Garden soil volume calculator for beds and containers

Quick answer: measure the space you need to fill

For a rectangular bed, multiply inside length × inside width × fill depth. Measurements in metres give cubic metres; multiply by 1,000 for litres. For a round pot, use its inside diameter and fill depth. A pot that narrows towards the base needs the tapered-pot calculation rather than a cylinder estimate.

This tool estimates volume, not soil weight, fertility or the root depth a crop needs. Use your garden bed plan or container plan to choose suitable dimensions first.

Calculate soil or potting mix volume

Enter inside measurements in one unit. Fill depth is only the empty layer you intend to fill, after allowing space for watering or mulch. For tapered pots, measure the upper diameter at the intended fill line. Changing units changes labels only: re-enter your measurements. Entries are not saved.

Enter your measurements and select Calculate soil volume.

Measure before you order

1. Measure inside the walls. Outside dimensions include material that does not hold growing medium. Exclude built-in reservoirs, false bottoms and other occupied space.
2. Choose the finished fill line. Leave room for watering and any planned mulch instead of measuring to the rim. The RHS container planting guide explains why a gap at the top helps prevent compost spilling during watering.
3. Measure only the missing depth. For a top-up, use the gap from the existing surface to your intended level. For uneven beds, take measurements at several points and use an average depth as an approximation.
4. Group identical sizes. Calculate different sizes separately and add their volumes before rounding to bags. Split an L-shaped bed into non-overlapping rectangles.

The tapered option assumes straight sloping sides and circular cross-sections. For a top-up in a tapered pot, the bottom diameter means the diameter at the existing soil surface. Curved bowls, oval pots and irregular beds are not exact matches; use a manufacturer's usable capacity when available or treat a geometric estimate cautiously.

Formulas and conversions

Use the same unit for every dimension. In the formulas below, h is fill depth, D is upper diameter and d is lower diameter; π is approximately 3.14159.

One cubic metre is 1,000 litres. Divide cubic centimetres by 1,000 to get litres. One cubic foot is 28.316846592 litres; one cubic yard is 27 cubic feet. The calculator converts inches using 0.0254 metres per inch and feet using 0.3048 metres per foot.

Buying volume = volume per item × item count × (1 + allowance ÷ 100). Divide buying volume by the labelled bag volume in matching units and round up once to a whole bag. Displayed volumes are rounded for readability; bag counts use the underlying calculation.

Worked examples: a bed, a pot and a top-up

For the first bed, a chosen 10% reserve would increase the buying volume to 950.4 litres, or 24 bags of 40 litres. This demonstrates the allowance control; 10% is not a universal recommendation. In imperial units, a bed 8 ft long × 4 ft wide with a 1 ft fill depth holds 32 ft³, about 1.185 yd³, requiring 16 bags labelled 2 ft³ with no allowance.

Choose the material as well as the quantity

Open-bottom raised beds: a suitable soil and compost blend is different from simply filling everything with potting compost. University of Maryland Extension's raised-bed guidance describes using garden soil or topsoil mixed with compost. Choose a blend for your existing soil and crops; the calculator does not prescribe amendment rates.

Pots and enclosed planters: use a growing medium intended for containers and suited to the crop. University of Maryland Extension's container guidance discusses potting media and advises against packing the medium tightly. A volume estimate does not make heavy garden soil a suitable potting mix.

Top-ups and repotting: existing soil and rootballs occupy space. This calculator does not automatically subtract them. For repotting, subtract an estimated rootball volume from the new container's fill volume before deciding what to buy, or accept that the empty-pot estimate leaves surplus. Keep plant crowns at the appropriate level rather than burying them to use up spare mix.

Bags, bulk delivery and settling

Compare bagged and bulk prices using the same volume unit, then include delivery and handling in your garden budget. Check the supplier's stated volume: a “bulk bag” is not automatically one cubic metre. Litres measure volume; kilograms and tonnes measure mass. Ask the supplier for a product-specific conversion if it sells by weight, because moisture and material affect density.

Material may settle after filling and watering. The amount depends on the product and how it is placed; ask the supplier about its fill guidance instead of assuming a fixed percentage. Avoid forcing extra material into a pot by heavy compaction. For balconies, roofs or elevated planters, confirm the permissible load before filling: this tool does not estimate wet weight or structural capacity.

Record dimensions, fill depth, product, labelled bag size, bags used and any surplus in your garden journal. Check the level after watering and before the next planting. Measure an actual gap when topping up; do not automatically rebuy the original full-bed quantity each year.

Sources and review

Reviewed by Grow Blueprint on 23 September 2026. Formulas, examples and the calculator are original geometric planning aids. Dimensions and allowances are user inputs, not crop recommendations. Growing guidance was checked against:

• University of Maryland Extension: Building Raised Beds for Vegetable Gardening — raised-bed filling materials.
• University of Maryland Extension: Growing Vegetables in Containers and Salad Tables — container growing media and filling.
• RHS: How to plant up a container — filling, planting and watering space.