Potting Soil Calculator
Calculate the exact volume of potting mix needed for any container. Enter pot dimensions for round, square, rectangular, or hexagonal planters to get volume in litres and gallons, bags needed, and estimated cost.
Potting Soil Calculator Tool
Why use this free potting soil calculator?
Built with the features most competitors miss — deeper inputs, benchmark data, and actionable guidance alongside the core calculation.
How to use this potting soil calculator
Potting soil needed for common round pot sizes
| Pot diameter | Depth | Volume (90% fill) | Bags needed (8 qt) |
|---|---|---|---|
| 6 inch | 6 inch | 0.7 qt / 0.7 L | 1 bag |
| 8 inch | 8 inch | 1.8 qt / 1.7 L | 1 bag |
| 10 inch | 10 inch | 3.1 qt / 2.9 L | 1 bag |
| 12 inch | 12 inch | 5.9 qt / 5.6 L | 1 bag |
| 15 inch | 14 inch | 11.0 qt / 10.4 L | 2 bags |
| 20 inch | 18 inch | 27.1 qt / 25.6 L | 4 bags |
How this calculator compares
LazyTools fills the gaps most competing tools leave open — deeper analysis, benchmark context, and actionable guidance alongside the core calculation.
| Feature | LazyTools | OmniCalculator | Gardeners Supply | Bonnie Plants |
|---|---|---|---|---|
| 4 pot shapes | ✓ Yes | ✓ | ✗ | ✗ |
| Litres and quarts/gallons | ✓ Yes | ✓ | Partial | ✗ |
| Multiple pots at once | ✓ Yes | ✓ | ✗ | ✗ |
| Fill percentage option | ✓ Yes | ✓ | ✗ | ✗ |
| Bag count and cost | ✓ Yes | ✓ | ✗ | ✗ |
| Free, no registration | ✓ Yes | ✓ | ✓ | ✓ |
Potting Soil Calculator: Complete Guide
Knowing how much potting soil to buy before visiting the garden centre saves trips, prevents over-buying, and ensures you have exactly the right amount for every container. This calculator handles round pots, square planters, rectangular window boxes, and raised beds.
Why container volume matters for plant health
Container size directly limits root volume, water-holding capacity, and nutrient availability. Undersized containers dry out quickly, restrict root development, and produce smaller plants and lower yields. A tomato in a 5-gallon container will produce significantly less than one in a 15-gallon container. Knowing the volume of your containers helps you match them to the right plant and plan watering frequency.
Potting soil bag sizes: US market
Common retail potting mix bag sizes: 1 quart (0.25 gallon), 2 quart, 8 quart (2 gallon), 16 quart (4 gallon), 32 quart (8 gallon). Large bags at 50 quarts (12.5 gallon) and 2 cubic feet (approximately 60 quarts) are also common at big-box retailers. Premium potting mixes are often sold in smaller bags at higher prices; budget mixes in large bags. Premium mixes typically have better texture, drainage, and initial nutrients.
Potting soil vs garden soil: never use garden soil in pots
Garden or native soil in containers compacts severely under repeated watering, creating a dense, poorly-draining mass that suffocates roots. Purpose-made potting mix is light, porous, and designed for the repeated wetting and drying cycle of container growing. It also drains correctly through container drainage holes. Even high-quality garden soil should never be used alone in containers — it can be used at 50% blended with perlite and compost as a cost-saving measure for very large raised beds.
How to reduce potting soil cost for large containers
For large decorative planters or raised beds where drainage is less critical: use horticultural perlite or wood chips to fill the bottom third of deep containers before potting mix. This reduces the volume of expensive potting mix needed without sacrificing root zone quality for shallow-rooted plants. For deep raised beds (over 18 inches): fill the bottom 6 to 8 inches with coarse compost, wood chips, or straw before adding premium potting mix or amended native soil.