What the number is, and why beds always take more than people expect
The quantity you need is a volume, not an area, and volume grows with every inch of depth. That is the whole reason raised beds catch people out. A 4 ft × 8 ft bed is 32 square feet of surface — a small patch of garden — but filled twelve inches deep it holds 32 cubic feet of soil, which is 1.19 cubic yards, twenty-two 1.5-cubic-foot bags, or — at the 35 to 45 lb per cubic foot a damp blended mix runs as a rule of thumb — somewhere between 1,100 and 1,400 lb.
Two beds like that will not fit in a car. They will not fit in most trunks even in bags. Knowing the number before you go to the garden centre changes what you buy, how you get it home, and whether you order bulk with a delivery fee instead.
The calculation itself is deliberately simple, and its honesty depends on three inputs people habitually get wrong. You measure the inside of the frame, because the boards hold no soil. You subtract freeboard, the inch or two you deliberately leave empty below the rim so that water and mulch stay in the bed rather than washing over the side. And you add a settling allowance, because a freshly poured mix is full of air that the first heavy watering drives out.
Each of the three moves the answer in its own direction, so getting them all right matters more than getting any one of them right.
The formula, term by term
Everything reduces to length × width × depth, with two corrections and a unit conversion:
V = L × W × (H − F)/12 × N × (1 + s)
L × W is the inside footprint in feet. If you measured in inches, divide by 12 first; if you measured in metres, the calculator's unit switch handles it.
(H − F) is the fill depth in inches. The height of your frame is not the depth of your soil. A bed built from two stacked 2×6 boards is about 11 in tall, and with 2 in of freeboard you are filling 9 in — nearly 20% less than the height suggests. Dividing by 12 turns that fill depth into feet so the three dimensions are consistent and the answer comes out in cubic feet.
N multiplies by the number of identical beds. Beds of a different size need their own run.
(1 + s) is the settling allowance. Bagged mixes and freshly screened bulk soil are fluffed with air; once watered and worked, they lose volume. Ten to twenty per cent is the range experienced gardeners order, and the amount depends almost entirely on how much of the mix is compost, since it is the organic fraction that both compacts and decomposes. A mostly mineral topsoil settles very little; a mix that is half compost can drop noticeably in the first season.
Two conversions finish the job. Cubic yards = cubic feet ÷ 27, because a yard is 3 ft and 3 × 3 × 3 = 27; this is the unit landscape suppliers sell in, and the cubic yards to cubic feet calculator converts it either way if you are checking a delivery. Bags = ⌈V ÷ B⌉, rounded up, because a partial bag is not something you can buy. That ceiling function is why a bed needing 30.7 cubic feet takes sixteen two-cubic-foot bags rather than 15.3.
The mix split is a straight proportional division of the same volume. Enter the compost and aeration shares and the remainder is topsoil. The shares are by volume, not by weight — a cubic foot of perlite weighs a small fraction of a cubic foot of topsoil, so a weight-based recipe would give a completely different mix.
Worked example: two 4 × 8 ft beds, 12 in frames, 2 in of freeboard
You have built two beds, each 4 ft × 8 ft inside, from 12 in frames. You want 2 in of freeboard, you are allowing 15% for settling, and you plan a 60/30/10 mix of topsoil, compost and perlite. Bags are 1.5 cu ft at $6.50; the local landscape yard quotes $55 a cubic yard.
- Fill depth. 12 in − 2 in = 10 in, which is 10 ÷ 12 = 0.8333 ft.
- Volume of one bed. 8 × 4 × 0.8333 = 26.67 cu ft.
- Both beds. 26.67 × 2 = 53.33 cu ft.
- Settling allowance. 53.33 × 1.15 = 61.33 cu ft.
- In cubic yards. 61.33 ÷ 27 = 2.272 cu yd.
- Bags. 61.33 ÷ 1.5 = 40.9, rounded up to 41 bags.
- Bagged cost. 41 × $6.50 = $266.50.
- Bulk cost. 2.272 × $55 = $124.94, before delivery.
