Calculator
Raised Bed Soil Mix Calculator
Filling a raised bed is not the same problem as filling a low spot in a lawn. A lawn just needs enough soil to reach a target depth; a raised bed needs a specific mix — topsoil, compost and something for drainage and aeration — blended in proportions that actually grow vegetables well, and it needs enough headroom below the top of the frame that watering doesn't wash soil straight over the edge. The soil calculator already handles the plain depth-fill question; this page answers a different one.
This calculator starts from the bed's volume, backs off a fill percentage for freeboard at the top, adds a settling allowance on top of that because freshly blended bed mix compacts as it beds in, and only then splits the resulting total into topsoil, compost and aeration material by the ratio you choose. Change the ratio and the total volume needed doesn't change — only how that volume is divided between the three ingredients does.
Calculate your quantity
Leave freeboard so watering does not wash soil out.
Total mix needed
33.12 cu ft
- Bed volume before settling
- 28.8 cu ft
- Topsoil
- 16.56 cu ft
- Compost
- 9.94 cu ft
- Aeration (bark, perlite, pumice)
- 6.62 cu ft
- Total in cubic yards
- 1.227 cu yd
- Beds
- 1 beds
- Filling to 90% of the frame leaves 1.2 in of freeboard so watering does not wash soil over the edge, and the 15% settling allowance is added on top of that.
Shopping summary
- • 12 x 1.5 cu ft bags of topsoil
- • 7 x 1.5 cu ft bags of compost
- • 5 x 1.5 cu ft bags of aeration material
This is an estimate — confirm structural work with a professional.
How this calculation works
The bed's gross volume comes from its length, width and depth, multiplied by the fill percentage — filling to 100% of the frame leaves no room at all for a thorough watering without soil washing over the edge, so a working fill target is usually somewhere under that, with the unfilled portion left as freeboard.
A settling allowance is then added on top of the fill volume, not folded into it, because freshly mixed soil is loose and full of air pockets that compress over the first several waterings — filling exactly to the target depth on day one reliably leaves the bed noticeably low by the following month once it settles. The final volume is then split by ratio into topsoil, compost and aeration material (bark fines, perlite, pumice or a similar amendment), each as a share of the same total rather than each calculated independently.
- Fill percentage and settling allowance are two separate deductions working in the same direction — freeboard leaves room at the top on purpose, settling accounts for volume the bed will lose on its own over time.
- The topsoil, compost and aeration percentages don't need to add to exactly 100 — they're treated as relative shares of the total volume, so a ratio of 50/30/20 and a ratio of 5/3/2 produce the same split.
- Multiple beds share the same per-bed ratio and fill settings in this calculation — a garden with beds built to different depths or mixes needs to be run once per distinct bed design.
- This page does not calculate how much material fills a bed to a target depth alone — that's the plain fill question the soil calculator answers; this page's whole purpose is splitting the volume into a mix by ratio.
The formula
gross volume = length × width × (depth ÷ 12) × (fill % ÷ 100) × beds; total mix = gross volume × (1 + settle % ÷ 100); each ingredient = total mix × (its share ÷ sum of all shares)
- fill percentage
- How much of the frame's full depth actually gets filled with mix, leaving the rest as freeboard so a heavy watering or rainfall doesn't carry loose soil over the top edge of the bed.
- settling allowance
- Extra volume added on top of the fill target to account for compaction as loose, freshly blended mix settles over its first weeks — without it, a bed filled exactly to the fill line is measurably lower within a season.
- topsoil / compost / aeration shares
- The relative proportion of each ingredient in the final mix, treated as ratios rather than percentages that must sum to 100 — a heavier compost share holds more moisture and nutrients, more aeration material improves drainage in a bed prone to staying soggy.
- beds
- The number of identical beds being filled at once, multiplying the single-bed volume directly — used for ordering material for a multi-bed garden built to one consistent design, not for beds of different sizes or mixes.
