Advanced options
All bagged compost in the UK retail market is now peat-free: multi-purpose suits most pots, with John Innes (loam-based) for anything staying potted over a year and ericaceous for blueberries, camellias and rhododendrons. Big pots don't need filling entirely with compost: crocks and up-ended smaller pots at the base save litres without hurting plants.
What The Compost Calculator Works Out
- Compost needed: number of containers x litres each, plus 10% because watering-in settles the compost below the rim. Pots are sold by litreage: a medium patio pot is about 10L, big statement pots 30L to 50L, hanging baskets 5L to 7L, so ten medium pots want a hundred-odd litres.
- Bags to buy: the litres divided into 40L bags, the common UK retail size, rounded up. The half-used bag folded shut in the shed is next month's top-up; a bag left open to rain becomes a saturated liability.
- Money-saver: the 3-for or 4-for multi-buy is the real price of compost and the single-bag rate is the penalty for shopping without a list.
- Custom pot size and settling (advanced): type an exact pot volume (most pots have it moulded underneath) and set your own settling allowance. Peat-free mixes settle more than the old peat ones, so 10% to 15% is realistic for deep pots.
- Void fill and reuse (advanced): crocks, up-ended small pots or bought void fillers can occupy the bottom third of giant pots that roots would never use, and last year's compost refreshed with a slow-release feed can make up part of the mix. Both cut the litres you buy, and the litres-saved card prices the saving.
- Bag size (advanced): compost comes in 20L to 100L bags. Bigger bags are cheaper per litre but a 70L bag is an awkward lift; the weight card shows what you are carrying.
- Cost and multi-buy saving (advanced): enter the single-bag price and the price for three to see the bill both ways and the saving. 40L peat-free multi-purpose is typically GBP 6 to GBP 9 single or 3 for GBP 15 to GBP 20; the calculator also flags when buying one extra bag to complete a triple is cheaper than paying singles.
- Compost type and weight (advanced): peat-free multi-purpose weighs about 0.35kg per litre, John Innes loam-based mixes nearer 0.9kg. Multi-purpose suits bedding and veg; John Innes serves anything staying potted beyond a season; ericaceous is for blueberries, camellias and rhododendrons.
- Pot size table (advanced): litres, bags and cost for the same number of containers at each common size, from 3L pots to a 100L half barrel. Bagged compost is a growing medium, not a mulch or soil improver by price: those jobs belong to the cheaper composted bark and manure tiers, and raised beds want a bulk-bag delivery of topsoil instead.