Raised Bed Soil Calculator

1. Choose the bed shape: rectangle, round (circle) or L-shape.
2. Enter the dimensions in feet. For inches, divide by 12 (for example, 8 inches is about 0.667 ft).
3. Enter the fill depth in feet.
4. Pick a bag size that matches what your garden center sells (0.75, 1, 1.5, 2 or 3 cubic feet).
5. Choose a mix recipe: Mel's Mix, Standard 60/30/10 or Custom/single soil.
6. Set a settling buffer (10 to 15 percent is typical) to account for soil compacting over time.
7. Read the total volume, bag count and mix breakdown for each component.

A raised bed soil calculator takes the dimensions of a garden bed and works out the volume of soil you need to fill it. It handles the common shapes that gardeners build: rectangular beds, round or circular beds and L-shaped beds with a rectangular cutout.

The tool also tells you how many bags to buy at a given bag size and breaks the total down by soil mix component, such as compost, topsoil and perlite. A settling buffer adds extra volume so you do not run short after the soil compacts in the first few weeks.

All of the math is basic geometry and proportions. No lookup tables, no volatile data. The only inputs are your measurements and your choices for bag size, mix and buffer.

The calculator measures volume with a formula that matches your bed shape, then divides by the bag size and splits the total across your chosen mix.

Unit Conversions

• 1 cubic yard = 27 cubic feet
• 1 cubic foot = 28.3168 liters
• 1 cubic foot = 7.48 US liquid gallons

Bag Count

Bags needed = volume (with buffer) divided by bag size, rounded up to the next whole bag. You cannot buy a partial bag, so always round up.

Mix Breakdown

Each component gets its share of the buffered volume based on the percentage in the recipe. Mel's Mix splits the volume into equal thirds. The Standard recipe gives 60 percent to topsoil, 30 percent to compost and 10 percent to perlite or vermiculite.

Settling Buffer

Fresh soil mix settles 10 to 15 percent in the first season. The buffer multiplies the base volume by 1 + buffer% / 100 so you start with enough.

Take a standard 4 ft x 8 ft rectangular bed, 1 ft deep.

Volume: 4 x 8 x 1 = 32 cubic feet = 1.185 cubic yards = 906.1 liters.

Bags (2 cu ft): 32 / 2 = 16 bags.

Mel's Mix: one third of 32 = 10.67 cu ft each of compost, peat moss (or coco coir) and vermiculite.

With 10% settling buffer: 32 x 1.10 = 35.2 cu ft, which needs 18 bags of 2 cu ft.

As a second check: a round bed 3 ft across and 1 ft deep holds pi x 1.5^2 x 1 = 7.069 cu ft, requiring 5 bags of 1.5 cu ft (since 7.069 / 1.5 = 4.71, rounded up to 5).

And an L-shaped bed that is 4 x 8 on the outside with a 2 x 3 cutout at 1 ft deep: (4 x 8 - 2 x 3) x 1 = 26 cu ft.

The two built-in recipes cover the most common raised-bed approaches. Choose the one that fits your plants and budget.

Mel's Mix comes from Mel Bartholomew's "Square Foot Gardening" method. It drains well and stays light, but vermiculite can be expensive. The Standard mix uses cheaper topsoil as the bulk ingredient and works well for most vegetables and flowers.

Garden soil and amendments come in several common bag sizes. Knowing which your store stocks saves a trip.

Bulk Delivery vs Bags

For beds larger than about 100 cu ft (roughly 4 cubic yards), buying by the cubic yard from a landscape supplier is usually cheaper than bagged soil. The calculator gives volume in cubic yards so you can get a bulk quote.

Freshly filled raised beds lose height as the soil compacts and organic matter breaks down. Most gardeners see 10 to 15 percent settling in the first growing season, and a little more over the next year or two.

Factors That Affect Settling

• Organic content: mixes with a lot of compost or peat settle more as the organics decompose.
• Watering: irrigation pushes soil particles closer together, especially in the first weeks.
• Soil type: lightweight mixes (Mel's Mix) tend to settle more than dense topsoil blends.

What to Do

Start with 10 to 15 percent extra. Top up with compost at the start of each new season. The calculator adds this buffer automatically so you buy enough from the start.

Rectangle

The most common raised bed. Standard sizes include 4 x 4, 4 x 8 and 4 x 12 feet. Keep the width to 4 feet or less so you can reach the center from either side.

Round or Circle

Round beds are popular for herb spirals, keyhole gardens and decorative plantings. The calculator uses the circle area formula: pi times the radius squared.