Splitting a fixed total volume by ratio, rather than calculating each ingredient's fill depth independently, is what keeps the three ingredients from silently adding up to more or less than the bed can actually hold. Changing the ratio reallocates the same total; it never changes how much total mix the bed needs.
Where these numbers come from
- 50/30/20 topsoil/compost/aeration starting ratio
- Trade convention among raised-bed vegetable gardening guidance — a mix weighted toward topsoil for structure, with enough compost for nutrients and organic matter, and aeration material to prevent the compacted, waterlogged texture straight topsoil-and-compost blends can develop over time. Specific crops and native soil conditions shift this ratio in either direction.
- 90% fill target
- Working default leaving roughly 10% of the frame's depth as freeboard — trade convention rather than a fixed rule, intended to keep a hard watering or a heavy rain from carrying loose surface soil over the edge of the bed.
- 15% settling allowance
- Trade convention for freshly blended bed mix; bagged and bulk topsoil-compost blends both settle measurably as they bed in and the loose structure from mixing and delivery compacts. Beds filled with well-aged, already-settled material can reasonably use a lower figure.
- 1.5 cu ft bagged material
- Trade convention — the most common bag size sold for topsoil, compost and soil amendments at garden centers, used here only to convert a bulk volume into a bag count for smaller beds where a bulk delivery minimum isn't worth ordering.
Worked examples
A single standard 8 by 4 foot bed at 12 in deep
| Bed length | 8 ft |
|---|---|
| Bed width | 4 ft |
| Frame depth | 12 in |
| Number of beds | 1 |
| Fill level | 90 % |
| Topsoil share | 50 % |
| Compost share | 30 % |
| Aeration share | 20 % |
| Settling allowance | 15 % |
| Total mix needed | 33.12 cu ft |
|---|---|
| Bed volume before settling | 28.8 cu ft |
| Topsoil | 16.56 cu ft |
| Compost | 9.94 cu ft |
| Aeration (bark, perlite, pumice) | 6.62 cu ft |
| Total in cubic yards | 1.227 cu yd |
| Beds | 1 beds |
The bed's frame holds 28.8 cu ft at a 90% fill target, and the 15% settling allowance on top of that brings the working total to 33.12 cu ft — nearly 4.5 cu ft more than the bare fill volume, which is exactly the margin that keeps this bed from reading visibly low by midsummer. Split 50/30/20, that comes to 16.56 cu ft of topsoil, 9.94 of compost and 6.62 of aeration material.
At 1.5 cu ft per bag, that's roughly 12 bags of topsoil, 7 of compost and 5 of aeration material — 24 bags total for one bed, which is a reasonable case for a bulk blended delivery instead if the local supplier offers one, since bagged material at this volume often costs meaningfully more per cubic foot than a bulk order.
Four deep beds for root vegetables, topsoil-heavy mix
| Bed length | 4 ft |
|---|---|
| Bed width | 4 ft |
| Frame depth | 24 in |
| Number of beds | 4 |
| Fill level | 95 % |
| Topsoil share | 60 % |
| Compost share | 25 % |
| Aeration share | 15 % |
| Settling allowance | 10 % |
| Total mix needed | 133.76 cu ft |
|---|---|
| Bed volume before settling | 121.6 cu ft |
| Topsoil | 80.26 cu ft |
| Compost | 33.44 cu ft |
| Aeration (bark, perlite, pumice) | 20.06 cu ft |
| Total in cubic yards | 4.954 cu yd |
| Beds | 4 beds |
Four identical 24 in deep beds multiply the per-bed volume directly, and the total mix comes to 133.76 cu ft — about 4.95 cu yd, well into bulk-delivery territory rather than a bagged purchase. The ratio here is topsoil-heavy (60/25/15) rather than the first example's more even split, which suits deep-rooted crops that want more structural soil depth and less of the lighter, faster-draining aeration material a shallow bed of leafy greens might favor.
Notice the settling allowance is lower here (10% against 15%) on the assumption this mix uses more finished, already-settled compost rather than fresh material — that single percentage point choice moves the total by several cubic feet across four beds, which matters when ordering bulk material in fractions of a cubic yard rather than whole bags.
