Compost Bin Volume & Output Calculator

Home Composting Guide

What do I need to know about Compostable Materials?

GREENS (high nitrogen, fast-rotting): grass clippings. Vegetable peelings. Fruit waste. Coffee grounds and used tea leaves (NOT teabags with plastic). Cut flowers, deadheads. Fresh weeds (not seed heads or perennial weeds). Plant prunings. BROWNS (high carbon, slow-rotting): cardboard (torn into small pieces). Newspaper and shredded paper. Dry leaves. Twigs and small branches. Egg shells. Wood ash (small amounts). Straw or hay. AVOID: cooked food, which attracts rats unless using a hot composter. Meat, dairy, and cooked food generally, since these attract pests and produce unpleasant smells in a standard cold composting system.

What should I know about The Brown:Green Ratio?

Ideal C:N ratio: approximately 30:1. Translates roughly to 2-3 parts brown to 1 part green by volume. Common problem: too much green (slimy, smelly compost). Add more brown — shredded cardboard works immediately. Too much brown (slow, dry compost). Add more green or water. Layering or mixing: traditional layered approach (alternating browns and greens) works. Mixing as you add also fine. Aim for moisture like a wrung-out sponge — squeeze a handful, should clump but not drip. Turning the heap introduces oxygen, which speeds up decomposition significantly — a heap turned regularly can finish in months rather than the year or more an untouched heap might take.

What should I know about Bin Sizing?

Approximate annual compostable waste: kitchen waste only: 70 kg/person/year. Plus garden waste: typical 50m² lawn = 200 kg/year cuttings. Larger garden with prunings: 500-1500 kg/year. Bin volume: 200-300L sufficient for small household + small garden. 400L (single Dalek bin): typical 4-person household + medium garden. 600L+: large garden producers. 1000L+: 2-3 bay wooden system for allotments. Compost shrinks dramatically — final compost is approximately 20% of original input volume. Two-bin systems let you keep filling one bin while the other finishes composting, which is more practical than a single bin for most regular composters.

What do I need to know about Hot Composting?

Hot composters (HotBin, Joraform, Aerobin): insulated bins reaching 60-70°C internal temperature. Bacterial action accelerates dramatically. Benefits: 30-90 days to finished compost (vs 12-18 months traditional). Kills weed seeds and pathogens. Compost cooked meat, fish, dairy (excluded from cold systems). Cost: £200-500 for serious hot composters. Worthwhile for: small gardens with limited space. Households producing food waste from cooking. People wanting compost quickly. Caveats: requires active management — regular turning and monitoring temperature — which is more effort than passive cold composting but produces usable compost in weeks rather than a year.

What's the key thing to understand about Getting the Carbon-to-Nitrogen Ratio Right?

Effective composting depends heavily on balancing 'brown' (carbon-rich) materials with 'green' (nitrogen-rich) materials, since the right mix feeds the microorganisms that break down organic matter efficiently, while an imbalanced mix slows decomposition or creates problems like unpleasant odours. Brown materials include dried leaves, cardboard, straw, and wood chippings, and generally provide the bulk structure and carbon that composting microbes need for energy. Green materials include grass clippings, vegetable peelings, and fresh plant trimmings, and provide nitrogen that helps microbes grow and multiply quickly. A commonly recommended target ratio is roughly two to three parts brown material to one part green material by volume, though exact ratios aren't critical to get precisely right — the more important practical guidance is to avoid the common mistakes of adding too much of one type at once, such as a large amount of fresh grass clippings alone, which can compact into a dense, smelly, slimy layer that excludes air and slows the whole process, or relying entirely on dry brown material with no green input, which can leave composting stalled for a very long time due to insufficient nitrogen. Turning the compost periodically to reintroduce air, and layering or mixing new additions rather than dumping a large amount of one material in one spot, helps maintain a reasonably balanced mix throughout the bin without needing to measure ratios precisely.

What should I know about What Not to Put in a Compost Bin?

Several common household and garden items should be avoided in a standard home compost bin, either because they decompose too slowly, attract pests, or can introduce problems to the finished compost. Meat, fish, dairy, and cooked food scraps generally shouldn't go in an open garden compost bin, since they attract rats and other pests and can create strong odours as they decompose — these are better suited to a sealed hot composting system or a council food waste collection service designed to handle them safely at higher temperatures. Diseased plant material should be avoided or disposed of separately, since home composting temperatures often aren't consistently high enough to reliably kill plant pathogens and weed seeds, meaning diseased material or persistent weeds composted at home can simply reintroduce the same problem into the garden when the finished compost is eventually used. Glossy or coated paper, and anything treated with chemicals (pressure-treated wood, glossy magazine paper, heavily printed cardboard) should also be avoided, since the coatings and treatments don't break down naturally and can introduce unwanted chemicals into compost that will eventually go back into garden soil. Pet waste from carnivorous pets (cats and dogs) is also generally not recommended for compost destined for use around edible plants, due to the risk of pathogens that survive typical home composting temperatures — these restrictions aren't about making composting harder, but about keeping the process working well and the finished compost safe to use.

Reference: Bin Size by Household and Garden Size

Running two smaller bins in rotation — filling one while the other sits and matures — is usually more practical than one large bin for most gardens, because compost needs time to break down undisturbed, and a single bin that is constantly being added to and dug into never gets that settled maturation period.

Worked Example: Estimating Volume From Weekly Waste

A household adds roughly 5 litres of kitchen vegetable peelings a week and, during the growing season (April-October, about 30 weeks), adds a further 15 litres of grass clippings and plant trimmings a week, with little garden waste the rest of the year. Kitchen waste over a year: 5 litres x 52 weeks = 260 litres Garden waste over the growing season: 15 litres x 30 weeks = 450 litres Total raw input over a year = 710 litres Composting materials shrink substantially as they break down and lose water and volume — typically to around 30-50% of their original volume once fully composted — but a bin needs to hold the material at its bulkier, freshly-added stage, not its final rotted-down volume, so a 330-litre bin would need emptying roughly twice during a busy growing season to keep pace with 710 litres of raw annual input, which is a reasonable argument for sizing up to a 450-600 litre bin, or running two in rotation, for a household producing this much green waste.

What do I need to know about Compost Volume Reduction Over Time?

Fresh garden and kitchen waste loses a large share of its volume as it composts, which catches out a lot of first-time composters who think their bin isn't working because the level keeps dropping. Typical volume reduction from fresh, loosely-piled material to finished compost is 50-70%, driven by moisture loss, microbial breakdown of structure, and settling as larger particles collapse. This means a bin that looked full in week one can look half-empty by week eight even while actively composting well, and it's one reason gardeners keep topping bins up rather than assuming a shrinking pile signals a problem to fix.

What should I know about Bin Material and Insulation?

A plastic 'dalek' bin retains heat better than an open wooden pallet bin, which speeds up the composting process in cooler months by keeping the pile's core temperature higher for longer. Wooden bins breathe and drain more freely, which suits wetter climates or gardeners prioritising airflow over speed, so the choice between bin materials is really a trade-off between faster composting and better moisture management rather than one being simply better than the other.