To calculate compost for a raised bed, multiply the length × width × depth in metres to get cubic metres, then multiply by 1000 to convert to litres. For example, a 2m × 1m raised bed filled to 30cm deep needs 0.6 cubic metres or 600 litres of compost — roughly six 100-litre bags. Most gardeners use a 70:30 mix of topsoil to compost for raised beds, which reduces the amount of compost needed to around 180 litres (two 100L bags).

A typical pot’s compost requirement depends on its size. A 30cm diameter pot needs around 15–20 litres, a 40cm pot needs 30–40 litres, and a 50cm pot needs 60–80 litres. For round pots, use the formula π × r² × depth (in cm) ÷ 1000 to get litres. Always leave 2–3cm at the top for watering, and remember that compost settles after the first few waterings — so fill slightly higher than you think you need.

The best mix for raised beds is 70% quality topsoil and 30% organic compost or well-rotted manure. Multi-purpose compost works well for most vegetables and flowers, while peat-free compost is better for the environment and retains water effectively. For acid-loving plants like blueberries, rhododendrons and camellias, use ericaceous compost. John Innes No. 2 or 3 is ideal for heavier feeding plants like tomatoes and roses because of its loam-based structure.

Measure the border’s length and width in metres, then decide on the depth (typically 5–10cm for mulching on top of existing soil, or 15–30cm for filling a new border). Multiply length × width × depth to get cubic metres, then multiply by 1000 for litres. A 5m × 1m border mulched to 5cm deep needs 0.25 cubic metres or 250 litres of compost — about four 70L bags or three 100L bags.

Divide your total litres by the bag size. Common UK bag sizes are 40L, 50L, 60L, 70L, 80L and 100L. For example, 600 litres needs 15 × 40L bags, 12 × 50L bags, 10 × 60L bags, 9 × 70L bags, 8 × 80L bags, or 6 × 100L bags. Buying larger bags is usually more cost-effective per litre. Always buy 10% extra to allow for settling, spillage and top-ups later in the season.

Multi-purpose compost has a neutral pH (around 6.0–7.0) and suits most vegetables, flowers, shrubs and container plants. Ericaceous compost is acidic (pH 4.5–5.5) and is specifically formulated for lime-hating plants like rhododendrons, azaleas, camellias, blueberries, heathers and pieris. Using the wrong type can stunt growth, cause yellowing leaves (lime-induced chlorosis) or prevent flowering. Always check plant labels before choosing your compost.