Common mistakes
- Filling a bed to 100% of its frame depth with no freeboard, then watching soil wash over the edge on the first heavy watering or rainstorm.
- Filling exactly to the target line with no settling allowance, and finding the bed noticeably low within the first month as loose material compacts.
- Treating the topsoil/compost/aeration percentages as though they must sum to exactly 100 and adjusting the wrong one to force the total — they're relative shares, and any consistent ratio produces the same split of the fixed total volume.
- Using this page for a simple top-up of an existing, already-settled bed rather than the soil calculator's plain depth fill — a top-up doesn't need a fresh ratio split or a full settling allowance the way an initial fill does.
- Assuming a heavier compost ratio is always better. Too much compost by volume can hold excess moisture and nutrients unevenly, and most raised-bed guidance treats compost as a substantial minority of the mix rather than the majority ingredient.
Shopping summary
At this page's default 90% fill and 15% settling allowance, the working total volume is meaningfully more than the bed's fill-line volume alone — order to the total mix figure, not the raw frame volume, or the bed will read low within weeks of filling.
Small single-bed jobs are usually cheaper and easier bought bagged, at roughly 1.5 cu ft per bag; multi-bed or deep-bed jobs crossing a few cubic yards are typically better priced as a bulk delivery, sometimes as a pre-blended garden mix rather than three separate ingredients. If native soil under the bed is heavy clay or very sandy, amending it before the bed frame goes down is worth doing at the same time — the soil calculator covers a straightforward depth-based amendment for ground-level beds.
Once a bed is established and its organic matter needs a boost in a later season rather than a full refill, that's a different job with its own arithmetic — the compost calculator works out how much compost actually moves an existing bed's organic matter percentage from where it is to where you want it, which this page's ratio-based initial fill doesn't attempt to answer.
FAQ
Why does this page add both a fill percentage and a separate settling allowance?
Because they solve two different problems. The fill percentage leaves room at the top of the frame on purpose, so watering doesn't wash soil over the edge; the settling allowance adds extra volume because loose, freshly blended mix compacts over its first weeks regardless of how much freeboard was left. Skipping either one leaves the bed either overflowing on day one or visibly low by midsummer.
Do the topsoil, compost and aeration percentages need to add up to 100?
No. A gardening book's recipe stated as '3 parts topsoil, 2 parts compost, 1 part perlite' and this page's 50/30/20 default describe nearly the same mix once each is reduced to a fraction of its own total — 3:2:1 works out to roughly 50/33/17. Enter whatever numbers a source gives you directly rather than rescaling them to sum to 100 first; the split happens on the ratio between the numbers, not on their total.
I already have an established bed that's just settled a bit — do I need to run the full calculation again?
Not really. A modest top-up of an already-settled bed is closer to the plain depth-fill question the soil calculator answers than to a fresh mix build — you're not blending a new ratio or accounting for a full settling cycle, just adding volume back to a target depth.
What's a reasonable aeration material to use, and does the type matter for the calculation?
Perlite, pumice, coarse bark fines and rice hulls are all common choices, and the calculation itself is indifferent to which one you pick since it works entirely in volume. What does matter outside this page's math is that some aeration materials (perlite especially) are much lighter by weight than others, which affects delivery and handling even though the volume ordered is the same.
Should every bed in my garden use the same ratio?
Not necessarily — beds growing deep-rooted crops like carrots or potatoes often do better with a topsoil-heavier mix for structure, while beds of shallow-rooted greens or a bed prone to staying wet can benefit from a higher aeration share. Run the calculation once per distinct ratio you're using rather than assuming one mix fits every bed in a mixed garden.
How much does bed depth actually change the total volume needed?
Directly and proportionally — doubling the depth doubles the fill volume before any ratio split happens, which is why a shift from a shallow 8 in bed to a deep 24 in bed for root vegetables changes the order by three times, not by some smaller adjustment. Depth is usually the single biggest lever on total volume in this calculation, ahead of length or width changes of a similar percentage.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.