Coco coir pith holds 7.5 to 11 times its own dry weight in water, and a container of wetted pith that has finished draining is typically 60 to 80% water by volume with 10 to 13% air. Those are figures for fine, pure pith. Add fibre or husk chips and the air fraction climbs to 20, 30 or 40% while water holding falls. Dry bulk density sits around 0.04 to 0.08 g/cm³, and total porosity is anywhere from 80 to over 94% depending on who measured it and how.
That spread is the whole story. "Coco coir" isn't one material with one set of numbers. It's a family of particle blends, and the physical properties follow the particle sizes. For what coco peat is and how it's made, see what coco peat is. For hydrating blocks and how many litres to expect, see the blocks guide. This article covers the numbers in between.
The four numbers, in plain terms
Total porosity is the share of the volume that isn't solid. In coir it's very high, because the particles are themselves spongy and there's space between them.
Water holding capacity (WHC) is the share of the volume still full of water after saturating and free drainage. In a pot it's also called container capacity. WHC is also quoted by weight, as a percentage of dry mass, and the two bases give wildly different numbers for the same material. Check which one a sheet is using.
Air-filled porosity (AFP), also called air space or air capacity, is the share of the volume that drains and fills with air. Roots breathe from this fraction.
Bulk density is dry weight divided by volume. Low bulk density goes with high porosity, and it tells you how much coir you're getting per kilogram.
Total porosity = WHC + AFP. Make a coir mix coarser and you move volume from the water column to the air column.
Published ranges for coco coir
The table pulls together figures from research on pure pith, mixed-grade coir dusts and factory blends. They disagree, and the disagreement is mostly particle size.
| Property | Fine, pure pith | Mixed grades and blends | Basis |
|---|---|---|---|
| Total porosity | 85–90% | 80–95%+ | volume |
| Water holding (container capacity) | 73–80% | 40–66% | volume |
| Water holding | 750–1100% of dry weight | lower with more fibre or chip | weight |
| Air-filled porosity | 9.5–13% | 13–28% typical, 24–89% across very coarse lots, 40% for a 70/30 pith–chip blend | volume |
| Dry bulk density | 0.04–0.08 g/cm³ | 0.057–0.064 g/cm³ for factory blends, about 0.2 g/cm³ measured on one fine commercial grade | — |
Pure pith is wet. Nine to thirteen percent air after drainage is at the bottom of what most guidelines accept for a pot. The "coir holds about 40% water" line you'll see quoted comes from a coarser horticultural grade on a volume basis. It doesn't contradict the 750% figure, it's a different basis and a different grade.
General guidelines for container media put total porosity at 50 to 85%, air space at 10 to 30% and container capacity at 45 to 65%. Pure pith overshoots the water target and undershoots the air target. Blended coir lands inside both.
How pith, fibre and chips shift the numbers
Coir comes off the husk as three fractions: pith (the fine spongy "dust"), fibre (long strands, cut short for substrates) and chips (cut husk pieces). Each does a different job in the pore network.
| Fraction | Effect on WHC | Effect on AFP | Other effects |
|---|---|---|---|
| Pith | Raises it. Each particle is a sponge | Lowers it. Fine particles pack into small pores | Fastest to decompose, so most shrinkage |
| Short fibre | Very low water holding on its own, lowers WHC a little | Raises it by propping particles apart | Long fibre causes dry pockets, so it's cut short |
| Husk chips | Lowers it. A 70% pith / 30% chip blend retained 0.28 cm³/cm³ against 0.66 for an 80/20 pith and short-fibre blend | Raises it a lot. The same 70/30 blend measured 40% AFP | Chips still hold water inside each piece, so a chip mix stays moister than bark |
Particle size works the same way within pure pith. Across 13 coir dusts, both easily available water and total water holding fell in proportion to a coarseness index while air content rose with it. A fine and a coarse coconut substrate compared side by side gave 637 litres of water per cubic metre for the fine material and 376 for the coarse, while aeration went from 71 to 495 litres per cubic metre. Mean particle size is one of the most reliable predictors of both WHC and AFP in any substrate.
The rule berry growers use: particles above 1 mm are the water-retaining fraction, coarser chip and husk pieces are the aeration fraction, and dust below 1 mm raises density and limits aeration. Fines below about 0.2 mm are best sieved out, because dust fills the gaps between bigger particles and collapses the macropore network. For chip grades and blends, see the husk chips guide.
