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Coir Brick Hydration Calculator

Enter your coir brick weight and get the exact water volume, expanded volume, and hydrated weight for any monotub or grow-bag batch.

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How the formula works

Coir bricks are dehydrated coconut husk fiber compressed into a hard block. Adding water re-hydrates the fiber and the brick swells back to its full fluffy volume. The math is a simple per-kilogram expansion factor that depends on how the brick was pressed:

  • Compressed (standard) — expands ~8 L per kg of dry brick, with ~5 L of water per kg.
  • Kiln-pressed (dense) — expands ~6 L per kg, with ~4 L of water per kg. Same coir, just compacted harder so each kg has fewer air gaps when expanded.
  • Hydrated weight = brick weight + water weight. 1 L of water ≈ 1 kg, so a 0.65 kg brick that absorbs 3.25 L of water weighs ~3.9 kg when fully hydrated.

Use the result's expanded volumeas the coir component when sizing a CVG or manure-based substrate batch — that's the usable volume after hydration.

Worked example

Standard 0.65 kg compressed coir brick (the small "650 g" block you see at every hydroponics shop):

  • Expanded volume: 0.65 kg × 8 L/kg = ~5.2 L
  • Water needed: 0.65 kg × 5 L/kg = ~3.25 L
  • Hydrated weight: 0.65 kg + 3.25 kg = ~3.9 kg

That's enough hydrated coir to be the bulk of a small monotub (~6-quart shoebox tub) or to mix with vermiculite into a CVG batch with room to spare. A 5 kg kiln-pressed block, by comparison, expands to roughly 30 L with ~20 L of water — the workhorse size for serious bulk substrate runs.

When to use this calculator

Coir comes in two common sizes: the small 650 g brick (sometimes labeled 600 g or 700 g — close enough) and the big 5 kg block. The small bricks are easier to handle and rehydrate quickly in a five-gallon bucket; the 5 kg blocks are cheaper per liter but need a larger vessel and more time to break apart. One 5 kg block isn't 5× the small brick's expanded volume — kiln-pressed blocks are denser, so the per-kg expansion factor is a bit lower. The calculator handles that for you.

Buffered vs unbuffered:coir naturally contains sodium and potassium salts left over from coconut processing. "Buffered" coir has been pre-rinsed with a calcium nitrate solution and re-dried — it's ready to hydrate and use. Unbuffered coir needs a calcium-nitrate soak first (12–24 hours) and a thorough rinse before it's safe for mycelium. For mushroom cultivation, buffered or pre-rinsed horticultural coir is the safer default. Cheap unbuffered coir from big-box stores can stall colonization.

Where this fits in your workflow: hydrated coir is the base of CVG (coir + vermiculite + gypsum) and most manure-based substrates. Run this calculator first to size your brick, then plug the expanded-volume number into the CVG substrate calculator as the coir component. Pasteurize the hydrated mix at 160–170 °F for 60–90 minutes (or use boiling water in a cooler — the "tek pasteurization" method) before mixing with grain spawn.

Frequently asked questions

  • Hot or cold water?

    Warm water (100–140 °F) is fastest — a compressed brick fully expands in about 30 minutes. Boiling water expands it in 10–15 minutes and gives a low-grade pasteurization while you're at it. Cold water works but takes 1–2 hours and you'll want to break the brick up by hand once it softens.

  • Can I reuse coir?

    Not for mushrooms. Once a substrate has fruited, it's either partially colonized with native microbes or spent of nutrients — re-pasteurizing won't reliably reset it for a second grow. Spent coir is excellent for the garden, though: mix it into compost or use it as a soil amendment for tomatoes and peppers.

  • Why does my coir smell?

    A faint earthy or coconut smell is normal. A sour or ammonia smell after pasteurization means bacterial contamination — usually from under-pasteurization (too cool, too short), or from leaving the hydrated coir sitting at room temperature for hours before pasteurizing. Mix and pasteurize the same day, and hold 160–170 °F for the full 60–90 minutes.

  • Does brick brand matter?

    Yes, more than people expect. Stick to horticultural-grade buffered coir from a hydroponics supplier (Mother Earth, Roots Organics, FoxFarm, Canna are common reputable brands). Cheap garden-center bricks are often unbuffered and can carry too-high salt levels for mycelium. Pet-store reptile coir is usually fine too — the husbandry standards are similar.

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