L-shape

An L-shaped bed is a rectangle with a rectangular corner cut out. This fits into corners, wraps around patios or creates a walkway. Enter the full outer dimensions and the cutout dimensions, and the calculator subtracts the cutout area before multiplying by depth.

Several factors change the real amount of soil a raised bed requires beyond the raw geometry.

Bed Depth

A deeper bed needs proportionally more soil. Going from 6 inches to 12 inches doubles the volume.

Soil Mix Type

Lightweight mixes like Mel's Mix settle more than dense topsoil blends, so you may want a higher buffer.

Bed Wall Thickness

Always measure the inside dimensions. Thick lumber, stone or block walls reduce the interior space.

Existing Fill

If the bottom of the bed already has gravel, logs (hugelkultur) or old soil, subtract that depth from the fill depth.

Moisture and Compaction

Wet soil compacts more than dry. If you fill with dry bagged soil and then water heavily, expect extra settling.

Building a New Bed

Use the calculator before your first trip to the garden center so you buy the right number of bags or order the right cubic yards of bulk soil.

Topping Up an Existing Bed

Measure the gap between the current soil level and the rim, enter that as the depth, and the calculator tells you how much to add.

Comparing Bag vs Bulk

The calculator shows volume in both cubic feet and cubic yards. Compare the bag total cost against a bulk delivery quote to find the cheaper option.

Planning a Mix

Choose a recipe and see exactly how many bags of each component to buy, so nothing is left over or missing.

1. Forgetting Settling

If you fill to the brim with no buffer, the soil level will drop noticeably within weeks. Add 10 to 15 percent extra.

2. Measuring Outside Dimensions

Measure the inside of the bed frame, not the outside. Lumber thickness eats into the planting area.

3. Mixing Inches and Feet

A bed that is 8 inches deep is 0.667 feet, not 8. Divide inches by 12 before entering.

4. Buying the Wrong Bag Size

Check the volume printed on the bag, not the weight. A 40-pound bag of topsoil is roughly 0.75 cu ft, but it varies by brand and moisture.

5. Skipping the Mix

Pure topsoil compacts and drains poorly in a raised bed. A blend of topsoil, compost and a drainage amendment (perlite or vermiculite) gives roots the air and water balance they need.

The volume math is exact geometry. The bag count is a simple ceiling division. The mix percentages are exact proportions. The only estimation is the settling buffer, which depends on your specific soil and conditions.

A 4 ft by 8 ft bed filled 1 ft deep needs 32 cubic feet of soil, or about 1.19 cubic yards. With a 10 percent settling buffer that becomes 35.2 cu ft, which is 18 bags of 2 cu ft soil.

Multiply length by width by depth for a rectangle, or use pi times the radius squared times depth for a round bed. Divide the total volume by your bag size and round up to get the number of bags.

Two popular options are Mel's Mix (equal parts compost, peat moss or coco coir and vermiculite) and a Standard blend of 60 percent topsoil, 30 percent compost and 10 percent perlite. Both work well for vegetables.

A 4 x 4 ft bed at 1 ft deep holds 16 cu ft. At 2 cu ft per bag that is 8 bags before settling buffer and 9 bags with a 10 percent buffer.

Yes. Fresh soil settles 10 to 15 percent in the first season as it compacts and organics decompose. Start with that much extra so you do not need to top up right away.

Most vegetables do well in 6 to 12 inches (0.5 to 1 ft) of soil. Deep-rooted crops like tomatoes and carrots prefer at least 12 inches.

One cubic yard equals 27 cubic feet. Landscape suppliers sell bulk soil by the cubic yard, so the calculator converts your volume for easy comparison.

Divide inches by 12. For example, 8 inches is 8 / 12 = 0.667 feet and 6 inches is 0.5 feet.

Yes. Choose the Round shape and enter the inside diameter and depth. The calculator uses the circle area formula to find the volume.

Mel's Mix is a soil recipe from Mel Bartholomew's "Square Foot Gardening" method. It calls for one-third compost, one-third peat moss or coco coir and one-third coarse vermiculite. It is lightweight and drains well.

• Volume of common geometric shapes (National Institute of Standards and Technology; unit definitions and metric conversion factors).
• Mel Bartholomew, Square Foot Gardening (Square Foot Gardening Foundation; Mel's Mix recipe: 1/3 compost, 1/3 peat moss or coco coir, 1/3 vermiculite).
• University of Minnesota Extension - Building a Raised Bed (Raised bed soil depth, drainage and mix recommendations from university research).