Compression ratio and moisture at pressing
Nearly all coir ships compressed. The usual commercial ratio for a 5 kg block is about 5:1 by volume, with 6:1 and 7:1 also sold, and patents on compressed coir cover anything from 2:1 to 10:1. Machine trials at 4:1, 5:1 and 6:1 found that both moisture and compression ratio significantly affect how well a block holds together and recovers its volume.
Moisture at pressing matters as much as the ratio. Published pressing work uses 10 to 18% moisture by weight, and export specifications usually say below 15 to 20%. Too dry and the block won't bind. Too wet and you ship water instead of coir, and mould risk rises in transit.
Compression also changes the physical properties of pith, not just its shape. Harder pressing breaks down structure and pushes the expanded material toward the fine, wet end of the table above, and a quick soak gives a denser result than slow, thorough hydration.
How ageing changes WHC and AFP
Coir decomposes slowly. Lignin content is 20 to 40%, which is why it outlasts peat, bark and sawdust in long crops. But the fractions don't age at the same rate. Pith has the lowest lignin and breaks down fastest. Fibre and chips last longer.
As a substrate ages, particles get smaller and the mix drifts toward the fine end: WHC rises, AFP falls. The volume shrinks, which lowers slab height and reduces drainage further. And roots fill the large pores during a crop, cutting air capacity before any decomposition happens. Growers watch for shrinkage above about 5%, because past that point the root zone starts staying wet after each irrigation.
Pre-ageing at the factory is different. Raw pith is held for months so its carbon to nitrogen ratio falls and phenolics break down, which reduces nitrogen drawdown in the crop and slows further decomposition. Aged pith is a more stable product, not a worn one. So match the grade to crop length: a one-season propagation mix can be pith-heavy, while a ten-month tomato slab or a multi-year blueberry pot wants a real share of fibre or chip.
How to measure WHC and AFP yourself
You don't need a lab, just a pot with the drain holes plugged, a measuring jug in millilitres, a tray, a scale and somewhere warm to dry the sample. Use the pot you actually grow in, because container height changes the answer: a modelled peat mix fell from 19% air in a 15 cm pot to under 1% in a 1.3 cm plug cell.
- Find the container volume. Fill the sealed pot with water to the line you normally fill with media. Record it as A.
- Fill with moist coir to the same line and firm it as you do at potting. Dry coir can take hours to wet, so start with damp material.
- Saturate it. Add water slowly until a film glistens on the surface. Tap the sides to let air bubbles out.
- Drain. Pull the plug, keep the pot level, and collect everything that runs out over 10 to 15 minutes. Record it as B.
- Weigh the drained coir (C), dry it completely, and weigh it again (D). One gram of water lost is one millilitre held.
Then:
- Air-filled porosity = B ÷ A
- Water holding capacity = (C − D) ÷ A
- Total porosity = AFP + WHC
- Bulk density = D ÷ A
Worked example from a 2,960 ml pot: 800 ml drained, 2,400 g wet, 900 g dry. AFP is 27%, WHC 51%, total porosity 78%. Run three replicates, because the air figure varies between fills. Labs do the same thing in a fixed-height porometer so results can be compared, so ask whether a supplier's figures come from a porometer or a pot, and at what height.
What to specify when you buy
Physical properties are the one part of a coir spec you can test cheaply on arrival, so put numbers on them. These targets come from published container-media guidelines and are starting points, not a standard.
| Use | Air-filled porosity | Water holding | Particle profile | Why |
|---|---|---|---|---|
| Propagation plugs and trays | 10–15% in the cell | High | Fine pith, long fibre screened out, dust below 0.2 mm removed | Shallow cells hold water anyway. The medium must still have air at that height |
| Grow bags and open-top slabs | 20–30% | 45–65% | Pith with 20–30% chip or short fibre | Long crops, dense roots, frequent fertigation. Air has to last ten months |
| Potting and nursery mixes | 15–25% | 45–60% | Medium pith with some fibre, or coir blended with bark or perlite | Taller pots need less help than shallow ones |
Alongside the numbers, ask for the fraction percentages of pith, fibre and chips and the sieve sizes used, the test method and container height behind any AFP or WHC claim, bulk density and moisture at the time of weighing, and the age of the raw pith before pressing.