- Difference. $266.50 − $124.94 = $141.56. Unless the delivery fee exceeds that, bulk is the cheaper route here.
- Mix split. Topsoil 60% of 61.33 = 36.80 cu ft; compost 30% = 18.40 cu ft; perlite 10% = 6.13 cu ft.
One detail worth noticing: buying the three components separately in 1.5 cu ft bags takes 25 + 13 + 5 = 43 bags, not 41, because each component is rounded up on its own. Rounding is not distributive, and the mix breakdown table under the results reflects that rather than hiding it.
How to read the result: depth, bags and the bulk threshold
Start with the fill depth, because it is the number that decides whether the bed will actually grow things. Six inches supports lettuce, radishes, herbs and most greens. Ten to twelve inches is the general-purpose depth that handles beans, peppers, bush tomatoes and garlic. Root crops that go deep — parsnips, long carrots, daikon — want closer to eighteen inches, or twelve inches of bed sitting over loosened native soil they can push into. A bed on a solid surface such as a patio or a roof has no native soil beneath it and needs the full depth bought in.
Then look at the bag count against the cubic yards. The crossover point where bulk starts winning is not a fixed volume; it is wherever the delivery fee falls below the bag premium. At the prices used in the worked example below, bags cost $6.50 ÷ 1.5 = $4.33 a cubic foot against $55 ÷ 27 = $2.04 in bulk, slightly more than double, which is why the gap over 61 cubic feet reaches $141. But a single bed under a cubic yard often cannot be delivered at all — many suppliers set a one-yard minimum. The rough decision rule: under one cubic yard, buy bags; over two, get it delivered; in between, phone the yard and ask for the delivered total rather than the yard rate.
The mix shares are worth a second look too. A raised bed mix that is 100% compost is not soil — it is too rich, too high in soluble salts for seedlings, and it keeps shrinking as the organic matter finishes breaking down. A mix that is 100% topsoil compacts into a brick. The 60/30/10 default gives you mineral structure that will not slump, enough organic matter for water holding, and enough coarse material to keep the pore space open. Adjust the shares to what you have locally; the arithmetic does not care what you call the components.
Soil volume for common raised bed sizes
| Bed size | Footprint (sq ft) | 6 in fill (cu ft) | 12 in fill (cu ft) | 12 in fill (cu yd) | 1.5 cu ft bags at 12 in |
|---|---|---|---|---|---|
| 2 × 4 ft | 8 | 4.0 | 8.0 | 0.30 | 6 |
| 4 × 4 ft | 16 | 8.0 | 16.0 | 0.59 | 11 |
| 3 × 6 ft | 18 | 9.0 | 18.0 | 0.67 | 12 |
| 4 × 8 ft | 32 | 16.0 | 32.0 | 1.19 | 22 |
| 4 × 12 ft | 48 | 24.0 | 48.0 | 1.78 | 32 |
| 5 × 10 ft | 50 | 25.0 | 50.0 | 1.85 | 34 |
Read across: the 4 × 8 ft bed at 12 in is the reference case almost every bed kit quotes — 32 cubic feet, a shade over a cubic yard.
Bags are sold by volume, and the volume on the bag is loose
Soils, composts and mulches are sold by cubic measure rather than by weight in the United States — the method of sale set out in NIST Handbook 130, the Uniform Regulation for the Method of Sale of Commodities, which is why the front of the bag says cubic feet and not pounds. Two consequences follow. First, you can compare a $6.50 bag of 1.5 cu ft directly against a $55 cubic yard: 27 ÷ 1.5 = 18 bags per yard, so that bag price is $117 a yard equivalent. Second, the stated volume is the loose, uncompressed volume at packing, which is precisely why the settling allowance exists. Potting mixes are often labelled in quarts instead; there are about 25.7 dry quarts in a cubic foot, so a 32-quart bag is roughly 1.25 cu ft.
Mistakes that leave you short
- Measuring the outside of the frame. A 4 × 8 bed built from nominal 2× lumber has an inside dimension about 3 in smaller each way. Measuring outside over-orders slightly; measuring the lumber's nominal size rather than its actual size under-orders. Measure the hole you are filling.
