Mushroom Casing Layer Calculator
Work out exactly how much casing mix, lime and water your beds or trays need — sized to your growing area and target depth.
Overview
The casing layer is the moist, low-nutrient top layer spread over fully colonised substrate. It is the trigger that tells the mycelium to stop feeding and start fruiting — essential for Agaricus species (white button, cremini and portobello) and helpful for several others. Get the depth, volume and pH wrong and you get patchy pinning, uneven flushes or a failed crop.
This calculator turns your bed dimensions and target depth into a practical shopping list: total casing volume, prepared weight, the split of dry ingredients (peat, coir, vermiculite), the lime needed to bring pH toward the ideal 7.0–7.5 range, and the water required to reach field capacity. It is built for hobby growers scaling up, market gardeners planning a grow room, and teachers or students who need defensible numbers rather than guesswork.
How It Works
- Choose Metric or Imperial units at the top of the tool.
- Enter the length and width of a single bed or tray, and how many beds you are casing.
- Set your casing depth. Most growers use 4–5 cm (about 1.5–2 in) for Agaricus.
- Pick your casing mix type. Each option uses realistic bulk density, moisture and ingredient ratios.
- Press Calculate to see the full breakdown, then Reset to start over.
Formula Explanation
The calculation is a straightforward volume-to-weight chain, using material properties measured at field capacity (fully moist, freely draining casing).
Assumed material properties are practical field values, not laboratory ideals — for example a peat-and-lime casing at field capacity is treated as roughly 0.60 kg/L with about 68% of its weight as water.
Practical Benefits
- Buy the right amount once. Order peat, coir and lime with confidence instead of over-buying or running short mid-casing.
- Uniform depth, uniform pinning. A consistent, calculated depth across every bed gives more even flushes and predictable timing.
- Correct lime dose. The ingredient split includes lime sized to nudge pH toward the 7.0–7.5 window that Agaricus prefers.
- Water planning. Knowing the water held at field capacity helps you moisten the mix without drowning it.
- Scales cleanly. Whether it is one tray on a shelf or forty beds in a grow room, the maths and the buffer scale with you.
Frequently Asked Questions
Casing is a moist, nearly nutrient-free layer placed over colonised substrate. It holds water, supports a healthy microbial community and creates the humidity gradient that signals the mycelium to form pins (baby mushrooms). Without a properly built casing layer, many species — Agaricus in particular — will keep growing vegetatively and never fruit well.
For Agaricus, 4–5 cm (roughly 1.5–2 in) is standard. Thinner casing dries out fast and pins unevenly; much thicker casing delays fruiting and can encourage stroma. This tool will still calculate any depth you enter, but it flags values far outside the usual range so you can double-check.
Peat + lime is the classic, reliable choice for button mushrooms and holds water beautifully. Coir + vermiculite is a peat-free alternative that many hobby growers prefer for sustainability and consistency. A peat–coir blend sits in between, combining peat's water retention with coir's structure. All three include lime for pH; peat-based mixes need a little more because peat is naturally acidic.
Field capacity is the target: squeeze a handful and a few drops release, but it does not stream. The water figure here estimates the total water the mix holds at that point, assuming you start from relatively dry ingredients. Baled peat and buffered coir already carry some moisture, so add water gradually and stop once you reach that squeeze test rather than pouring in the full figure blind.
