Raised Bed Soil & Mix Calculator
Turn raised bed dimensions into soil, bags and mix
Raised bed soil and mix FAQ
How do you calculate raised bed soil volume?
The calculator treats your bed as a simple rectangular box: length × width × effective soil depth. If you enter measurements in feet, the result comes out in cubic feet first, with conversions to cubic yards and liters for comparison. Metric entries work the same way but start from cubic metres.
What units should I measure in?
For most US home projects it is easiest to measure length, width and depth in feet, because bag sizes at the store are usually in cubic feet. If you prefer metric, switch to meters and measure in metres; the tool still shows cubic feet alongside cubic metres and litres so you can match local bags.
Why do I need a bag size?
Soil yards and garden centres sell in bags (or by the cubic yard if you order bulk). Your bed volume alone does not tell you how many bags to buy. The bag size lets the calculator convert total volume into bag counts, rounding up so you don’t end up short when you are halfway through filling.
What mix ratio should I use for raised beds?
A common starting point is about 60% topsoil, 30% compost and 10% other material such as sand, bark or perlite for drainage. Some gardeners like a simple 50/50 topsoil–compost blend instead. The presets cover both ideas, and the custom parts option lets you match any recipe you like.
Can I change the ratio for different beds?
Yes. Each calculation is independent. You can run one bed with a richer compost mix for heavy feeders such as tomatoes, and another with more mineral topsoil for herbs or Mediterranean plants. Just adjust the mix style or custom parts and re-run the calculator.
Does this include paths or mounded soil?
No. The calculator assumes a fairly flat surface, filled to your chosen depth. If you plan raised paths, very mounded beds or decorative berms, you may want to add a small extra margin to the bag count to cover those shapes.
Why do the numbers not match another calculator exactly?
Small differences appear because some tools convert between cubic yards, metres and litres with slightly different rounding, or assume packed vs. fluffed soil. This calculator keeps the math transparent and rounds to practical numbers so you can comfortably buy an extra bag if you are on the edge.
Should I buy loose bulk soil instead of bags?
For very large beds, the bag count may suggest that bulk delivery by the cubic yard is more economical. You can still use the total cubic feet or cubic yards from this tool when talking to a local soil yard and ask them to convert it into their truck or trailer loads.
How to use this raised bed soil & mix calculator
This calculator turns your raised bed’s inside dimensions into total soil volume, approximate bag counts and a simple ingredient breakdown by mix ratio. The idea is to help you walk into the garden centre with a clear shopping list instead of guessing in the aisle.
1. Pick units and bag size
Start by choosing whether you want to work in feet or meters. For US projects, feet plus cubic feet usually line up best with bag labels. Then either enter your bag size (for example 1.5 cu ft or 40 L) or leave it blank so the calculator uses a typical bag size. You can always tweak this if your store uses something different.
2. Measure inside bed length, width and depth
Measure from inside board to inside board so you are only filling the soil cavity, not the timber thickness. Enter the length and width, then the effective soil depth you want to fill. For many beds, 10–12 inches (about 0.25–0.3 m) of good soil over native ground is enough; deeper beds for root crops may need more.
3. Choose a mix style or custom parts
Use the Mix style box to pick a quick recipe:
- 60/30/10 — roughly 60% topsoil, 30% compost and 10% extra material.
- 50/50 — simple topsoil–compost blend.
- Equal thirds — even split between topsoil, compost and other.
If you have your own recipe, choose Custom parts and enter any part numbers you like (for example 3 parts topsoil, 2 parts compost, 1 part sand). Only the ratio matters; the tool converts those parts into real-world volumes and bags.
4. Read volume, bags and ingredient breakdown
Tap Calculate soil & mix to see:
- Total bed volume in cubic feet, cubic yards and litres.
- Approximate number of bags needed, rounded up so you are not short.
- How that volume splits into topsoil, compost and other by your chosen ratio.
- Suggested bag counts for each ingredient so you can shop by component.
The calculator keeps the numbers practical and rounded so you can easily decide whether to grab an extra bag or two as a margin.
5. Copy the summary and adjust on site
Use the Copy summary button to paste the plan into your notes app or share it with whoever is picking up materials. Once you start filling the bed, you can quickly see whether you were generous or conservative and adjust your mix or bag count the next time you fill another bed.
Think of this page as your quick math helper for raised beds. Your eye on the actual soil level, your budget and the materials available locally always win; the calculator just keeps the volume and ratios consistent between projects.
How the raised bed soil & mix math works
The calculator follows the same core steps most garden planners use: work out the volume of the bed, convert it into useful units, and then split that volume by your mix ratio and bag size.
1. Box volume from your dimensions
First, your length, width and depth are converted into a consistent unit (feet or metres). For a rectangular raised bed, the volume is:
Volume = length × width × depth
If you enter feet, the result is instantly a volume in cubic feet. If you enter metres, the result is in cubic metres, which is then converted into cubic feet.
2. From cubic feet/metres to litres and cubic yards
To make comparing quotes and bag sizes easier, the calculator also converts between units:
- 1 cubic metre ≈ 35.31 cubic feet
- 1 cubic foot ≈ 28.32 litres
- 1 cubic yard = 27 cubic feet
Using these factors, you get one clean line showing cubic feet, cubic yards and litres so you can match whatever your supplier uses.
3. Bag counts from total volume
Once the total volume in cubic feet is known, it is divided by the bag size in cubic feet (or converted from litres if you are working in metric):
Bags needed = total_cubic_feet ÷ bag_cubic_feet
The calculator rounds up to the next whole bag because you cannot buy fractions of a bag and it is safer to have a little extra mix than not quite enough to reach the top board.
4. Splitting by mix ratio
For the mix, the tool uses parts rather than percentages so you can enter any recipe you like. If your parts are:
Topsoil parts = T, Compost parts = C, Other parts = O
then the total parts are:
Total parts = T + C + O
Each ingredient then gets:
Ingredient volume = total_volume × (parts ÷ total_parts)
and the same bag-size logic is used to turn that volume into bags of each ingredient, rounded up so you have enough to blend a consistent mix.
All of the equations are designed to be simple enough to check on a handheld calculator if you like. That way you can trust the numbers, adapt them to different beds and always see exactly how the soil and mix pieces fit together.
References and further reading on raised beds and soil mixes
Use these guides alongside this raised bed soil & mix calculator when planning projects:
- University of Minnesota Extension — Raised bed gardens — guidance on sizing beds, soil depth and site choice.
- Oregon State University Extension — Raised Bed Gardening (FS 270) — practical notes on constructing and filling raised beds.
- RHS — How to make your own potting mix — background on blending soil ingredients and organic matter.
Always adapt mix ratios and depths to your specific climate, plant choices and any recommendations from local nurseries or experienced gardeners in your area.