Coir against peat, perlite and rockwool
| Property | Coco coir | Sphagnum peat | Perlite | Stone wool (rockwool) |
|---|---|---|---|---|
| Total porosity (vol.) | 80–95% | 90–95% | ~75% | 93–96% |
| Water holding after drainage (vol.) | 40–80%, grade dependent | ~40–65% | low, ~25–30% | 85–91% in saturated slabs and cells |
| Air-filled porosity (vol.) | 10–13% fine pith, 13–28% horticultural grades, up to 40% with chips | 18–25% | ~30% | 5–11% at container capacity, higher once a slab drains to its set point |
| Dry bulk density | 0.04–0.08 g/cm³ | 0.07–0.11 g/cm³ | ~0.1 g/cm³ | ~0.08 g/cm³ |
| How it releases water | Large share released below 1 kPa suction, the rest held more slowly inside pith | Similar curve, holds some water at higher suction | Drains almost completely | Nearly all water released by 5 kPa, very little buffer |
| Change over a crop | Slow decomposition, pith fastest | Decomposes faster than coir, shrinks | Inert | Inert |
Peat and coir hold similar amounts of water and release most of it at low suction. Coir pith is wetter and less airy than peat at the same particle size, which is why blended grades exist. Perlite is an aeration component, not a water store. Stone wool holds the most water and holds it loosely, so it responds fast to each irrigation and dries back fast. In a drained slab, growers run water content anywhere from about 50 to 85% depending on crop stage. For growing comparisons, see coir vs rockwool.
Getting the blend right
Ask what fractions are in the coir, not just that it's "coco peat". CVM Fiber and Industries in Kanyakumari, Tamil Nadu, India, makes 5 kg blocks, briquettes, grow bags and open-top slabs, husk chips and custom blends of pith, fibre and chips. Tell us the crop, the container and the irrigation system, or run the pot test above on a sample and send us the numbers. Contact us to get started.
FAQ
What is the water holding capacity of coco coir?
By weight, dry coir pith holds about 7.5 to 11 times its own mass in water. By volume, a drained container of fine pith is 73 to 80% water, while coarser horticultural grades and chip blends come in at 40 to 66%. Check whether a figure is by weight or by volume before comparing suppliers.
What is the air porosity of coco coir?
Fine, pure pith measures about 9.5 to 13% air after drainage, which is at the low end of what pots need. Typical horticultural grades sit at 13 to 28%, and a 70% pith / 30% chip blend measured about 40%. Across very coarse coir dusts, values up to 89% have been recorded. The particle mix sets the number.
What is the bulk density of coco peat?
Dry bulk density is usually 0.04 to 0.08 g/cm³, or 40 to 80 kg per cubic metre. Factory blends with chips tested at 0.057 to 0.064 g/cm³. Fine or heavily compressed material can run higher, and one fine commercial grade measured about 0.2 g/cm³, so a high figure is worth a question about sand content and particle size.
Does coco coir hold more water than peat moss?
At the same particle size, fine coir pith holds a little more water and a little less air than sphagnum peat. Both release most of their water below 1 kPa of suction, so plants can use it easily. Coir also rewets more readily after drying out. Coarser coir grades hold less water than peat.
How does particle size affect coco coir?
Smaller particles mean more water and less air, larger particles the opposite. In one comparison, fine coconut substrate held 637 litres of water per cubic metre with 71 litres of air, while the coarse version held 376 litres of water with 495 litres of air. Dust below about 1 mm raises density and limits aeration, and fines below 0.2 mm should be sieved out.
What compression ratio should a coco peat block have?
Most 5 kg blocks are pressed at about 5:1 by volume, with 6:1 and 7:1 also common. Moisture at pressing is usually specified at 10 to 20% by weight. Higher compression saves freight but breaks down pith structure, so the expanded material tends to be finer and wetter. Ask for the ratio, the moisture at pressing and the expected expanded volume.
How do I test the air porosity of my coco coir?
Seal a pot's drain holes, measure its volume, fill it with moist coir and saturate it slowly. Then unplug it and collect the water that drains over 10 to 15 minutes. Drained volume divided by pot volume is your air-filled porosity. Weigh the drained coir, dry it and weigh it again to get the water holding capacity. Do it in the pot you actually use, because container height changes the result.
Does coco coir lose air porosity over time?
Yes, slowly. Pith decomposes faster than fibre or chips, particles get smaller, and the mix shrinks and holds more water. Roots also fill the larger pores during a crop. For crops lasting more than a season, choose a blend with 20 to 30% chip or short fibre so air capacity lasts to the end.