G&B Soil Calculator: Mix Your Own Garden Soil

The G&B soil calculator works out how much of each base ingredient you need to build a custom garden soil mix, typically for raised beds or containers. You enter your bed’s dimensions and the mix ratio you want (for example, 60% topsoil, 30% compost, 10% aeration), and it returns the volume of each component in cubic feet or cubic yards. This helps you avoid overbuying bags or underfilling your bed, and it works for any blend you can express as percentages.

Formula

The calculator uses a simple volume formula followed by percentage splits. First, find the total volume of the bed in cubic feet:

Total volume (cu ft) = Length (ft) × Width (ft) × Depth (ft)

Since depth is usually given in inches, convert it first: Depth (ft) = Depth (in) ÷ 12. Then for each ingredient:

Ingredient volume = Total volume × (Percentage ÷ 100)

Variables:

• Length, Width – bed dimensions in feet (ft)
• Depth – bed depth in inches (in), converted to feet
• Percentage – the share of that ingredient in your mix, from 0 to 100%

The result is in cubic feet (cu ft). To convert to cubic yards, divide by 27 (1 cu yd = 27 cu ft).

Worked Example

Using the default values: a bed 8 ft long, 4 ft wide, 12 in deep, with a mix of 60% topsoil, 30% compost, 10% aeration.

1. Convert depth: 12 in ÷ 12 = 1 ft
2. Total volume = 8 × 4 × 1 = 32 cu ft
3. Topsoil = 32 × 0.60 = 19.2 cu ft
4. Compost = 32 × 0.30 = 9.6 cu ft
5. Aeration = 32 × 0.10 = 3.2 cu ft

So you need 19.2 cu ft topsoil, 9.6 cu ft compost, and 3.2 cu ft aeration, totaling 32 cu ft. If buying bagged soil (usually sold in 1.5 or 2 cu ft bags), divide each number by the bag size to get the number of bags – for 2 cu ft bags, that’s 10 bags topsoil, 5 bags compost, and 2 bags aeration.

Why Mix Your Own Soil?

Pre-mixed “raised bed soil” often costs more per cubic foot than buying components separately, and it may not match what you’re growing. Mixing your own lets you adjust the blend:

• Vegetables – prefer a richer mix with more compost (up to 50%)
• Perennials and shrubs – do well with less compost (around 20-30%)
• Succulents and cacti – need much more aeration (up to 50% perlite or coarse sand)
• Seed starting – uses a lighter, fine-textured mix with no large chunks

The G&B calculator lets you experiment with these percentages before you buy anything.

Common Mix Ratios

While there’s no single “official” G&B recipe, a few ratios are widely used by gardeners:

• 1:1:1 – equal parts topsoil, compost, and aeration (e.g., perlite or pumice). Good general-purpose mix.
• 60/30/10 – 60% topsoil, 30% compost, 10% aeration. Heavier, holds moisture well; good for water-loving plants.
• 50/50 – half topsoil, half compost. Very rich; use sparingly or for heavy feeders.
• No topsoil – for containers, many gardeners use a soilless mix: 1 part compost, 1 part peat moss or coco coir, 1 part perlite. This calculator works for that too – just set topsoil to 0% and split the rest.

You can enter any percentages as long as they add to 100. The calculator doesn’t check the sum, so double-check your numbers.

Common Mistakes and How to Avoid Them

Getting the math right is only half the battle. Here are the typical errors people make:

• Mixing units – A bed that’s 8 ft long and 4 ft wide with a depth of 12 inches: you must convert inches to feet. Forgetting this gives you 384 cu ft instead of 32 cu ft – a 12x overestimate.
• Forgetting to convert bag sizes – Bags are often 1.5 or 2 cu ft, not 1. If you order by the yard, remember 27 cu ft per yard.
• Assuming all ingredients are the same density – The calculator gives volume, not weight. A cubic foot of compost weighs less than a cubic foot of topsoil. When buying in bulk by weight (e.g., per ton), you’ll need to ask your supplier for the density conversion.
• Not accounting for settling – Fresh soil settles after watering. It’s wise to add 10-15% extra, or plan to top up after a few weeks.

