What this calculator does
The Flower Pot Soil Volume Calculator estimates how much potting soil fills a tapered flower pot. Most pots are wider at the top than at the bottom, so the soil cavity is shaped like a truncated cone (a conical frustum) rather than a simple cylinder. By entering the top diameter, bottom diameter, and the height of the soil column, you get the exact soil volume in cubic centimeters (cm³, equal to milliliters) and in liters. This is pure geometry, so it works anywhere — no country-specific rules apply.
How to use it
Measure three things with a ruler or tape, all in centimeters: the diameter across the top circular opening of the soil, the diameter across the bottom circle of the soil, and the vertical height of the soil from base to surface. Enter each value and read the result. If your pot has a uniform width, just enter the same value for both diameters and the tool treats it as a cylinder.
The formula explained
The volume of a conical frustum is $$V = \frac{\pi \times h}{3} \times \left(R_{top}^{2} + R_{top} \times R_{bottom} + R_{bottom}^{2}\right)$$ where \(R_{top}\) and \(R_{bottom}\) are the radii (half of each diameter) and \(h\) is the height. Because diameters are easier to measure, the equivalent diameter form is $$V = \frac{\pi \times h}{12} \times \left(D_{top}^{2} + D_{top} \times D_{bottom} + D_{bottom}^{2}\right)$$ When the top and bottom are equal it reduces to the cylinder formula \(\pi R^{2} h\); when the bottom diameter is zero it becomes a full cone.
Worked example
Suppose the top diameter is 12 cm, the bottom diameter is 10 cm, and the soil height is 8 cm. The radii are 6 cm and 5 cm. Then \(R_{top}^{2} + R_{top} \times R_{bottom} + R_{bottom}^{2} = 36 + 30 + 25 = 91\). So $$V = \frac{\pi \times 8}{3} \times 91 \approx 8.3776 \times 91 \approx 762.36 \text{ cm}^{3}$$ which is about 762 mL or 0.762 liters of soil.
FAQ
Is cm³ the same as milliliters? Yes. One cubic centimeter equals exactly one milliliter, so 762 cm³ is 762 mL.
How many liters does my pot need? Divide the cm³ result by 1000. The calculator shows the liter figure for you, which is handy for matching bagged potting soil sizes.
Should I leave space below the rim? Yes — measure the height only up to where you want the soil surface, not to the top edge of the pot, so you account for a watering gap.
