Raised Bed Soil Calculator: How Much Soil Do I Need?

How to Use the Raised Bed Soil Calculator

The calculator gives you two easy ways to estimate your soil requirements.

Calculating Soil for a Birdies Raised Garden Bed

If you already know which Birdies bed you would like, use one of the built-in product options:

1. Select Choose a Garden Bed Model.
2. Choose your Birdies bed height.
3. Select Medium, Large, Jumbo or Round.
4. Choose your preferred bed configuration.
5. Select Calculate Soil Volume.

The Birdies presets use the dimensions and effective capacity of the selected garden bed. There is no need to measure the bed or convert the result yourself.

For 15-inch Birdies beds, calculate enough soil to fill the bed to the full 15-inch depth. For 29-inch beds, the calculator defaults to Birdies' recommended method: approximately 15 inches of soil over logs or other suitable organic material in the lower section. You can switch the calculator to a full 29-inch soil fill if preferred. Some settling is normal, so the bed may need topping up with soil or compost over time.

Calculating Soil for a Custom Raised Bed

Select Enter Dimensions if you want to calculate the soil volume for another rectangular or round raised bed.

Choose imperial or metric measurements and enter the bed's length, width and the soil depth you intend to use. If you plan to fill a bed completely with soil, use its full height as the soil depth. The result does not need to account for every curve, brace or small variation in the bed. It is a practical estimate for ordering soil, and natural settling is likely to have a greater effect than minor differences in measurements.

Understanding Your Soil Calculator Result

The calculator displays the same estimated soil volume in four useful units:

• Litres are helpful when purchasing metric bags of soil or compost.
• Cubic metres are commonly used for larger bulk orders in metric markets.
• Cubic feet are helpful when purchasing bagged soil in the United States.
• Cubic yards are commonly used by US bulk landscape suppliers.

Soil is usually sold in fixed bag sizes or bulk-delivery increments. Once you have your estimate, round up to the next quantity your supplier can provide.

Remember that organic-based mixes naturally settle. Filling the bed to the top at the start gives the mix room to settle, but you should still expect to add more soil or compost as the level drops over the first two to three growing seasons.

How Is Raised Bed Soil Volume Calculated?

For a rectangular raised bed, the basic calculation is:

Length × width × soil depth = soil volume

All measurements must use the same unit. If your length and width are measured in feet but the soil depth is measured in inches, divide the depth by 12 before completing the calculation. For Birdies 29-inch beds, the recommended calculator setting uses a 15-inch soil depth rather than the full 29-inch bed height.

For a round raised bed, the formula is:

Pi × radius² × soil depth = soil volume

The radius is half the diameter of the bed.

The Birdies calculator handles these formulas and unit conversions for you.

How Much Soil Does a 4 × 8 ft Raised Bed Need?

The amount depends on the soil depth you plan to use. For example, a 4 × 8 ft bed filled with 15 inches of soil requires:

4 × 8 × 1.25 = 40 cubic feet of soil

One cubic yard contains 27 cubic feet, so:

40 ÷ 27 = approximately 1.48 cubic yards

This is the estimated amount required to fill the bed to the top. Because organic-based soil mixes settle after watering and over time, expect the level to drop and plan to top it up during the first two to three growing seasons.

Should I Fill My Raised Garden Bed to the Top?

For a 15-inch Birdies bed, fill with soil to the full 15-inch depth. For a 29-inch bed, Birdies recommends filling the lower section with logs or other suitable organic material and adding approximately 15 inches of soil on top. You can also choose to fill the entire 29-inch depth with soil.

Whichever method you use, the bed can be filled to its intended level with the combined fill materials and soil. Some settling is normal as soil and organic materials break down.

Mulch can be mounded slightly above the top rim of the bed. It will usually settle and drop fairly quickly after watering and exposure to the weather.

Check the soil level from time to time and top the bed up with suitable soil or compost as required.

How Deep Should the Soil Be in a Raised Garden Bed?

For 15-inch Birdies beds, use the full 15-inch soil depth. For 29-inch beds, Birdies recommends approximately 15 inches of soil in the upper section, with logs or other suitable organic material below. If you prefer to fill the entire 29-inch bed with soil, the calculator also provides that option.

The upper 15 inches provides a substantial growing zone for a wide variety of vegetables, herbs and flowers. Whichever filling method you use, some settling is normal and soil or compost may need to be topped up over time.

Birdies raised garden beds can be installed in a variety of garden settings, including over soil, lawn and suitable hard surfaces when Birdies' current installation guidance is followed.

If you are choosing a bed height, compare Birdies' 15-inch raised garden beds and 29-inch raised garden beds. A new 20-inch option is also coming soon.

What Is the Best Soil for a Raised Garden Bed?

