Garden Bed Soil Calculator
Calculate exactly how much soil, compost, and mulch you need for your raised garden beds.
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How to Calculate for Garden Bed Soil Calculator
The Formula
We calculate the cubic volume of your bed and then add a percentage for "shrinkage" (compaction) to ensure you have enough soil after watering.
Measure Accurately
Measure in feet. For depth, measure in inches and our tool handles the conversion (divide by 12).
Check Your Shape
Most garden bed-fill-calculator projects are rectangles. If irregular, break it into smaller shapes.
Add Compaction
Garden soil settles by 15-20% after watering. Our tool auto-calculates this buffer so your beds don't end up half-empty.
Filling a raised garden bed is more than just dumping dirt in a box. It's about creating a living ecosystem that will feed your plants for years to come. Whether you're building a simple 4x8 cedar box or an elaborate keyhole garden, calculating the exact amount of soil you need is the first critical step to success.
In this comprehensive guide, we'll walk you through everything from the basic volume physics to advanced soil recipes like "Mel's Mix" and the "Hugelkultur" method. We'll also help you navigate the confusing world of bulk vs. bagged soil costs, so you don't overspend on your project.
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How to Calculate Soil for Raised Beds (The Accurate Way)
The foundation of any garden project is accurate measurement. Guessing can lead to two frustrating outcomes: a half-filled bed that shades your seedlings, or a driveway pile of expensive dirt you have nowhere to put. To get it right, we need to think in three dimensions.
Accurate measurement of Length, Width, and Depth is critical.
The Basic Formula for Rectangular Beds
Most raised beds are rectangles or squares. The math here is straightforward geometry. You are calculating volume, which is the amount of space inside the box.
Volume (Cubic Feet) = Length (ft) × Width (ft) × Depth (ft)
Example: Let's say you have a standard 4x8 foot raised bed that is 12 inches (1 foot) deep.
Calculation: 4 × 8 × 1 = 32 Cubic Feet.
Calculating for Circular Garden Beds
Fabric grow pots, galvanized fire rings, and keyhole gardens are often circular. The math changes slightly here, relying on the radius (half of the diameter).
Volume = π × Radius² × Depth
Example: A 4-foot diameter galvanized round bed, 2 feet deep.
Radius is 2 feet. Radius squared (2×2) is 4.
3.14 (π) × 4 × 2 (depth) = 25.12 Cubic Feet.
The "Shrinkage Factor": Why You Need 15% More Soil
Here is the classic novice gardener mistake: You calculate you need 32 cubic feet, you buy exactly 32 cubic feet, and one week after watering, your soil level is 3 inches below the rim. Why? Compaction.
What is Soil Settlement?
Soil is fluffy when you buy it. It contains pockets of air that artificially inflate its volume. As soon as you add water and gravity takes effect, those air pockets collapse. This is natural and healthy, but it ruins your volume calculations if you don't account for it.
How Much Extra Should You Buy?
We recommend a buffer of 10% to 15%. If your math says you need 1 yard, buy 1.15 yards. Is having a little leftover dirt a problem? Never. You will always find a use for it—topping off pots, filling low spots in the lawn, or just saving it for next year.
Best Soil Recipes for Raised Beds (Mel's Mix & More)
Filling a bed with 100% topsoil is a recipe for disaster. It turns to concrete in the summer and mud in the winter. Raised beds need a custom blend that holds moisture but drains well. Here are the top three recipes used by pro gardeners.
Mel's Mix
The Gold Standard for Square Foot Gardening.
- ✓ 1/3 Peat Moss or Coir
- ✓ 1/3 Coarse Vermiculite
- ✓ 1/3 Blended Compost
Hugelkultur
Best for deep beds (>18") to save money.
- ✓ Bottom: Logs & Branches
- ✓ Middle: Leaves & Twigs
- ✓ Top 6-12": Good Soil
The 50/50 Split
Economical and effective for most crops.
- ✓ 50% High Quality Topsoil
- ✓ 50% Organic Compost
- ✓ Optional: Add Perlite
Bulk vs. Bagged: The Cost Calcuation
The biggest question in garden economics: Should you drag 50 plastic bags from the big box store, or pay for a dump truck delivery? The answer almost always comes down to Volume Thresholds.
The Rule of "One Cubic Yard"
One cubic yard is the magic number. Why? For easy conversion help, use our cubic yards calculator.
- 1 Cubic Yard = 27 Cubic Feet.
- A standard bag of garden soil is usually 1.5 Cubic Feet.
