the richer layers below.
Finally, the potting soil top layer — roughly the top 20 cm of the bed — is where seeds germinate and young transplants establish themselves. This layer needs to be loose, weed-free, and nutrient-balanced, which is exactly why bagged, quality-controlled potting soil is recommended here rather than garden soil straight from the ground.
Example 1 – A standard rectangular bed: A raised bed measuring 200 cm long, 100 cm wide, and 60 cm high has a volume of (200 × 100 × 60) ÷ 1000 = 1,200 litres. Using the layered method: coarse material 420 L, turf and leaves 264 L, coarse compost 240 L, garden soil/fine compost 156 L, and potting soil 120 L. If you buy 40-litre bags, you'd need 3 bags (120 ÷ 40) for the top layer alone.
Example 2 – A small square bed filled only with potting soil: A bed measuring 100 cm × 100 cm × 40 cm gives 400 litres total. In potting-soil-only mode, that's the full 400 litres needed, which equals 8 bags of 50 litres each.
Example 3 – Planning for settling: Using the 1,200-litre bed from Example 1, expect roughly 25% settling, meaning around 300 litres of volume will compact down over the first year. It's wise to keep a bag or two of extra compost and potting soil on hand for spring top-up rather than buying it all at once.
| Layer | Share of Volume | Purpose |
|---|---|---|
| Coarse material (branches, chippings) | 35% | Drainage and rotting heat |
| Turf and leaves | 22% | Slow nutrient release |
| Coarse compost | 20% | Main nutrient store |
| Garden soil / fine compost | 13% | Water-holding buffer |
| Potting soil | 10% | Planting layer, top approx. 20 cm |
You can simplify the structure, but each layer serves a purpose. If you skip the coarse bottom layer, drainage suffers and roots may sit in waterlogged soil. If you're short on organic waste for the middle layers, you can compensate with extra coarse compost and garden soil, but try to keep at least three distinct layers for the best long-term results.
Expect around 25% of the total filled volume to settle during the first year as organic material decomposes and compacts. This is completely normal — simply top up the bed from above with fresh compost or potting soil once you notice the level dropping, usually in early spring.
Bagged products are convenient but relatively expensive per litre compared to bulk materials like garden compost, chippings, or turf, which you can often source for free or cheaply from your own garden or local suppliers. Since the potting soil layer is small (only about 10% of total volume) but needs to be clean, weed-free and nutrient-balanced for planting, it's the one layer worth buying in bags rather than sourcing in bulk.