A good raised bed soil mix should hold enough moisture for your plants while still allowing air and excess water to move through it.

The soil should feel loose and workable rather than heavy, compacted or waterlogged.

There is no single mixture that suits every garden, climate and crop. A useful starting point is a blend of quality screened topsoil and mature compost.

Iowa State University Extension suggests approximately 70% topsoil and 30% compost as one option for raised garden beds.

A reputable local supplier may also offer a prepared mix designed specifically for raised vegetable gardens. Ask what the mix contains, especially if manure-based compost is included.

Compost is valuable, but more is not always better. The University of Minnesota Extension explains that excessive compost or manure can sometimes create nutrient and salt problems.

Filling a 15-Inch Birdies Raised Garden Bed

A 15-inch Birdies bed provides a practical growing depth for vegetables, herbs, flowers and many root crops.

Fill the entire bed to the top with a quality, complete raised bed soil mix. Organic-based mixes will settle as they are watered and the organic material breaks down, so expect to top up the bed during the first two to three growing seasons.

Mulch can be mounded above the rim when first applied. It should settle quickly while helping the soil retain moisture and feeding the soil as it breaks down.

Filling a 20-Inch Birdies Raised Garden Bed

Birdies has a new 20-inch raised garden bed coming soon. It will provide a useful middle option for gardeners who want more height and soil capacity than a 15-inch bed without moving up to the full height of a 29-inch bed.

When the range becomes available, use the calculator preset for your chosen configuration and fill the bed to the top. As with other organic-based soil mixes, expect some settling and plan to top up the bed over the first two to three growing seasons.

Filling a 29-Inch Birdies Raised Garden Bed

A 29-inch Birdies bed provides a comfortable working height and considerably more total capacity.

Birdies recommends filling the lower section of a 29-inch bed with logs or other suitable organic material, then adding approximately 15 inches of quality soil on top. This is the default method used by the calculator and requires roughly half the soil of a full 29-inch fill.

If you prefer, you can instead fill the complete 29-inch depth with soil. Use the calculator's Fill completely with soil option to see that volume.

Use only clean, untreated materials. Avoid treated timber, contaminated soil, diseased plant material and anything likely to introduce persistent weeds or unwanted chemicals.

The woody and organic layers will break down and settle over time. Check the level regularly and expect to add more soil or compost during the first two to three growing seasons. You can learn more about the principles behind hügelkultur from Oklahoma State University Extension.

Should I Buy Raised Bed Soil in Bags or in Bulk?

Bagged soil is convenient for smaller projects, occasional top-ups and gardens where access is difficult.

The Birdies Elevated Planter is specifically designed to be filled with bagged potting mix, making it a convenient option for patios, balconies and smaller growing spaces.

Check the volume printed on each bag rather than relying on its weight. Moisture levels and ingredients can make two bags of the same volume weigh very different amounts.

For one large bed or several raised beds, ordering soil in bulk may save money, packaging and a great deal of carrying.

Before arranging a delivery, ask your supplier:

• Is the soil sold by the cubic yard or cubic metre?
• What ingredients are included in the mix?
• Is there a minimum order?
• Is there a separate delivery fee?
• Where can the soil be unloaded?
• Can you reach your beds with a garden cart or wheelbarrow from the delivery point?

It is much easier to answer these questions before a truck full of soil arrives at your gate.

Common Raised Bed Soil Calculation Mistakes

Chasing Unnecessary Measurement Precision

The calculator provides a practical estimate for ordering soil. For a custom bed, use its stated length and width together with the soil depth you intend to use. Small differences caused by rounded corners, braces or minor measurement variations are usually less important than the natural settling of an organic-based soil mix.

Using the Wrong Fill Depth

For 15-inch beds, calculate the full 15-inch soil depth. For 29-inch beds, the default Birdies recommendation is approximately 15 inches of soil above logs or other suitable organic material, unless you choose to fill the entire 29-inch depth with soil. Some settling is normal, so top up with soil or compost as needed over time.

Mulch can be mounded above the rim and will usually settle fairly quickly.

Forgetting About Every Bed

The soil calculator estimates one bed at a time.

For several identical beds, multiply the result by the number of beds. For different bed sizes, calculate each one separately and add the results together.

Forgetting About Access Around the Beds

A bed may technically fit into a space while leaving too little room to garden comfortably.

Allow at least enough space for a garden cart or wheelbarrow between beds, structures and boundaries.

Purchasing Soil by Weight

Use the volume shown on the soil bag. Weight can change depending on the ingredients and moisture content.

Filling the Bed With Compost Alone

Compost is a useful soil amendment, but it is not automatically a complete growing medium. Too much compost may create nutrient, salt or drainage problems.

Calculate the Soil, Then Plan the Garden

The soil calculator helps you understand how much soil one raised bed may need.