Spike's Grow School
How to Mix a Coco-Perlite Blend to a Target Ratio
Why blend coco and perlite at all
A growing medium has two jobs that pull against each other. It has to hold water and dissolved nutrients close to the roots, and it has to hold air, because roots breathe. Push a medium too far towards water and the roots suffocate; push it too far towards air and you're watering three times a day. Almost every media decision is really a negotiation between those two jobs.
Coco coir is very good at the water side. It holds a large amount of water and, unlike inert aggregates, it carries a genuine cation exchange capacity, so it hangs onto nutrients rather than letting everything rush straight through. Peer-reviewed work on coir dusts describes exactly this profile: high total porosity with a strong water-holding fraction (Abad et al., 2002). The trade-off is that fine, water-retentive coco on its own can sit wetter than some roots like, especially in shorter pots where less water drains away.
Perlite solves the other half. It's a lightweight, rigid, sterile volcanic aggregate with a near-neutral pH, and its value in a mix is almost entirely physical: it props the medium open and adds large, air-holding pores (Owen & Lopez, 2015; Perlite Institute, n.d.). It holds very little plant-available water inside itself and carries essentially no nutrient-holding capacity, which is the point. You're not adding perlite for what it gives; you're adding it for the space it makes.
Blend the two and you get to place the mix wherever you want on the water-versus-air line. That's the whole game. If you want the full background on each ingredient before you start scooping, our coco coir explainer and perlite explainer cover them properly.
What the ratio actually controls
When you change a coco-perlite ratio, the number you're really moving is air-filled porosity: the share of the medium's volume that drains to air after you water it and let it settle. Everything else, the drought buffer, how often you water, how forgiving the mix is, follows from that.
Horticultural sources put a workable floor for container media at about 10 percent air-filled porosity after it drains, with the rest of the pore space holding water (Cabrera & Johnson, 2014), while other extension guidance aims higher, at around 20 to 25 percent air space, for strong root growth (Nemali, 2018). The exact target depends on the crop and the container, and the honest answer is that there's no single magic number; the two respected sources above don't even fully agree, which tells you to treat these as a sensible window rather than a law. Below roughly 10 percent air, most roots start to struggle for oxygen (Cabrera & Johnson, 2014).
Perlite is how you move within that window. Because it's a coarse, rigid aggregate, adding more of it increases the proportion of large pores that drain to air and lowers the total water the mix holds (Owen & Lopez, 2015; Abad et al., 2002). More perlite means more air and a faster-draining, more forgiving mix that you'll water more often. Less perlite means more water held, longer gaps between waterings, and less margin if you tend to overwater. That relationship is steady and predictable, which is exactly what lets you mix to a target instead of guessing.
Spike's Tip: Don't chase a lab number you can't measure at home. Instead, read the pot after you water. Water thoroughly, let it drain for a few minutes, then squeeze a handful from the middle of the pot: a well-balanced coco-perlite mix should feel like a wrung-out sponge, damp and springy, never dripping and never dusty-dry. If it clumps tight and soggy in your hand, add more perlite next batch. If it runs dusty-dry and won't hold shape, work in a splash of water and a touch more coco.
The sensible default: 70/30, and when to move off it
At Hydro Oasis the blend we point most people to first is 70/30, that's seventy percent coco to thirty percent perlite by volume. It lands comfortably inside the air-and-water window above for the majority of container and hydroponic setups, it suits our cool-climate Tasmanian conditions where media dries more slowly than in the tropics, and it's a genuinely hard ratio to get wrong. If you take one number away from this post, take 70/30.
The reason to move off it is always about how wet your particular situation runs. Reach for more perlite (a 60/40 or even 50/50 blend) when your mix will trend wet: short or wide pots that hold more water, humid or cool rooms with slow evaporation, heavy hand-watering, or crops that hate wet feet. Reach for less perlite (a 90/10 blend) when your mix will trend dry: tall narrow pots that drain hard, hot or breezy conditions, automated drip feeding little and often, or when you simply want a longer buffer between waterings. A useful way to think about it: the drier your environment and watering style, the more coco you want holding a reserve; the wetter they are, the more perlite you want opening things up.
