If you're trying to figure out exactly how much coco coir for 5 gallon pot grows you need, the short answer is that you'll want about 0.67 cubic feet of expanded material per container. That translates to roughly 19 liters of coco if you're measuring by volume. However, it's never quite as simple as just grabbing a bag and pouring it in, because coco usually comes in two very different forms: compressed bricks or loose, pre-buffered bags.
Getting your measurements right matters more than you might think. If you buy too little, you're stuck halfway through a messy transplant with a plant that has its roots exposed to the air. If you buy too much, you've got a heavy, damp bag of substrate taking up space in your garage. Let's break down how to calculate your needs so you can get your grow started without a hitch.
The basic math of volume and pots
When we talk about a "5-gallon pot," we're usually talking about the nursery trade standard, which isn't always exactly five liquid gallons. In the gardening world, these pots often hold a bit less than their name suggests—usually closer to 4.4 or 4.5 gallons of actual soil or coco.
To be safe and ensure you have enough to fill the pot to the brim (allowing for some settling), you should aim for about 0.7 cubic feet per pot. If you're buying loose coco in those big 50-liter bags, one bag will typically fill about two and a half 5-gallon pots. If you're planning on a four-plant grow, you'll definitely want to pick up two of those 50L bags to make sure you don't run short.
It's also worth noting that fabric pots—those "smart pots" everyone loves—tend to bulge a little when they're filled. They can actually take a bit more material than a rigid plastic pot of the same "size." If you're using fabric, definitely lean toward that 0.7 cubic feet estimate rather than trying to skimp.
Bricks vs. loose coco: What are you buying?
This is where most people get tripped up. If you walk into a hydro store and buy a bag of loose coco, the math is easy. The bag says 50 liters; your pot takes about 19 liters. Done.
But if you're buying those dry, heavy bricks, you have to account for expansion. A standard 5kg (11lb) coco coir brick is the most common size you'll find online. Once you add water to one of these, it's going to expand like crazy. Usually, a single 5kg brick will yield somewhere between 60 and 70 liters of usable substrate.
If we do the math on that, one 5kg brick is enough to fill roughly three to four 5-gallon pots. If you're just doing a small two-plant grow, a single brick is more than enough. Just remember that you have to hydrate it first, and it's always better to have a little left over than to have a pot that's only three-quarters full.
Factoring in the perlite mix
Hardly anyone grows in 100% straight coco coir these days. Most growers prefer a blend, usually a 70/30 mix of coco and perlite. This helps with drainage and prevents the substrate from staying too wet, which can lead to root issues.
If you're planning on mixing in perlite, you don't need as much coco. For a 5-gallon pot using a 70/30 ratio: * You'll need about 13-14 liters of coco. * You'll need about 5-6 liters of perlite.
This is a great way to stretch your budget if you're filling a lot of pots. Perlite is relatively cheap and it adds that much-needed oxygen to the root zone. When you're standing in the store trying to figure out how much coco coir for 5 gallon pot containers you need to buy, remember that the perlite is going to take up a significant chunk of that volume.
How to hydrate your bricks without the mess
If you decided to go with the bricks (good choice, they're way easier to ship), you've got to expand them. Don't just throw the brick in the pot and start watering. That's a recipe for a disaster.
The best way to do it is to get a massive plastic tote or a clean trash can. Drop the brick in and slowly add about 4 to 5 gallons of lukewarm water. Warm water actually helps the fibers break apart faster than cold water. Let it sit for about 20 to 30 minutes.
Once it's all fluffed up, give it a good stir. You want it to feel like a wrung-out sponge—damp, but not swimming in a puddle. If you see big chunks that are still hard, add a little more water. This is also the perfect time to mix in your perlite. Throw the perlite in while the coco is still damp so the dust doesn't fly everywhere and get in your lungs.
Why 5 gallons is the "Goldilocks" size for coco
You might be wondering if 5 gallons is even the right choice. In soil, 5 gallons is a standard size for a medium-to-large plant. In coco, however, plants grow much faster because it's a hydroponic medium.
A plant in a 5-gallon pot of coco can get massive. Because coco holds so much oxygen even when wet, you can feed the plants more often than you would in soil. Many growers find that a 3-gallon pot of coco actually performs similarly to a 5-gallon pot of soil.
That said, if you have the space, sticking with the 5-gallon pot gives you a bigger "buffer." If you miss a watering in a 1-gallon pot of coco, your plant might be dead by dinner time. In a 5-gallon pot, you have a bit more wiggle room. It's the perfect size for someone who wants big yields but doesn't want to be tied to a watering can three times a day.
Filling the pot the right way
When you're finally ready to fill those pots, don't just dump the coco in and stomp it down. You want to keep it relatively loose. Coco is loved for its aeration, and if you pack it down too tightly, you're defeating the purpose.
Fill the pot to the very top, then give the pot a gentle tap on the floor to let the material settle naturally. You'll notice the level drops an inch or two. Add a little more until it's about an inch from the rim. This "headspace" at the top is important so that when you water your plants, the water doesn't just spill over the sides before it has a chance to soak in.
Also, keep in mind that coco tends to settle even more after the first few heavy waterings. Don't be surprised if, a week into the grow, your 5-gallon pot looks like it's only 80% full. You can always top it off with a bit more coco later if the roots aren't too high up.
Quick reference guide for shopping
To make it easy when you're at the store or ordering online, here's a quick cheat sheet for how much coco coir for 5 gallon pot projects:
- 1 Pot: You need ~20 liters. Buy one small 650g brick or a small bag.
- 2-3 Pots: You need ~40-60 liters. One 50L bag of loose coco or one 5kg brick will do it.
- 4-5 Pots: You need ~80-100 liters. Buy two 50L bags or two 5kg bricks.
- 10 Pots: You need ~200 liters. Four 50L bags or three 5kg bricks (you'll have some leftover).
A final word on buffering
One thing I have to mention—even though it's not strictly about volume—is buffering. If you bought the cheap bricks from a hardware store instead of a dedicated grow brand, they might be loaded with salt or have a weird pH.
Before you fill your 5-gallon pots, it's a smart move to rinse your coco and maybe even soak it in a Cal-Mag solution. Coco has a habit of "stealing" calcium and magnesium from your plants. Buffering the coco beforehand fills those sites so the nutrients actually go to your plants instead of getting locked up in the fibers. It's a bit of extra work, but when you're dealing with 5 gallons of substrate, you want to make sure it's high-quality stuff from the start.
Filling your pots correctly is the foundation of the whole grow. Now that you know exactly how much to get, you can stop guessing and start planting. Good luck!