- Therefore, 1 Cubic Yard = 18 Bags of Soil.
| Metric | Bagged Soil | Bulk Delivery |
|---|---|---|
| Cost Per Yard | $100 - $150 | $40 - $60 (+ delivery) |
| Effort | High (loading/unloading) | Low (dumped in driveway) |
| Plastic Waste | Massive (18+ bags) | Zero |
| Quality Consistency | Variable (check bag contents) | Look at the pile before buying |
Pro Tip: If you need less than 0.5 cubic yards (about 9 bags), buy bagged for convenience. If you need more than 1 cubic yard, bulk delivery is almost always cheaper, even with the $50-$100 delivery fee added.
Step-by-Step Guide to Filling Your First Bed
Prep the Site
Grass will grow through 12 inches of soil easily. Lay down thick cardboard (remove tape first) at the bottom of your bed. This smothers the grass and eventually decomposes into worm food.
Layering (The Lasagna Method)
Don't mix everything at once. Put your "browns" (leaves, twigs) at the bottom, then your native soil or cheaper topsoil, and save your expensive compost/Mel's Mix for the top 6-8 inches where the roots live.
The Water-In
Fill the bed halfway, then soak it thoroughly. Fill to the top, and soak again. This jumpstarts the settling process so you can top it off immediately rather than discovering a sunken bed a week later.
Ready to Grow?
A successful garden starts with the soil. By measuring accurately and choosing the right ingredients, you are setting your plants up for massive yields. Don't forget to bookmark this calculator for your annual "top-off" calculations next spring!
Frequently Asked Questions
Can I just use dirt from my yard in a raised bed?
Generally, no. Garden soil is often too heavy and compacts in a raised bed, suffocating roots. Raised beds require a lighter, fluffier mix (like compost and vermiculite) to ensure proper drainage and aeration.
How many bags of soil do I need for a 4x8 box?
A 4x8 bed that is 1 foot deep is 32 cubic feet. A standard soil bag is 1.5 cubic feet. You would need approximately 22 bags (32 ÷ 1.5 = 21.3) to fill it.
What is the cheapest way to fill a raised bed?
The 'Hugelkultur' method is the cheapest. Fill the bottom 50% of the bed with free organic debris like logs, branches, and leaves. Then, only buy quality soil for the top 12 inches.
Does potting soil work in raised beds?
Yes, potting soil works great but is very expensive. It is designed for containers. For large raised beds, it's better to mix your own blend using bulk components to save money.
Should I put rocks in the bottom for drainage?
No! This is a gardening myth. Rocks at the bottom actually raise the water table (perched water table effect) and can cause root rot. Use permeable organic matter like sticks instead.
How often do I need to replace the soil?
You never need to replace it entirely. However, you must 'top it off' with fresh compost every year. Soil life breaks down organic matter, causing the level to drop 1-2 inches annually.
What is vermiculite used for?
Vermiculite is a mineral expanded by heat that acts like a sponge. It holds water and nutrients in the soil, making them available to plants slowly. It is a key ingredient in 'Mel's Mix'.
Can I plant immediately after filling?
Yes, but it's better to wait a few days if you can. Watering the bed settles the soil. If you plant immediately, your seedlings might sink deeper than you intended as the soil collapses.
What is the best depth for a raised bed?
12 inches (1 foot) is standard and sufficient for most veggies. However, root crops like carrots or parsnips benefit from 18 inches. Deep beds (24"+) are easier on your back but cost more to fill.
Is bulk delivery cheaper than bags?
Yes, if you need more than 1 cubic yard (about 1 pickup truck load). Below that amount, the delivery fees often make bags the more economical choice.
Do I need fabric liner for my raised bed?
Only if you have a massive weed problem (like Bermuda grass) or gophers. Otherwise, cardboard is better because it allows earthworms to move up into your new soil.
How do I calculate volume for a U-shaped bed?
Break the 'U' shape into three separate rectangles. Calculate the volume of each rectangle (Length x Width x Depth) and add them together for the total volume.
What is the difference between topsoil and garden soil?
Topsoil is generic mineral soil from the earth's surface. 'Garden Soil' is typically topsoil enriched with compost and organic matter. Garden soil is better for immediate planting.
Should I test my soil pH?
If you are buying a quality commercial blend, the pH should be balanced (6.0-7.0). If you are mixing your own using local peat moss (acidic) or mushroom compost (alkaline), a cheap test kit is a good safety measure.
How much does a cubic yard of soil weigh?
A cubic yard of dry topsoil weighs about 2,000 lbs (1 ton). If it is wet or has high organic matter, it can vary. Always check weight limits if hauling it yourself in a truck!