Figuring out how much garden soil do i need is a common first step for any planting project. Getting the right amount saves you time, money, and the hassle of returning bags or ordering more. This simple guide will walk you through the easy calculations for any garden bed, raised box, or container.
You don’t need to be a math expert. With a tape measure and a few minutes, you’ll know exactly how much soil to buy. We’ll cover simple formulas, quick tips, and common mistakes to avoid. Let’s get your garden off to a great start.
How Much Garden Soil Do I Need
This is the core question. The answer comes down to volume, which is just length, width, and depth. We measure garden soil in cubic feet or cubic yards. Bagged soil is usually sold in cubic feet, while bulk deliveries come by the cubic yard.
First, you need to know the shape of your garden space. The calculations are slightly different for a rectangular raised bed versus a circular flower pot. Don’t worry, we’ll cover both.
The Basic Formula: Length x Width x Depth
This formula works for any square or rectangular area. It’s the foundation for all your calculations.
1. Measure the Length: Use a tape measure to find the longest side of your garden bed in feet.
2. Measure the Width: Find the shorter side in feet.
3. Determine the Depth: Decide how deep you want the soil to be. For most gardens, 6 to 12 inches is standard. Remember to convert inches to feet for the calculation (6 inches = 0.5 feet, 12 inches = 1 foot).
Now, multiply them together: Length (ft) x Width (ft) x Depth (ft) = Volume in cubic feet.
Example: You have a raised bed that is 8 feet long, 4 feet wide, and you want 10 inches of soil.
* Convert depth to feet: 10 inches ÷ 12 = 0.83 feet.
* Calculation: 8 ft x 4 ft x 0.83 ft = 26.56 cubic feet.
Adjusting for Existing Soil
Are you filling a raised bed on top of ground? You might not need to fill it to the very top if you have native soil underneath. If you plan to mix with the ground below, subtract 2-3 inches from your total depth before calculating. This can save you a significant amount of soil.
Calculating for Circular Pots and Gardens
For round containers, you need a different formula. It involves pi (π), but you can just use 3.14 as a simple number.
The formula is: 3.14 x (Radius x Radius) x Depth.
1. Find the Radius: Measure the distance across the pot at its widest point (the diameter). Divide that number by 2 to get the radius.
2. Measure the Depth: How deep is the pot, or how deep do you want the soil?
Example: You have a large round pot with a 3-foot diameter and a 2-foot depth.
* Radius = 3 ft ÷ 2 = 1.5 ft.
* Calculation: 3.14 x (1.5 x 1.5) x 2 = 3.14 x (2.25) x 2 = 14.13 cubic feet.
Converting Cubic Feet to Bags or Bulk Yards
Now that you have your total cubic feet, you need to translate it into something you can buy.
For Bagged Soil:
Most bagged soil and compost comes in 0.75, 1, or 2 cubic foot bags. Check the bag label carefully. Simply divide your total cubic feet by the bag size.
* From our raised bed example: 26.56 cubic feet needed.
* Using 1.5 cubic foot bags: 26.56 ÷ 1.5 = 17.7 bags.
* Always round up. You should buy 18 bags.
For Bulk Soil (Cubic Yards):
Bulk soil is sold by the cubic yard. There are 27 cubic feet in one cubic yard. To convert, divide your total cubic feet by 27.
* From our raised bed example: 26.56 cubic feet needed.
* Calculation: 26.56 ÷ 27 = 0.98 cubic yards.
* You would order 1 cubic yard from a supplier.
The “Little Extra” Rule
Soil settles after being poured and watered. It’s also smart to have a bit left over for topping up later in the season. A good rule is to add 10-15% to your final total. For our 26.56 cubic foot example, adding 10% means buying about 29.2 cubic feet worth.
Special Considerations for Different Garden Types
Not all gardens are created equal. The type of plants your growing affects how deep your soil needs to be.
Raised Garden Beds
* Vegetables & Annuals: Aim for a minimum depth of 12 inches. 18-24 inches is ideal for deep-rooted plants like tomatoes and carrots.
* Herbs & Lettuce: 6 to 8 inches can be sufficient.
Remember to calculate the entire volume of the bed frame if you’re filling it on a hard surface.
In-Ground Garden Beds
When amending existing soil, you’re not filling a volume but adding a layer on top. The calculation is similar, but the depth will be much smaller.
* Measure the length and width of the bed area.
* Decide how thick a layer of compost or topsoil you want to add (usually 2-4 inches).
* Example: A 10ft x 6ft bed with a 3-inch layer: 10 x 6 x 0.25 ft = 15 cubic feet.
Containers and Pots
You rarely fill a pot completely to the brim with soil. Leave 1-2 inches of space at the top for watering. Subtract this from the pot’s total depth before you calculate. A 12-inch deep pot should be calculated with a 10-inch soil depth.
Common Mistakes to Avoid
A few simple errors can lead to a big soil shortage or surplus. Here’s what to watch for:
* Measuring in inches, not feet: This is the most common error. Always convert your depth measurement to feet before multiplying.
* Forgetting about settlement: Fresh, fluffy soil will compact. That 12-inch depth might become 10 inches after a good rain.
* Ignoring the shape: Using the rectangle formula for a circular bed will give you a very wrong answer.
* Underestimating bag coverage: The bag might say it covers a certain area, but this is often for a very thin layer. Rely on your cubic volume calculation instead.
* Not checking what’s included: Some “garden soils” have a lot of wood chips that will break down over time, causing more settlement.
A Quick Reference Table
Here’s a handy cheat sheet for common raised bed sizes, assuming a 12-inch (1-foot) soil depth.
| Raised Bed Size (LxW) | Soil Needed (Cubic Feet) | 1.5 cu ft Bags Needed |
| :——————– | :———————-: | :——————–: |
| 4 ft x 4 ft | 16 | 11 |
| 6 ft x 3 ft | 18 | 12 |
| 8 ft x 4 ft | 32 | 22 |
| 10 ft x 4 ft | 40 | 27 |
Remember, if you want less than 12 inches of depth, you’ll need less soil than shown here.
FAQ: Your Soil Questions Answered
What’s the difference between topsoil and garden soil?
Topsoil is the base layer, often screened for rocks. Garden soil is usually a blend of topsoil, compost, and other organic matter designed for planting. For raised beds and containers, use garden soil or a specific raised bed mix.
How do I calculate soil for multiple beds?
Calculate the soil need for one bed first. Then, simply multiply that number by the total number of identical beds you have. If the beds are different sizes, calculate each one separately and add the totals together.
Should I buy bags or bulk?
For small projects under about 20 cubic feet, bags are convenient. For large projects like filling big raised beds or amending entire garden plots, bulk delivery is almost always more cost-effective and creates less plastic waste.
What if my garden is an odd shape?
Break it down into simpler shapes. An L-shaped bed can be treated as two separate rectangles. Calculate the soil for each rectangle, then add them together for the total.
Do I need to fill my entire raised bed with expensive soil?
No! You can use a technique called “hugelkultur” or simple filler. The bottom one-third to one-half of a deep bed can be filled with logs, sticks, leaves, or grass clippings. Then, fill the top portion with your quality garden soil mix. This saves money and improves drainage.
Figuring out your soil needs is a simple but essential task. With your tape measure and these formulas, you can confidently order the perfect amount. No more guessing, no wasted trips to the garden center, and no half-empty beds. Now you’re ready to focus on the fun part—choosing your plants and watching them grow.