Monotub Grain & Substrate Calculator

Use our Monotub Grain & Substrate Calculator to determine the perfect mix of grain spawn and CVG substrate for your grow setup. Just enter your tub’s internal length and width, pick a substrate depth (standard options or custom), and select your grain type. The tool then calculates the precise weights you need and recommends the matching refill kit from our range (30L or 45L bundles). Ideal for growers who want to scale accurately and avoid waste.

Spawn + CVG Calculator

Enter tub footprint and pick a depth. Choose your grain. We’ll calculate exact amounts and recommend the matching kit.

Advanced (densities)
CVG (calculated): kg
Spawn (calculated): kg
Total mix volume: L · Target fill: L · Depth: cm
Recommended kit

Assumes the final mixed volume (spawn + CVG) equals your chosen depth. Typical depths are 7–10 cm.

Why use the Monotub calculator?

This calculator is designed to help growers in the UK accurately measure the grain spawn and CVG substrate needed for monotub cultivation. By entering your tub’s internal dimensions and preferred substrate depth, the tool applies real-world density values for popcorn, rye, millet, or milo grain. It then calculates the correct spawn-to-substrate ratio and recommends the closest refill kit from our range. This ensures your monotub is filled to the ideal depth, whether you are using BagTek or traditional monotub methods, and helps avoid wasted materials. All refill kits are prepared in the UK and shipped nationwide for reliable results every time.

Refill Bundles

Monotub Grain & Sub Calculator FAQ:

What substrate depth should I choose for a monotub?

Most growers use 7–10 cm. The calculator provides 7 cm and 9 cm presets, with a custom option if you prefer a specific depth.

Does the calculator work for different grain types?

Yes. We use measured kg/L densities for each grain (popcorn, milo, millet, rye) so results are tailored to the grain you select.

How do you decide which refill kit to recommend?

We compute the total required weights of grain spawn and CVG from your tub footprint, depth, and ratio. If the total is at or below 3kg we recommend the 30 L kit; above that we recommend the 45 L kit. If you’re close to the boundary, we’ll let you know both are suitable.

Can I use the recommended bundle for BagTek as well as monotubs?

Yes. The bundles are sized for monotubs but are equally suitable for BagTek. The same grain-to-substrate ratio applies.

Monotub Refill Collection:

These bundles are designed for both monotub and BagTek ‘fruit in the bag’ style cultivation. Each bundle includes a sterile grain bag of your choice and sterilised bulk substrate, which are perfect for refilling your monotub or preparing a new grow bag for your fruiting chamber. These refill kits are much more cost effective than buying separately. Plus, they’re available with next-day UK delivery as standard.

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