Pot Soil Volume Calculator - Litres of Soil for Tapered Pots

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How much soil does a pot need

Pots look simple until you stand in the garden centre guessing how many bags to buy. Buy too little and you are back at the shop mid-planting. Buy too much and bags sit in the shed, splitting and spilling. Pots are tapered, wider at the top than the bottom, so guessing from the top diameter alone overstates the volume. This calculator uses the real tapered shape.

Enter the top diameter, the bottom diameter, and the height, measured inside the pot. The calculator gives the soil volume in litres, quarts, and gallons. Use the unit switch for inches or centimetres. The math treats the pot as a frustum, which is the shape you get when you cut the top off a cone, and that matches most round pots well.

A worked example

Take a pot 12 inches across the top, 8 inches across the bottom, and 10 inches tall. The tapered volume works out to about 13 litres, or roughly 3.4 gallons. A pot that size needs about one third of a 40-litre bag. If you had guessed from the top diameter alone, treating it as a straight cylinder, you would have estimated over 18 litres and bought far too much.

Try a bigger pot: 20 inches at the top, 14 inches at the bottom, and 16 inches tall. That pot needs about 57 litres, or one and a half 40-litre bags. Large pots surprise people most, because volume grows with the square of the diameter. Doubling the width quadruples the volume, so a pot twice as wide needs roughly four times the soil.

Measure inside, not outside

Pot walls have thickness, and thick terracotta walls can steal an inch of diameter. Measure across the inside at the rim for the top diameter, and across the inside bottom for the bottom diameter. Measure height from the inside bottom to the rim. If you measure outside, you overstate the volume and buy extra soil you do not need.

Leave room below the rim for watering. Soil filled to the very top spills water every time you water, washing soil and food out of the pot. Leave two to three centimetres of headroom. The calculator gives the full volume, so subtract a little in your head, or simply accept that one bag covers slightly more pots than the raw numbers suggest.

Drainage and pot feet

Every pot needs drainage holes. Without them, water pools at the bottom and roots rot, no matter how good the soil is. Cover holes with a piece of broken pot or mesh to keep soil in while letting water out. Do not fill the bottom with gravel for drainage: gravel raises the water table inside the pot and makes waterlogging worse, not better.

Raise pots on feet or bricks so water drains freely and the pot does not stain the patio. Raised pots also dry faster in wind, so check them more often in summer. A pot on feet in full sun can need watering twice a day in a heatwave.

Common mistakes to avoid

The most common mistake is using garden soil in pots. Garden soil compacts in a pot, drains badly, and can carry pests and weed seeds. Use potting mix, which is lighter and drains better. Another mistake is reusing old potting mix without refreshing it: mix in fresh compost and check that it still drains freely.

People also underwater big pots and overwater small ones. Big pots hold water for days, while small pots dry in hours. Check soil with a finger two inches deep: water only when it feels dry at that depth, and water until it runs from the drainage holes.

Frequently asked questions

What about square pots? Square pots hold a little more than round pots of the same width, because corners add volume. The calculator assumes round pots, so add about ten percent for square ones. Rectangular planters are closer to boxes: multiply length by width by height and convert to litres.

How often should I repot? Most potted plants need fresh mix every one to two years, or a bigger pot when roots circle the bottom. Spring is the best repotting time, just before growth restarts.

Can I mix my own potting soil? Yes, and our potting mix ratio calculator gives the ingredient amounts. A simple mix of peat or coir, perlite, and compost suits most plants.

Use the calculator above before you shop, measure inside the pot, and leave headroom for watering. Garden results are general guidance estimates only. Your local climate, your soil, and the instructions on your seed packets govern what actually grows, and results vary with weather and conditions.

Pot Soil Volume Calculator – Litres of Soil for Tapered Pots