- Confusing frame height with fill depth. A '12 inch' bed built from two 2×6 boards is really 11 in tall, and you are only filling 9 in of it once freeboard is left. That is 25% less soil than the name implies.
- Forgetting settling. Order the geometric volume exactly and the bed will be visibly sunk after the first week of watering. The allowance is not padding; it is the difference between loose and settled volume.
- Using weight ratios for the mix. Recipes are by volume. Perlite is so light that a mix blended to 10% by weight would be mostly perlite by volume.
- Buying 'topsoil' for a bed on a hard surface. On a patio or roof nothing drains into native ground below, so heavy screened topsoil compacts and waterlogs. Use a lighter mix with a larger aeration share.
- Assuming bulk is always cheaper. It usually is per cubic foot, but a delivery fee on a half-yard order can reverse the comparison entirely. Compare delivered totals, which is what the cost outputs here are for.
- Filling a very deep bed entirely with purchased mix. Beds over about 24 in are commonly filled part-way with coarse material and topped with mix. Enter only the depth you actually intend to buy for.
Key terms
- Freeboard
- The unfilled depth deliberately left between the soil surface and the top of the frame, so watering and mulch stay inside the bed. One to three inches is usual.
- Settling allowance
- Extra volume ordered to account for the compaction a loose mix undergoes once watered and worked. Expressed as a percentage of the geometric volume.
- Aeration amendment
- Coarse, low-density material — perlite, pumice, coarse vermiculite or fine bark — added to keep large pores open so the mix drains and roots get oxygen.
- Cubic yard
- 27 cubic feet, the unit landscape suppliers sell bulk material in. A cubic yard of damp blended mix weighs around half a ton, and of straight screened topsoil closer to a full ton, which is why it arrives on a truck.
- Hügelkultur
- A deep-bed method in which the lower portion is filled with logs and woody debris that decompose slowly, reducing the purchased mix needed and holding water. It also means the bed will sink for several years.
Mix recipes, alternatives and where this calculation stops
The 60/30/10 default is a general-purpose garden-soil recipe, not the only accepted one. The best-known alternative is Mel Bartholomew's 'Mel's Mix' from Square Foot Gardening: one-third compost, one-third peat moss or coir, and one-third coarse vermiculite, all by volume. It is a soilless mix, which makes it light, fast-draining and expensive, and it works well in shallow 6 in boxes where a mineral soil would compact. Enter 33 for compost and 33 for aeration here and the remaining 34% is your peat or coir line. Many gardeners now substitute coir for peat on the grounds that peat extraction damages carbon-storing bogs.
If you are converting an existing lawn or bed rather than filling a new frame, the calculation changes shape entirely. You are then amending, not filling: a 2 in layer of compost spread over the same 32 sq ft is only 5.3 cu ft, one-sixth of the fill quantity. Work out the depth you are adding, not the depth of the bed.
Three things this calculator does not attempt. It does not estimate weight, which as a rule of thumb runs about 75 to 90 lb per cubic foot for damp screened mineral topsoil, 35 to 45 lb for a blended raised-bed mix, and 10 to 20 lb for a peat- or coir-based soilless mix — check with your supplier before loading a trailer or a roof. It does not model the slow annual volume loss from mineralising organic matter, which is real but is handled by a spring top-up rather than by ordering more up front. And it says nothing about fertility: volume and nutrition are independent, and a soil test is the only way to know what your mix actually needs.
For related planning, water use is the other recurring cost of a raised bed — beds dry faster than ground soil because they drain on all sides, and if you are on a metered supply the water and sewer bill calculator will show what a summer of daily irrigation adds. If you are working from a metric plan, convert the footprint with the square feet to square metres calculator before entering dimensions, and if you are mixing liquid feed by the gallon or litre the gallons to litres calculator handles the conversion. The same round-up-to-whole-units arithmetic that makes 40.9 bags into 41 governs most home projects that buy material in indivisible pieces — the carpet and padding calculator is the same problem in two dimensions instead of three.