Garden soil calculator

How to use the garden soil calculator

This calculator estimates the soil volume for one or more identical raised beds. It divides the total into topsoil, compost and aeration material, then estimates the bags needed for each component.

1. Choose Feet / inches or Metres / cm.
2. Enter the number of identical beds and each bed’s inside dimensions.
3. Enter the fill depth, leaving the desired freeboard below the bed rim.
4. Set the topsoil, compost and aeration percentages so they total 100%.
5. Choose a bag size and settling allowance, then review the total volume and component bag counts.

What to enter

Unit system

Choose Feet / inches or Metres / cm. Feet / inches is selected by default.

Number of beds

Enter a positive whole number of identical beds. The default is 1.

Inside length

Enter the bed’s inside length. Use feet in imperial mode or metres in metric mode. The default is 8.

plus inches

Add a fractional part to the inside length. Use inches in imperial mode or centimetres in metric mode. The default is 0.

Inside width

Enter the bed’s inside width. Use feet in imperial mode or metres in metric mode. The default is 4.

plus inches

Add a fractional part to the inside width. Use inches in imperial mode or centimetres in metric mode. The default is 0.

Fill depth

Enter the soil depth. Use inches in imperial mode or centimetres in metric mode. The default is 12 in in imperial mode and 12 cm in metric mode.

Topsoil

Enter the topsoil share by volume as a percentage. The default is 60%.

Compost

Enter the compost share by volume as a percentage. The default is 30%.

Aeration (perlite, bark fines)

Enter the aeration-material share by volume. This represents materials such as perlite, vermiculite or bark fines. The default is 10%.

Bag size

Choose the bag volume used for every component. Available sizes are 0.75, 1, 1.5, 2 or 3 cu ft. The default is 1.5 cu ft.

Waste %

Add material for settling. Enter 0–50%. The default is 5%.

Example: one 8 ft by 4 ft bed

Using the default imperial settings: 1 bed, 8 ft inside length, 4 ft inside width, 12 in fill depth, a 60/30/10 mix, 1.5 cu ft bags and 5% settling. Volume per bed is 8 × 4 × (12 ÷ 12) = 32 cu ft. Net volume is 32 × 1 = 32 cu ft. The settling factor is 1 + (5 ÷ 100) = 1.05, so the order volume is 32 × 1.05 = 33.6 cu ft, displayed as 1.24 cu yd because 33.6 ÷ 27 = 1.2444 and volumes are rounded to two decimals. Topsoil is 33.6 × 0.60 = 20.16 cu ft; 20.16 ÷ 1.5 = 13.44, so the calculator rounds up to 14 bags. Compost is 33.6 × 0.30 = 10.08 cu ft; 10.08 ÷ 1.5 = 6.72, so it requires 7 bags. Aeration material is 33.6 × 0.10 = 3.36 cu ft; 3.36 ÷ 1.5 = 2.24, so it requires 3 bags.

How the soil volume is computed

The calculator converts the length and width to decimal feet internally. In imperial mode, each minor value is divided by 12 and added to its major length. In metric mode, the major values are metres and the minor values are centimetres before conversion. Fill depth is converted to inches. For each bed, the calculation is length × width × (fill depth ÷ 12). That volume is multiplied by the number of beds, then by the settling factor 1 + (waste percentage ÷ 100). Each component volume is total volume × (component percentage ÷ 100). The calculator divides each component volume by the selected bag size and rounds up to a whole bag separately. Displayed volumes are rounded to two decimals. Imperial order volume is shown in cu yd, while metric order volume is shown in m³. The breakdown also includes cubic yards and cubic metres.

Reading the results

The main “Soil mix to order” result shows the total volume after the settling allowance: cu yd in imperial mode or m³ in metric mode. The results also show total volume in cu ft, volume per bed, net volume before settling, cubic yards, cubic metres and the selected fill depth. Component results show the topsoil, compost and aeration volumes plus whole-bag counts. Bag counts are rounded up for each component, so you do not get a partial bag in the estimate.

Limitations and common mistakes

• The beds must be identical rectangular beds. The calculator does not combine beds with different dimensions or calculate irregular shapes.
• The three mix percentages must total 100%, with a tolerance of 0.01 percentage points. Values outside the 0–100% range are rejected.
• The selected bag size is applied to topsoil, compost and aeration material. Real products may use different package sizes, so check each product label before buying.
• The settling allowance is an adjustable estimate from 0% to 50%. Actual settling can differ, and the calculator does not model later top-ups.

Frequently asked questions