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Soil and substrate calculator: liters and bags
Estimate how many liters of soil or substrate and how many bags you need to fill your garden beds or pots. The tool calculates the volume of rectangular beds and round pots, shows the result in liters or cubic feet (with its equivalent in m³ or yd³), and works in metric (Europe/LatAm) or American (US) units, with a waste margin included.
Last reviewed: July 2026 · How we calculate
Input data
Results
Enter the data and press Calculate.
Results are estimates. Always verify manufacturer or supplier specifications before buying.
How the soil or substrate you need is calculated
Buying soil or substrate "by eye" almost always ends in an extra trip to the nursery or in leftover bags that spoil. Doing it right is, at its core, calculating a volume and translating it into the number of bags you need to carry. The process has three steps and this calculator solves them for you, but understanding them helps you check that the number makes sense:
- Volume of the space. For a rectangular garden bed you multiply its three dimensions: length × width × depth. In metric that result comes out in cubic meters, so it is converted to liters by multiplying by 1000. For a round pot you use the area of the circle, π × (diameter ÷ 2)², multiplied by the height.
- Waste adjustment. A percentage (5% by default) is added to the theoretical volume to cover the settling of the substrate after the first watering, the volume taken up by roots and drainage stones, and small losses while filling.
- Conversion to bags. The adjusted volume is divided by the size of the bag you are going to buy and rounded up, because in practice you can't buy half a bag of substrate.
Formula
For a rectangular garden bed (metric), with measurements in meters:
For a round pot, with the diameter and height in centimeters:
And from there, the waste adjustment and conversion to bags are the same in both cases:
Bags = round up( Adjusted volume ÷ Bag size )
Remember the key equivalence: 1 m³ = 1000 L. In American units, length and width are entered in feet and depth in inches (which are converted to feet by dividing by 12), so the volume comes out in cubic feet (ft³). That volume is also shown converted to cubic yards (yd³ = ft³ ÷ 27), the unit in which substrate is usually sold in bulk.
Practical example
Suppose a garden bed 2 m long × 1 m wide × 0.30 m deep, which you want to fill with substrate in 50-liter bags and with 5% waste:
| Volume of the space | 2 × 1 × 0,30 = 0,6 m³ = 600 L |
|---|---|
| Volume with 5% waste | 600 × 1,05 = 630 L |
| 50 L bags (630 ÷ 50) | 630 ÷ 50 = 12,6 → 13 bags |
This is exactly the result you'll see if you press "Calculate" with the default values. If instead of 50 L bags you buy 70 L bags, you'll need 9 bags for the same bed; larger bags hold more volume, so fewer are needed.
Soil vs. substrate and recommended depth
Although used as synonyms, they are not the same. Garden soil comes from natural ground: it contains minerals, clay, and microorganisms, but weighs more and tends to compact, especially inside a pot. Substrate (or potting mix) is a light, airy blend of materials such as peat, coconut fiber, perlite, and compost, formulated to drain well, retain moisture, and not cake. The rule of thumb: in pots and planters always use substrate; in garden beds you can start from the existing soil and improve it by mixing in compost.
The depth is the most underestimated dimension. In a pot, fill it almost to the rim and leave only one or two centimeters free so the water doesn't overflow when watering. In a garden bed, 20 to 30 cm of quality substrate is enough for most vegetables and flowers; deep-rooted plants appreciate 30 cm or more. For lawn, seedlings, or ground covers, a layer of 10 to 15 cm is usually enough.
The mix matters: pots, raised beds, and lawns don't ask for the same
Buying plain "soil" usually goes wrong: each use calls for a different mix, and the volume you calculated above may split across several components:
| Pots and containers | 60% light potting mix + 20% compost + 20% perlite — plain garden soil compacts and chokes roots |
|---|---|
| Raised beds | 50% topsoil + 30% compost + 20% aerating material (perlite, coco coir) |
| Lawn leveling | Screened soil mixed with sand — layers of at most 1–2 cm (½ in) per application |
| In-ground vegetable garden | Your soil + 5–8 cm (2–3 in) of compost worked into the top 20 cm (8 in) |
With this table you can turn the calculator's total volume into the actual shopping list: for example, 300 L for a raised bed ≈ 150 L of topsoil + 90 L of compost + 60 L of perlite or coir.
Common mistakes when estimating soil and substrate
- Confusing liters and cubic meters. This is the most common mistake: remember that 1 m³ is 1000 liters. A 0.6 m³ bed doesn't need "0.6 bags", it needs 600 liters of substrate.
- Filling the pot to the exact rim. If you calculate the volume up to the edge of the container, the water will overflow when watering. Leave a small margin and calculate the usable height, not the total.
- Compacting the substrate too much. Pressing the soil down so "more fits" expels the air the roots need and worsens drainage. Fill gently and let it settle on its own.
- Forgetting the settling. After the first watering, the substrate drops in level. That's why the waste margin is worthwhile and, often, a small top-up a few weeks later.
Frequently asked questions
How many liters of substrate does a pot need?
It depends on the size. A round pot 30 cm in diameter and 25 cm tall needs about 17.7 liters of substrate. The formula is π × (diameter ÷ 2)² × height ÷ 1000, with the diameter and height in centimeters. Fill it almost to the rim and leave one or two centimeters for watering.
What soil depth do I need for a garden?
For most vegetables and flowers, a garden bed 20 to 30 cm deep is enough. Deep-rooted plants like tomatoes or carrots appreciate 30 cm or more, while lawn and seedlings do fine with 10 to 15 cm of quality substrate.
What is the difference between soil and substrate?
Garden soil comes from the ground and contains minerals and microorganisms, but it can compact. Substrate is a light, fluffy blend (peat, coconut fiber, perlite, compost) formulated for pots: it drains well, retains moisture, and doesn't cake. In pots substrate is always preferable; in beds you can mix soil with compost.
Why add a waste percentage?
A 5% margin covers the settling of the substrate after watering, small losses while filling, and the volume taken up by roots and drainage stones. It's better to have a little left over than to fall short and have to open another bag halfway.
How many 50 L bags do I need per m³?
One cubic meter equals 1000 liters, so 1 m³ ÷ 50 L = 20 bags of 50 liters. If you buy 40 L bags you'll need 25 bags per m³, and with 70 L bags about 15 are enough. Remember to add the waste before dividing.
Can I mix soil and compost?
Yes, it's a recommended practice for garden beds. A mix of 60-70% soil or substrate with 30-40% compost adds nutrients and improves structure. For pots, use substrate as the base and add a handful of compost or humus for every few liters.
Settling and drainage: why the bed "sinks" within weeks
Freshly placed soil is fluffed up: with the first waterings it compacts and loses 15–20% of its apparent volume. It's normal for a raised bed filled to the rim to sit 5 cm (2 in) lower a month later. Two solutions: fill in layers, watering between each, or simply buy that 15–20% extra from the start (the calculator's waste field exists for this).
In pots and planters, mind the drainage too: enough holes and, in very deep containers, a more aerated mix at the bottom. And a fact almost nobody considers: moist soil weighs about twice as much as dry (~1,200–1,700 kg/m³). Before filling large planters on balconies or rooftops, check how much weight the structure can take.