Soil Calculator for Raised Garden Beds and Containers

How to Use This Soil Calculator

Whether you are filling your first raised bed or topping up a container collection on the patio, the goal is the same: order the right amount of soil the first time. Too little and you make multiple trips to the garden centre; too much and you have leftover bags taking up space all season. Here is the four-step process we use ourselves.

1. Measure inside the bed or container. Always measure inside dimensions — not outside — because the wood, plastic, or fabric walls take up real space. Use a tape measure for length and width, then measure depth from the soil-line you actually want (not always all the way to the bottom).
2. Choose the right shape tab. Use "Raised Bed (Rectangular)" for any 4-sided bed, "Round Planter" for galvanized rings or whisky barrels, and "Container / Pot" for individual pots. The math changes for each, but this calculator handles it for you.
3. Enter your numbers — keep an eye on units. If you measure in inches, the calculator handles the conversion. If you have a metric tape measure, click the metric toggle. Multiple beds of the same size? Use the counter to multiply the result instantly.
4. Order with the 10% buffer applied. Soil settles dramatically after the first few waterings — sometimes by 2–3 inches in a 12-inch bed. The 10% buffer (already on by default) covers this so you are not adding more soil two weeks later.

The Best Soil Mix for Raised Beds

Our family has tested commercial "raised bed mixes," straight topsoil, and homemade blends across more than two decades. The mix that consistently produces the best vegetables — without breaking the budget — is a simple 60/30/10 blend. Here is exactly what goes into it and why.

The 60/30/10 Recipe

60% quality topsoil — the bulk ingredient. Good topsoil holds moisture, supplies minerals, and gives roots a stable medium to grow in. Buy by the cubic yard from a reputable landscape supplier if you are filling more than one bed; bagged topsoil works fine for one or two small beds.

30% finished compost — the engine of the mix. Compost feeds soil biology, slowly releases nutrients, and improves the texture of heavy topsoil. Our family makes our own from kitchen scraps, but bagged compost from a reputable brand works too.

10% soilless growing mix — the lightener. A small amount of peat-based or coco-coir-based potting mix loosens the blend and improves drainage. We never go above 10% for raised beds because too much makes the soil dry out too quickly in summer.

What We Avoid

We do not recommend filling raised beds with pure landscape "topsoil" alone — it is often heavy, salty, and short on organic matter. We also do not use peat moss as more than 20% of any mix; it is naturally acidic and can lock out nutrients for vegetables that prefer neutral soil. Finally, we never use chemically treated wood mulches as a soil component.

For a deeper dive into soil chemistry and amendments, the University Extension network is the most reliable free source. The USDA Cooperative Extension also publishes excellent regional guidance for soil testing and amendment.

Quick Reference: Common Bed Sizes

Already know your bed size? Here are the most common dimensions and the soil they need at 12-inch depth, with a 10% settling buffer applied.

Calculating Soil for Containers and Pots

Containers behave differently from raised beds in two important ways. First, they hold much less soil, so even small measurement errors cost you. Second, container plants depend entirely on the soil you give them — there is no native ground beneath them to fall back on.

Soil for Round Pots

For a round pot, the math is straightforward: multiply π (3.14) by the radius squared by the depth. The calculator does this automatically — just enter the inside diameter and the depth. For a quick rule of thumb, a 14-inch round pot at 12 inches deep needs about 1 cubic foot of mix, or roughly 16 quarts.

Soil for Rectangular Planters

Long rectangular planters — the kind you sit on a balcony rail or a porch — are calculated the same way as a raised bed: length × width × depth. Use the "Container / Pot" tab and choose "Rectangular Planter" from the shape menu.

Container Mix vs Raised Bed Mix

Containers need a much lighter mix than raised beds because they drain through small holes and the entire root zone is above ground. For pots, our family uses a 50/50 blend of quality potting mix and finished compost, with no added topsoil. The 60/30/10 raised-bed recipe is too heavy for most containers.

Related Garden Guides

Soil is just the start. Here are other family-tested guides to help your raised bed thrive: