Potting Soil Calculator
Work out how much potting mix your containers need. Pick a round pot, a rectangular planter or a grow bag and get litres, cubic feet, dry quarts and bags to buy.
🪴 What is a Potting Soil Calculator?
A potting soil calculator converts the physical dimensions of a container into the volume of growing medium it takes to fill it. Potting mix is sold by volume, in litres, cubic feet or US dry quarts, never by weight, so the only way to buy the right amount is to compute the volume first. This calculator handles the three container shapes that cover almost every real planting job: a tapered round pot, a rectangular planter box or window box, and a cylindrical fabric grow bag. It reports the volume per container, the total across however many containers you are filling, the same figure in three different units, the number of bags you need at your chosen bag size, and roughly what that soil will weigh once it is in place.
The situations are familiar to anyone who grows in containers. You are potting on twenty seedlings into 15 cm pots and want to know whether one 50 litre bag covers it. You are filling a run of window boxes along a balcony and need a delivery quantity before the shop closes. You are setting up eight fabric grow bags for tomatoes and would rather not make a second trip. You are repotting a large specimen and want to know whether you can lift the finished pot. Each of these is the same arithmetic problem in a different costume, and each goes wrong in the same expensive way when it is guessed.
The single biggest source of error is treating a tapered pot as though it were a straight-sided cylinder. Nursery pots almost always narrow toward the base, and using the rim diameter all the way down inflates the answer badly. A pot 30 cm across the top, 20 cm across the base and 25 cm deep is a cone frustum holding about 12.4 litres, whereas a 30 cm cylinder of the same depth would hold 17.7 litres, an overshoot of more than 40 percent. This calculator uses the proper frustum formula for round pots for exactly that reason. The second common error is filling to the rim: soil that reaches the top of the pot means water runs off the surface instead of soaking in, so a fill level of 85 to 92 percent is standard practice and is what the fill-level control is for.
Beyond the headline number, the page shows how volume responds to container size. Because volume scales with the square of the diameter, pot sizes escalate much faster than they appear to on the shelf, and the chart makes that relationship visible. The bag comparison table then shows how much soil you would end up buying, and how much would be left over, at every common bag size, which is often the difference between one trip and two. For filling raised beds or borders in the open ground rather than containers, the companion Soil Calculator works in cubic metres and cubic yards instead.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — One Tapered Patio Pot
Round pot 30 cm top, 20 cm base, 25 cm deep, filled to 90%
Example 2 — Three Window Boxes
Rectangular planters 80 × 25 cm, filled 20 cm deep, three of them
Example 3 — Four Tomato Grow Bags
Fabric grow bags 40 cm across and 40 cm tall, four of them
Example 4 — Six Small Pots for Potting On
Round pots 20 cm top, 14 cm base, 18 cm deep, six of them at 95% fill
❓ Frequently Asked Questions
🔗 Related Calculators
How much potting soil do I need for my pot?
Multiply the pot volume by your fill level. A tapered pot 30 cm across the top, 20 cm across the base and 25 cm deep holds about 12.44 litres brim-full, so at a 90 percent fill it takes about 11.19 litres, roughly 0.4 cubic feet or 10.2 US dry quarts.
What is the formula for the volume of a flower pot?
A tapered pot is a cone frustum, so V = pi x h / 12 x (Dt squared + Dt x Db + Db squared), where Dt is the top inside diameter, Db the base diameter and h the inside depth. Divide cubic centimetres by 1000 to get litres.
Why not just treat a pot as a cylinder?
Because almost every pot narrows toward the base, and a cylinder formula uses the widest diameter all the way down. On a pot that tapers from 30 cm to 20 cm the cylinder shortcut overstates the volume by more than 40 percent, which means buying far more mix than you need.
How many litres is a cubic foot of potting soil?
One cubic foot is 28.317 litres. Bagged mix in the United States is usually labelled in cubic feet or US dry quarts, and one US dry quart is 1.1012 litres, so a 2 cubic foot bag holds about 56.6 litres or roughly 51.4 dry quarts.
How much does a litre of potting mix weigh?
Dry, a typical peat-based mix runs 0.2 to 0.3 kg per litre and a compost-rich mix 0.4 to 0.6 kg per litre. Wet, it can be two to three times heavier. The calculator uses a default bulk density of 0.4 kg per litre, which you can change.
How full should I fill a container with soil?
Stop 2 to 3 cm below the rim on a small pot and 4 to 5 cm below on a large planter. That gap holds water long enough for it to soak in rather than sheeting off the top. On most containers that works out at 85 to 92 percent of the total volume.
How do I calculate soil for a rectangular planter box?
Multiply the inside length by the inside width by the fill depth, all in centimetres, then divide by 1000 for litres. An 80 by 25 by 20 cm planter is 40,000 cubic centimetres, which is 40 litres brim-full and 36 litres at a 90 percent fill.
How much soil does a grow bag hold?
Treat it as a cylinder: pi divided by 4, times the diameter squared, times the height. A 40 cm wide, 40 cm tall bag is about 50.3 litres brim-full, so around 45.2 litres of mix once you leave a 10 percent gap at the top.
Should I reuse last season's potting mix?
You can stretch it, but not indefinitely. Old mix compacts, loses structure and holds spent roots and possible pathogens. A common approach is to refresh one third to one half of the volume with new mix plus compost each season rather than replacing everything.
Is potting soil the same as garden soil or topsoil?
No. Potting mix is a soilless blend of peat or coir, bark, perlite and similar materials, engineered to drain freely in a container. Garden soil and topsoil compact badly in pots, hold too much water and suffocate roots, so they should not be used as a direct substitute.
What units does this calculator use?
All container dimensions are entered in centimetres and bag size in litres. Results are shown in litres, cubic feet and US dry quarts at once, so you can match whichever unit your local bags are labelled in.