Two ratios worth naming because growers ask about them: 50/50 is popular for propagation and for run-to-waste drip systems where you feed frequently and want fast drainage and lots of air. Straight coco with no perlite can work under precise, frequent fertigation, but for hand-watered pots it leaves very little room for error, which is why a blend is the safer default.
Measure by volume, not by weight
This is the step that trips people up. Coco and perlite have very different densities, and perlite is extraordinarily light, so a ratio measured by weight looks nothing like the same ratio measured by volume. Physical properties of a mix are governed by how much space each ingredient occupies, not how much it weighs, so growing-media ratios are always given by volume. When you read "70/30," read it as seven buckets of coco to three buckets of perlite, not seven kilos to three.
The easy way to do it is the "parts" method. Pick any container as your unit, a scoop, a bucket, a nursery pot, and count parts. For 70/30, that's 7 parts coco to 3 parts perlite. For 60/40, 6 and 4. For 50/50, equal scoops. Same scoop every time, loosely filled and levelled off rather than packed, and the ratio takes care of itself. Measure the coco after it's hydrated and fluffed up, not as a dry brick, because that's the volume it'll actually occupy in the pot.
Working out a batch
Once you can measure by volume, hitting a target for a specific job is just arithmetic. Multiply your total finished volume by each fraction:
- A single 30 L pot at 70/30 needs about 21 L of hydrated coco and 9 L of perlite (30 multiplied by 0.7 and by 0.3).
- A 50 L batch at 60/40 needs about 30 L coco and 20 L perlite.
- A 10 L propagation tub at 50/50 needs 5 L of each.
The one thing that catches people is that coco arrives compressed and expands a lot once wet, so you have to plan around the hydrated volume, not the brick. As a rule of thumb, compressed coco expands to roughly five to nine times its compressed size when fully hydrated, depending on the product and how finely it's milled. A standard 650 g brick typically yields somewhere around 7 to 9 L of usable coco, and a 5 kg block around 60 to 75 L, but these vary by brand, so check the figure on the bag and treat the first batch as your calibration. Hydrate first, measure second.
To hydrate, add warm water and give it time; the block will drink far more than you expect and break apart into loose, fluffy coco. Fluff it through with your hands or a fork so it's even and not clumped before you measure and blend.
Step by step: mixing a clean, even blend
- Check whether your coco needs buffering. Unbuffered coco can grab calcium and magnesium out of your feed early on and hand back sodium and potassium, which throws off your first few weeks. If you're using a pre-buffered product you can skip this, but if not, plan to condition it before you plant: do the buffering soak once the coco is hydrated and loose (see step 2), because loose coco is far easier to buffer. Our guide to buffering coco coir walks through it.
- Hydrate and fluff the coco. Expand your blocks or bricks in warm water, let them drink fully, then break up any clumps so the texture is even. Measure the coco only once it's fully hydrated.
- Dampen the perlite before it goes near the mix. Dry perlite throws a fine dust that's a nuisance to breathe, and lightly wetting it knocks that dust down almost completely (Perlite Institute, n.d.; National Institute for Occupational Safety and Health, n.d.). A light mist in the bag or bucket is enough. We cover this properly in our note on handling perlite dust-free, and it's worth the extra minute every time.
- Measure both by volume and combine. Using the same scoop, add your counted parts of coco and perlite to a clean tub. Do the perlite in a couple of additions rather than all at once so it distributes evenly.
- Fold, don't beat. Turn the mix over gently with your hands or a trowel until the white perlite is spread evenly through the brown coco with no pockets of either. Folding rather than vigorous stirring keeps the perlite granules intact; smashing them just makes dust and destroys the very pores you added them for.
