Raised Bed Soil Calculator — Complete Guide for Depth, Mix & Volume
Find out how much soil your raised beds need. Covers soil depth, mix ratios, shrinkage, layering, and using a raised bed soil calculator accurately.
Raised beds grow healthier plants because they give roots loose, fertile, well‑drained soil. But raised beds also require accurate soil depth, the right soil blend, and enough volume to fill them properly. Miscalculations lead to wasted money or half‑filled garden beds.
This guide explains everything you need: soil depth, mix ratios, shrinkage, layering methods, and how to calculate exact soil volume for any raised bed size.
Key Benefits of Understanding Raised Bed Soil Needs
- Accurate soil volume prevents overbuying
- Supports deeper, healthier root growth
- Improves drainage and nutrient flow
- Reduces soil settling and long‑term compaction
- Helps choose the right soil blend for vegetables or flowers
How Much Soil Do You Need for Raised Beds?
Raised beds are measured using length × width × height. The result gives the total volume in cubic feet or cubic yards. A raised bed soil calculator does this instantly.
Example:
4 ft × 8 ft × 1 ft = 32 cubic feet
That equals 1.18 cubic yards.
If you have multiple raised beds, calculate each one and add them together.
Recommended Soil Depth for Raised Beds
Different plants need different depths.
| Plant Type | Ideal Depth |
|---|---|
| Shallow-root vegetables (lettuce, spinach) | 6–8 inches |
| Medium-root vegetables (peppers, beans) | 12 inches |
| Deep-root vegetables (tomatoes, carrots) | 18–24 inches |
| Flowers & herbs | 8–12 inches |
Deeper soil gives roots better access to water, nutrients, and oxygen.
Pro Tip #1
Always measure the inside dimensions of your raised bed. Outside measurements give inflated volume.
Best Soil Mix Ratios for Raised Beds
Raised beds perform best with blends that stay loose, fertile, and well‑drained.
Popular Raised Bed Mix (3‑Part Blend)
- • 1/3 topsoil
- • 1/3 compost
- • 1/3 aeration material (coarse sand, perlite, pine fines)
Vegetable Garden Mix
- • 50% screened topsoil
- • 30% compost
- • 20% coarse sand
Premium Mix for Heavy-Feeding Crops
- • 40% loam soil
- • 40% compost
- • 20% biochar + aeration additives
These blends support moisture retention while preventing waterlogging. For a deeper dive into soil quality, read our guide on Topsoil vs Fill Dirt vs Garden Soil.
Key Benefits of Balanced Soil Mix
- Prevents soggy or compacted soil
- Boosts microbial activity
- Supports deep, strong roots
- Improves plant nutrition and growth
Pro Tip #2
Use screened topsoil only. Unscreened soil contains debris and compacts quickly.
Why Raised Bed Soil Shrinks After Filling
Soil settling is normal. Expect 10–20% shrinkage during the first month.
Reasons for Shrinkage
- • Compost decomposes
- • Air pockets collapse
- • Watering compresses loose soil
- • Soil blends settle into corners
This is why raised beds should be slightly overfilled above the rim during installation.
How to Prevent Excessive Soil Settling
- Add compost each season
- Avoid peat-heavy mixes (they shrink more)
- Water slowly and evenly during first week
- Mix soil thoroughly before filling
Pro Tip #3
If your bed height is over 18 inches, use the layered fill method to save money.
Budget-Friendly Layering for Tall Raised Beds
Filling deep raised beds with premium soil is expensive. Layering reduces cost without harming plant growth.
Layering Method:
- Bottom layer: Logs, branches, sticks
- Middle layer: Compostables: leaves, grass, mulch
- Top 8–12 inches: High-quality soil mix
This mimics hugelkultur and improves moisture retention.
Multi-Bed Calculations (For Multiple Garden Boxes)
If your garden has several raised beds:
- Calculate each bed separately
- Add all cubic feet together
- Convert total to cubic yards or bags
A calculator simplifies this for large gardens.
Bag Conversion Guide
Many gardeners buy soil in bags. However, for larger projects, you might want to consider buying in bulk to save money.
Common Bag Sizes
- • 25 liters
- • 40 liters
- • 50 liters
Conversions
- • 1 cubic foot = 28.3 liters
- • 1 cubic yard = 27 cubic feet = ~765 liters
Use these conversions to estimate exact bag counts for your raised beds.
Pro Tip #4
If buying bags, always buy 10–15% extra for settling and top-ups.
Soil Drainage & Aeration for Raised Beds
Good drainage prevents root rot and disease.
Improve Drainage By:
- Adding coarse sand or perlite
- Avoiding clay-heavy topsoil
- Using compost rich in organic matter
- Mixing soil thoroughly before filling
Signs Your Raised Bed Has Poor Drainage
- Water stays on top for several minutes
- Soil feels muddy or heavy
- Plants wilt despite wet soil
Fixing drainage early improves plant survival rates.
Real Examples (Common Raised Bed Sizes)
Example 1
4×8 Bed (12 inches deep)
Volume: 32 cu ft → 1.18 yd³
Example 2
2×6 Bed (18 inches deep)
Volume: 18 cu ft → 0.66 yd³
Example 3
Two 4×4 Beds (10 inches deep)
Total Volume: ~27 cu ft → 1.0 yd³
Small changes in bed depth make a big difference in soil volume.
FAQs
A 4×8×1 ft bed needs around 32 cubic feet or 1.18 cubic yards.
A 1/3 topsoil, 1/3 compost, and 1/3 aeration blend works for most plants.
Soil settles due to watering, compost breakdown, and air-pocket collapse.
Yes. Layering saves money and improves moisture retention.
Yes. Good drainage prevents waterlogging and root rot.
One cubic yard is about 765 liters, depending on bag size.
No. Just top up with compost each season.
Shallow vegetables yes, but deep-root crops need 12–24 inches.
Optional. Lining helps with weed control and moisture retention.
Yes. Mixing prevents pockets of poor drainage and uneven nutrients.
Final Words
Raised beds need the right soil depth, mix ratio, and volume to produce strong, healthy plants. A raised bed soil calculator simplifies the process by giving exact soil measurements for any garden box size. With accurate depth planning, balanced soil blends, and smart layering techniques, you can build fertile, productive raised beds that last for years.
Grow confidently. Measure correctly. Fill wisely.