Limitations of the Formula

This calculator is a geometric volume estimate. It assumes your bed is a rectangular box with straight sides. If your bed is tapered (wider at top than bottom), or if it’s a raised bed on a slope, the actual volume will differ. For irregular shapes, break the bed into rectangles, calculate each, and add them together.

It also doesn’t account for the fact that soil compresses. When you fill a bed, the first watering will settle it by an inch or two. The calculator gives you the theoretical fill volume; you may need more in practice.

Finally, the calculator only handles the volume of the mix itself – it doesn’t tell you about nutrients, pH, or drainage. Those depend on the quality of your ingredients, not the proportions.

Bulk vs. Bagged: How to Order

Once you have your volumes, you have two purchasing options:

• Bulk delivery – Sold by the cubic yard. A typical dump truck holds 10-15 yards. Best for large beds (over 1 yard total). You’ll need to specify the mix to the supplier, or buy components separately and mix on site.
• Bagged – Sold at garden centers in 1.5 or 2 cu ft bags. Easier for small projects, but more expensive per cubic foot. A 2 cu ft bag covers a 4×8 ft bed at 3 inches deep (8 cu ft total, so 4 bags).

To convert cubic feet to cubic yards, divide by 27. For the example above (32 cu ft), that’s about 1.2 cu yd. In practice, you’d order 1.5 yd to account for settling and spillage.

Frequently Asked Questions

What does “G&B” stand for in soil?

In gardening contexts, G&B usually stands for “Garden & Bloom” or “Garden & Bed” – it’s not a standard industry abbreviation. More commonly, it refers to a brand name or a general label for garden soil mixes. The calculator itself just needs your dimensions and percentages; the label doesn’t change the math.

Can I use this calculator for a raised bed that’s not rectangular?

Not directly. The formula assumes a rectangular box. For L-shaped or circular beds, break the shape into rectangles (or use the circle area formula πr²) and calculate each section separately, then add the volumes. For a circle, use: Volume = π × radius² × depth.

How deep should my raised bed be?

Most vegetables need at least 12 inches of soil, but 18-24 inches is better for deep-rooted crops like tomatoes or carrots. The calculator lets you adjust the depth to see how much more soil you’ll need. Remember that the bottom of the bed may be open to native soil, so you only need to fill the bed itself.

Is it cheaper to mix my own soil?

Generally yes, especially for larger beds. Buying topsoil, compost, and aeration in bulk costs less per cubic foot than buying pre-mixed bagged soil. However, if you only need a few cubic feet, the convenience of bags may be worth the extra cost. The calculator helps you compare by giving exact volumes.

What is the best ratio for vegetable gardens?

A common recommendation is 60% topsoil, 30% compost, and 10% aeration (perlite or pumice). This provides good structure, nutrients, and drainage. Some gardeners prefer a 50/30/20 mix with more aeration for root crops. The calculator lets you try different ratios and see the volume difference instantly.

Can I use this for potting mix in containers?

Yes, but containers often need a lighter mix. A soilless blend (no topsoil) of 1 part compost, 1 part peat moss or coco coir, and 1 part perlite works well. Set topsoil to 0% and split the other percentages accordingly. The calculator will still give you the correct volumes for your pot dimensions.

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in l | Length | ft | 8 | 0
in w | Width | ft | 4 | 0
in d | Depth | in | 12 | 0
in p1 | Topsoil % | % | 60 | 0
in p2 | Compost % | % | 30 | 0
in p3 | Aeration % | % | 10 | 0
out total | Total volume | cu ft | (l * w * (d / 12)) | 2
out topsoil | Topsoil needed | cu ft | total * (p1 / 100) | 2
out compost | Compost needed | cu ft | total * (p2 / 100) | 2
out aeration | Aeration needed | cu ft | total * (p3 / 100) | 2