- Check moisture and pH before planting. The finished blend should feel like a wrung-out sponge. Coco naturally sits a touch acidic and buffered coco is usually in a friendly range, but it's worth checking your runoff pH before you plant, especially in a run-to-waste system. Our dedicated guide to pH-ing your medium walks through it in full, but as a quick rule, aim your feed at the range your plants prefer and watch the runoff.
That's it. A well-folded 70/30 tub is ready to pot straight up.
Common mistakes
The usual errors are small and easy to avoid. Measuring by weight instead of volume gives you a mix that's nothing like the ratio you intended. Measuring dry coco instead of hydrated coco underestimates how much you'll end up with. Packing the scoop tight one time and loose the next makes your ratio drift between batches. Stirring hard enough to crush the perlite quietly undoes the aeration you paid for. And skipping the buffering step on raw coco causes nutrient problems that look like a feeding issue but are really a media issue. None of these are dramatic on their own, but together they're the difference between a mix that behaves and one that keeps you guessing.
Frequently asked questions
Is 70/30 coco to perlite the best ratio?
It's the best default, not a universal answer. 70/30 lands inside the sensible air-and-water window for most container and hydroponic growing and is very forgiving. Move to 60/40 or 50/50 if your setup runs wet (short pots, humid or cool rooms, heavy hand-watering), and to 90/10 if it runs dry (tall pots, hot rooms, frequent drip feeding).
Should I measure coco and perlite by weight or by volume?
Always by volume. Perlite is far lighter than coco, so a ratio by weight would contain a completely different amount of perlite than the same ratio by volume. Use the same scoop for both and count parts: 7 scoops coco to 3 scoops perlite for 70/30.
Do I measure the coco before or after adding water?
After. Compressed coco expands roughly five to nine times when hydrated, so measure it fully wetted and fluffed, which is the volume it'll actually take up in the pot. Check the expansion figure on your specific brick or block.
How much perlite do I add to make a mix drain faster?
Add more perlite. Because it adds large, air-holding pores, increasing the perlite fraction raises air-filled porosity and lowers water retention, so the mix drains faster and dries sooner. Going from 70/30 to 60/40 is a noticeable step; 50/50 is faster again.
Do I need to buffer coco before mixing it with perlite?
If your coco isn't already pre-buffered, yes. Buffering conditions the coco so it doesn't strip calcium and magnesium from your early feeds. Do it while the coco is loose, before or after fluffing, and before you blend and pot. Perlite itself is inert and needs no buffering, only a light dampening to control dust.
Can I reuse a coco-perlite blend?
Often, with care. Perlite is reusable and coco can be reused if it's cleaned and re-conditioned, though it breaks down over time. The general steps for cleaning and sterilising a used medium are covered in our reuse and sterilising guide.
Get your coco and perlite sorted
If you'd rather not gamble on a mystery bag, we stock coco coir and horticultural perlite as separate ingredients so you can mix to your own target, plus ready blends if you want to skip the tub entirely. We're Tasmanian growers ourselves, not a drop-shipper, so the advice comes from people who actually use this stuff, and you're welcome to talk it through at the Youngtown store any day of the week. Found it cheaper locally? Our Price Beat Guarantee means we'll do our best to beat it, and we dispatch orders same day, so you're not left waiting to get mixing. Browse the growing media range to get started.
References
- Abad, M., Noguera, P., Puchades, R., Maquieira, A., & Noguera, V. (2002). Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology, 82(3), 241-245. https://doi.org/10.1016/S0960-8524(01)00189-4
- Cabrera, R., & Johnson, J. (2014). Fundamentals of container media management: Part I (Fact Sheet FS812). Rutgers Cooperative Extension.
- National Institute for Occupational Safety and Health. (n.d.). Perlite (1988 OSHA PEL Project documentation). Centers for Disease Control and Prevention.
- Nemali, K. (2018). Greenhouse and indoor production of horticultural crops: Understanding the pores of a soilless substrate (HO-287-W). Purdue University Extension.
- Owen, W. G., & Lopez, R. G. (2015). Evaluating container substrates and their components (HO-255-W). Purdue University Extension.
- Perlite Institute. (n.d.). Horticulture.