Potting Soil Calculator

How the Potting Soil Calculator Works

The calculator converts the inside dimensions of your container into a soil volume — in litres and cubic feet — and then divides that volume by your chosen bag size to produce an exact bag count, rounding up to the next full bag so you always have enough. A configurable settling allowance is added automatically.

Container Volume — How Each Shape Is Calculated

Each container type uses a standard geometric formula applied to the inside dimensions you enter:

• Round pot — volume is calculated from the inner diameter and depth using the cylinder formula. A 30 cm pot filled to 28 cm holds roughly 19.8 litres.
• Square pot — volume is the side length squared, multiplied by the depth. A 30×30 cm pot filled to 28 cm holds approximately 25.2 litres.
• Rectangular planters and window boxes — volume is length × width × depth. A standard 60 cm window box, 25 cm wide and 25 cm deep, holds 37.5 litres.
• Grow bags — calculated identically to rectangular planters. Use the grow bag presets for standard 5-gallon, 7-gallon, 10-gallon, and 20-gallon sizes in metric equivalents.
• Custom volume — if the container has a volume printed on its label, enter it directly in litres, cubic feet, or US gallons and the calculator converts it automatically.

Fill Mode — Full Fill, Leave Top Gap, and Top-Up Existing

Most gardeners do not fill containers to the absolute rim. The Fill Mode selector accounts for three real-world scenarios:

• Full Fill uses the complete stated height of the container. This is useful for calculating maximum capacity or for manufacturers' volume specifications.
• Leave Top Gap reduces the fill depth by the margin you specify. Leaving 3–5 cm (1–2 inches) from the rim creates a watering reservoir that lets water soak in rather than run off immediately, and prevents soil washing over the edge.
• Top-Up Existing calculates only the volume of new soil needed to refill a container that has already settled. Enter the current soil depth and the target depth, and the calculator subtracts the existing soil automatically.

Settling Allowance — Why 10% Extra Is Standard

Potting mix typically loses 10–15% of its volume after the first one or two waterings, as air pockets collapse and the organic matter compresses under its own weight. The default 10% settling allowance adds this buffer so the pot stays full once the mix has settled. Bark-heavy mixes used for large shrubs and trees can settle by up to 20%; fine peat-based or coir-based mixes typically settle by 10–12%. Adjust the allowance in Advanced Options to match your specific product.

Bag Sizes and Unit Conversion

Garden centres sell potting mix in bags ranging from 10 litres to 50 litres, as well as in cubic-foot bags common in North America and Australia (1 cu ft ≈ 28.3 litres; 2 cu ft ≈ 56.6 litres). Select your bag size from the preset chips — or enter a custom size — and the calculator converts your total volume to a bag count. Results are always rounded up to the next full bag so you never run short.

Potting Soil Volume Formula

The calculator uses standard geometric volume formulas for each container shape. All dimensions are entered in your chosen unit and converted internally to centimetres before calculation.

Round Pot Formula

The volume of a cylindrical pot uses the circle area formula multiplied by the fill height:

Where:

• V = volume (cm³)
• D = inside diameter of the pot (cm)
• H = fill depth (cm) — the height to which soil will be added
• π ≈ 3.14159

To convert cm³ to litres, divide by 1,000. The calculator handles this automatically — the formula is shown here for transparency.

Rectangular Planter, Window Box, and Grow Bag Formula

Where L = inside length, W = inside width, and H = fill depth — all in centimetres.

Converting Litres to Cubic Feet

One cubic foot equals 28.317 litres. To convert a litres result to cubic feet:

For example, 21.8 L ÷ 28.317 = 0.77 cu ft. A standard 1 cu ft bag of potting mix contains 28.3 litres — slightly more than a 25 L bag.

How Much Potting Soil Do Common Containers Need?

The table below shows soil requirements for frequently used container sizes at full fill, with a 10% settling allowance included. Use it as a quick shopping guide when you know your pot size but do not have a tape measure to hand.

All figures assume straight-sided containers filled to the stated height. Tapered pots hold roughly 8–12% less than the equivalent straight-sided dimensions suggest. To calculate soil for raised beds and ground-level growing areas, use our Raised Bed Soil Calculator.

How to Read Your Potting Soil Result

The total-to-buy figure shown in the result card already includes the settling allowance. Here is how to act on your result across different volume bands:

Under 10 litres — a single small container. One 10 L bag covers it with soil to spare, or use a 20 L bag across two small pots. Avoid buying a 40–50 L bag for a single small pot unless you have other containers ready to fill in the same session.

10–30 litres — one to two standard bags. For results between 25 and 30 litres, a single 25 L bag may be 1–3 litres short after settling; buy a 40 L bag or two 20 L bags to be safe rather than returning to the store.

30–80 litres — two to four bags. At this scale, 40 L and 50 L bags offer a lower cost per litre at most garden centres compared to 20 L bags of the same brand. Compare price-per-litre before choosing your bag format.

80–200 litres — five to ten bags. Consider whether splitting a bulk bag (typically sold as 1 cubic metre = approximately 1,000 litres) across your growing season is more economical. One cubic metre of moist potting mix weighs roughly 400–600 kg, so access and handling should factor into your decision.

Over 200 litres — bulk delivery from a garden centre or horticultural supplier is almost always cheaper per litre than individual retail bags at this scale. Use this calculator to confirm your total volume before requesting a bulk quote.

Tips for Buying and Using Potting Mix

• Buy one extra bag — even with the settling allowance included, containers gradually lose soil volume across the season as organic matter decomposes. A spare bag in the shed saves a return trip mid-season.
• Never use garden soil in containers — outdoor soil compacts tightly in pots, cutting off air to roots and blocking drainage. Potting mixes are formulated to stay open and drain freely, even after repeated watering. Using garden soil typically halves the effective life of a planted container.
• Measure inside dimensions — pot diameter is usually stated as the outside rim size. The inside is typically 1–3 cm narrower due to wall thickness, which reduces actual soil volume by up to 10% in thick-walled terracotta or glazed ceramic containers.
• Check the bag weight, not just the volume — potting mix bags are sold by volume, not weight. A 50 L bag of lightweight perlite-heavy mix weighs roughly 12 kg; a 50 L bag of dense loam-based compost can exceed 25 kg. Volume is what matters for this calculation.
• Top-up annually — containers typically need topping up by 10–20% of their depth each year as organic matter breaks down and washes out. Use the Top-Up Existing mode to calculate only what you need to add, rather than replacing the entire soil volume.
• Refreshing used mix — potting mix from the previous season can be extended by blending in 20–30% new mix by volume and adding slow-release fertiliser. The calculator can help you find the volume of new mix needed to bring a part-filled container to its target fill level.

For mulch, ground cover, and path surfacing calculations, refer to our Mulch Calculator.

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