How Much Soil Do I Need For My Raised Bed

Figuring out how much soil you need for your raised bed is a common first step for gardeners. Getting the right amount saves you time, money, and a last-minute trip to the garden center. This simple calculation ensures your plants have the deep, fertile foundation they need to thrive.

Ordering too little soil leaves your bed incomplete, while too much leads to waste and extra cost. With a quick formula and a tape measure, you can find the perfect answer in minutes. Let’s break down the easy math so you can get back to the fun part—planting.

This question is all about volume. We’re calculating the three-dimensional space inside your bed’s frame. The good news is you only need two things: the dimensions of your bed and a basic formula. You don’t need to be a math expert at all.

Tools You’ll Need for the Calculation

A tape measure (or you can use the bed’s planned design specs).
A calculator (the one on your phone is perfect).
A notepad or your phone’s notes app to jot numbers down.

The Simple Soil Volume Formula

The magic formula is: Length x Width x Height = Volume. You’ll measure in feet to calculate cubic feet, which is how bulk soil and bags are often sold. Here’s how to do it step-by-step.

Measure the Length: Use your tape measure to find the inside length of your raised bed in feet. For example, a bed that’s 8 feet long.
Measure the Width: Next, measure the inside width in feet. A common width is 4 feet, which allows easy reaching to the center from either side.
Measure the Height: Finally, measure how deep your bed is in feet. A height of 1.5 or 2 feet is great for most vegetables. Remember, this is the depth of the soil, not necissarily the full height of the frame if you have a bottom layer of drainage material.

Now, multiply these three numbers together. For our example: 8 ft (Length) x 4 ft (Width) x 1.5 ft (Height) = 48 cubic feet. That’s the total volume of your raised bed.

Adjusting for Soil Settlement and Top-Ups

Fresh soil, especially fluffy compost, will settle over time. It’s smart to account for this by adding 10-15% to your total volume. This gives you a little extra for topping up after the first few waterings and ensures you have enough.

For our 48 cubic foot bed: 48 x 0.15 = 7.2 extra cubic feet. So, your adjusted total would be about 55.2 cubic feet. It’s better to have a small amount left over than to come up short.

What About Irregular Shapes?

For a circular bed, the formula changes slightly. Measure the distance across the center (the diameter). Half of that is the radius. The formula is: 3.14 (π) x Radius x Radius x Height. For an L-shaped bed, break it into two or more rectangles, calculate each volume, and add them together.

Converting Cubic Feet to Bags or Bulk

Now that you have your volume, you need to know how to buy it. Soil is sold in bags (usually 1-2 cubic feet) or in bulk by the cubic yard.

For Bags: Check the bag label for its volume in cubic feet. Divide your total cubic feet by the bag size. For 55.2 cubic feet using 1.5 cu ft bags: 55.2 / 1.5 = 36.8 bags. You’d round up and buy 37 bags.
For Bulk (Cubic Yards): There are 27 cubic feet in a cubic yard. Divide your total cubic feet by 27. For 55.2 cubic feet: 55.2 / 27 ≈ 2.04 cubic yards. You’d order 2 cubic yards, knowing the small extra from your settlement buffer covers the 0.04.

Bulk is often cheaper for large projects, but bags are easier to handle if you don’t have a truck or a place to dump a large delivery.

The “Soil Recipe” for a Healthy Raised Bed

Filling your bed with just plain topsoil isn’t ideal. A balanced mix provides nutrients, drainage, and good structure. A classic recipe is called “Mel’s Mix,” popularized by square foot gardening. It’s equal parts:

Compost: Provides nutrients and beneficial microbes. Use multiple sources if you can (e.g., mushroom, manure, plant-based).
Peat Moss or Coco Coir: Helps retain moisture and lighten the soil. Coco coir is a more sustainable alternative to peat.
Vermiculite: Aids in moisture retention and aeration, keeping the soil loose.

If your using a simpler mix, aim for about 60% high-quality topsoil, 30% compost, and 10% coarse sand or perlite for drainage. Your local garden center can also make a custom bulk blend for you based on there recommendations.

Common Mistakes to Avoid

Not Measuring Accurately: Guesswork leads to problems. Take the two minutes to measure properly.
Forgetting to Convert Units: If you measure inches, convert to feet (divide by 12) before multiplying. A 24-inch height is 2 feet.
Ignoring the Frame Thickness: Measure the inside dimensions. The wood or metal sides take up space you can’t fill with soil.
Filling with Poor Quality Material: Don’t use soil from your yard if it’s heavy clay or fill dirt. Investing in a good mix is key to your sucess.

FAQ: Your Soil Questions Answered

How deep should a raised bed be for vegetables?

Most vegetables need 12-18 inches of soil depth. Root crops like carrots and parsnips prefer 18-24 inches. A depth of at least 12 inches is a good general rule.

Can I put logs or leaves at the bottom to save soil?

Yes, a technique called “hugelkultur” uses logs, branches, and yard waste as a base. It saves soil, improves drainage, and breaks down to feed plants over years. Just ensure you have at least 12 inches of proper soil on top for plants to start in.

How much does raised bed soil cost?

Cost varies widely. Bagged soil mix might cost $5-$10 per cubic foot. Bulk soil is cheaper per unit, often $30-$50 per cubic yard, plus delivery fees. Getting quotes from local suppliers is the best approach.

Do I need to replace the soil every year?

No, you don’t need to replace it. But you should replenish it. Each season, add 2-3 inches of fresh compost on top and mix it in gently. This replaces nutrients and maintains soil volume as organic matter decomposes.

Calculating your soil needs is a simple but essential task. With your volume in hand, you can shop confidently and build a fantastic foundation for your garden. Taking the time to do this right from the start prevents headaches and sets your plants up for a strong, healthy growing season. Now that you know exactly what to do, you’re ready to fill that bed and get planting.