Soil Mix Calculator

How to estimate total soil volume

1. Measure total area (length x width) and choose an average depth for the bed.
2. Convert depth to feet (in / 12) or meters (cm / 100) before volume math.
3. Add waste to cover settling, uneven beds, and rounding to delivery minimums.
4. If you're filling multiple beds, measure each and sum volumes—depth and settling can vary per bed.
5. If you have a ratio (like 60/40), multiply the total volume by each ratio to get the component volumes.
6. If your ratios do not add to 100, scale them so the total equals 100% before splitting volumes.
7. If you are buying bagged materials, convert yards to cubic feet first (1 yd³ = 27 ft³) and then divide by bag size.
8. If you are filling a raised bed on uneven ground, measure the deepest and shallowest points and use an average depth instead of one number.

Assumptions to double-check

• Compost and topsoil properties vary; a 50/50 mix is a starting point, not a universal rule.
• Raised beds may settle after watering—buffers help avoid running short.
• Some mixes need amendments (perlite, peat, lime) beyond these three components.
• If your soil is compacted clay, improving drainage may require more than adding compost on top.
• Peat and coir expand when hydrated; plan volume with that expansion in mind if you use them.
• Bulk materials can arrive wetter or drier than expected, which changes weight and sometimes how much you can fit in a bed when you rake and water it in.
• If you are planting directly, local soil pH and nutrient balance matter more than the exact ratio. A soil test can prevent over-amending.
• If you are topping off an existing bed, the new mix may blend with the old soil; effective ratio changes after incorporation.

Buying tips

• Ask whether the topsoil is screened and what the compost source is.
• If drainage is an issue, consider adding sand or other drainage-friendly amendments.
• Order a little extra for topping off after the first few waterings.
• If you're growing vegetables, consider a soil test or local extension guidance to avoid over-amending.
• For bagged components, compare by volume (cu ft or liters), not by weight—moisture changes weight a lot.
• If you are filling a lot of volume, compare bulk delivery to bags; bags are convenient but labor and cost add up fast.
• Ask about contaminants and texture: rocks, sticks, and uncomposted chunks slow spreading and reduce usable volume.
• If you need consistent texture, buy all components at once; mixing batches over time can create uneven drainage and settling.

Common mistakes to avoid

• Using a deep fill mix when you only need a thin topdressing (or vice versa).
• Forgetting settling—beds often drop after the first few waterings.
• Assuming all compost/topsoil products behave the same; quality varies widely.
• Over-amending without a goal (too much compost can reduce structure in some soils).
• Adding sand to heavy clay without understanding proportions (it can create a dense, cement-like texture).
• Treating the mix ratio as universal for every plant and every climate instead of adjusting based on drainage and water needs.