The practical toolbox
Soil & compost calculator for pots and planters
Work out how much soil or compost to buy for pots and planters. Calculate litres and whole bags for rectangular, round or tapered containers with a draggable diagram.
Save this version with its URL. Bookmark this page or copy its link to reopen your inputs later. Save a new link after making changes. Anyone with the link can read your inputs. Results are recalculated on reopening; print a copy if you need a fixed record.
Create a saved link with AI
Give your AI assistant your measurements or figures and the instructions below. It can create a ready-to-open link for this planter soil volume calculator. Open the link, check the inputs, then bookmark it or copy the URL to reuse it later.
Copy this prompt and add your information:
Read https://www.lejnel.com/soil-volume-url-guide.txt and generate a clickable saved-plan URL for my planter soil volume calculator. Convert my units to the guide’s required units. Ask for missing inputs instead of guessing, and explain any assumptions. Return the link and a summary of the inputs used.
Read the URL format and examples · AI instructions for all tools
The URL contains your inputs. Anyone with the link can read them. Future calculator updates may change results; retain a printed copy when you need a fixed record.
Soil to buy
How much soil or compost do I need for a pot?
Calculate the empty space up to your intended soil level, multiply by the number of identical pots, then divide the total litres by the litres in one bag. Round up to whole bags. Choose a rectangular planter, straight-sided round pot or tapered round pot to match your container.
How to measure a pot for soil
- Measure inside the container. Outside dimensions include walls and can overestimate the space.
- For a rectangular planter, measure length left–right and width front–back. For a round pot, measure diameter through the centre of the opening.
- Measure inside height from the flat base to the rim, then enter the percentage of that height you want to fill.
- For a tapered round pot, enter its side angle from vertical. If you know the inside top and base diameters, angle = arctan((top diameter − base diameter) / (2 × inside height)), converted to degrees.
- Enter the container count and the bag’s labelled litres. Add an allowance only if you choose one, then review the diagram and shopping total.
Soil volume formulas
For a rectangular planter, multiply inside length × inside width × filled height. For a straight-sided round pot, use π × radius² × filled height. Filled height is inside height × fill percentage / 100. Divide cubic millimetres by 1,000,000 to get litres.
Tapered pots and the draggable drawing
Choose Tapered round pot to enter a side angle measured from vertical: 0° is straight-sided. Bottom diameter = top diameter − 2 × height × tan(angle), and must stay positive. At the soil surface, radius = bottom radius + filled height × tan(angle). Soil volume is π × filled height × (bottom radius² + bottom radius × soil-surface radius + soil-surface radius²) / 3. The top gap therefore removes a wider slice of soil in a tapered pot.
Drag handles 1 at the rim to change length or diameter, 2 at the base to change the taper, 3 below the pot to change height, and 4 at the soil line to change the fill percentage. Keyboard users can focus a handle and use arrow keys; the numeric inputs provide the same adjustments. The drawing uses the same horizontal and vertical scale. For rectangular planters, input 5 is width (front to back), shown as a measurement below the side view.
From litres to bags
Multiply one container’s litres by the number of identical containers, then by (1 + allowance / 100). Divide by the bag’s labelled litres and round up to whole bags. Bags are pooled across these identical containers.
Compare container shapes
Each example uses one empty container, 300 mm inside height, 90% fill height (270 mm, leaving 30 mm below the rim), no extra allowance and 40-litre bags. These are arithmetic examples, not planting or product recommendations.
| Shape | Inside measurements | Soil volume | 40-litre bags |
|---|---|---|---|
| Rectangular planter | 800 × 300 mm | 64.8 litres | 2 |
| Straight-sided round pot | 400 mm diameter | 33.93 litres | 1 |
| Tapered round pot | 400 mm top diameter, 10° from vertical | 24.94 litres | 1 |
Are litres of soil the same as kilograms?
No. Litres measure volume; kilograms measure mass. This calculator uses bag volume in litres and cannot convert a bag’s weight into usable soil volume without material-specific information.
Can this calculate compost as well as potting soil?
Yes, when the product is sold by volume and you are estimating the space to fill. It does not choose a growing medium or decide how much compost to mix into existing soil.
Why does a tapered pot need less soil?
The base is smaller than the opening. Treating the whole pot as a cylinder at its top diameter overestimates its volume. Use the tapered shape when the circular walls slope evenly inward toward the base.
Worked example
An 800 × 300 mm planter with 300 mm inside height and 90% fill height (270 mm, leaving 30 mm below the rim) has 270 mm filled height: 64.8 litres. Two need 129.6 litres. With no extra allowance and 40-litre bags, buy four bags, providing 160 litres and a nominal 30.4 litres left over.
Where should I measure?
Measure the inside soil space, not the outside dimensions or decorative rim. For round pots, use inside diameter across the centre. The rectangular and straight-sided round models assume vertical walls and a flat base. For a tapered round pot, measure the inside top diameter and the side angle from vertical. The pot must have a circular flat base and straight, evenly sloping walls. Curved or irregular pots need a different model.
What about existing root balls or soil?
The result assumes empty containers. It does not subtract plants, root balls, existing soil, reservoirs, false bottoms or drainage layers. Those reduce the space to fill. Choose a model or measurement that represents the actual empty space; do not treat the result as an exact purchase requirement.
Should I add extra for settling?
No automatic extra is applied. Enter your own percentage if appropriate for your material and project. Bag volume, moisture and compaction affect actual yield; follow the supplier’s instructions for the chosen growing medium.
Does 50% fill mean half the volume?
Fill percentage describes soil height from the inside base to the rim. In a rectangular or cylindrical container, 50% height also gives half its volume. In a tapered pot, the lower half is narrower and holds less than half the full volume. Changing height keeps your chosen fill percentage.
Can I use inches?
Yes. The shared units control converts dimensions while preserving their physical sizes. Volume and bag size remain in litres so they can be compared directly with a litre-labelled bag. Saved links always store dimensions in millimetres.
Can I save or print this estimate?
Copy the estimate link or bookmark the complete URL after saving. Anyone with that URL can read its inputs. Print for a fixed record. The drawing is a labelled schematic, not a full-size template.
Limitations and responsibility
All containers share the same dimensions. The calculator uses ideal rectangular prisms, cylinders or circular conical frustums, without internal obstructions. Allowance is user-selected and bag surplus is nominal. This tool does not specify soil suitability, drainage, planting depth, structural capacity or wet weight. Check your measurements, product label and location requirements before buying or filling. Results may contain errors and are not a guarantee of fit, yield, suitability or purchase quantity. Independently verify inputs and quantities before purchase or filling.
To the extent permitted by applicable law, lejnel.com disclaims warranties and liability for loss, damage, wasted materials and costs arising from use of or reliance on this calculator. Nothing in this notice excludes or limits liability or statutory rights that cannot lawfully be excluded or limited. See the Terms of Use.
For more context, read growing vegetables in pots or growing herbs indoors. For indoor projects, try the paint calculator or curtain width calculator.