How to Calculate a Masters Mix Ratio: Hardwood, Soy Hulls and Water

What this guide covers

This is the arithmetic behind building a batch of masters mix from scratch: splitting dry ingredients 50/50 by weight, then adding the right amount of water to reach a target moisture level. If you want the background on why this recipe works and which species suit it, read the masters mix substrate guide first — this article is the calculation, not the theory.

Step 1 — decide your total dry weight

Start from the batch size you actually want, in dry ingredients, before any water goes in. Dry weight is the number that matters for ratios and for later biological efficiency comparisons, so weigh it before hydrating anything.

Step 2 — split 50/50 by weight

Masters mix is traditionally hardwood fuel pellets and soybean hull pellets in equal parts by dry weight, not by volume. The two materials have different densities, so a 50/50 split by volume will not give you a 50/50 split by weight.

hardwood (g) = total dry weight ÷ 2 soy hulls (g) = total dry weight ÷ 2

Step 3 — work out water needed for a target moisture

Total finished (wet) weight and moisture percentage are related by:

wet weight = dry weight ÷ (1 − target moisture)

water to add = wet weight − dry weight

This assumes your dry pellets start essentially bone dry, which is a reasonable approximation for kiln-dried hardwood and soy hull pellets straight from the bag.

Worked example — a 4 kg dry batch at 65% moisture

1. Total dry weight: 4,000 g
2. Hardwood: 4,000 ÷ 2 = 2,000 g
3. Soy hulls: 4,000 ÷ 2 = 2,000 g
4. Target wet weight: 4,000 ÷ (1 − 0.65) = 4,000 ÷ 0.35 = 11,430 g
5. Water to add: 11,430 − 4,000 = 7,430 g (roughly 7.4 litres, since water is close enough to 1 g/ml for this purpose)

That water goes in gradually, with time for the pellets to absorb and expand, not all at once — pellets can take twenty minutes or more to fully break down and soak.

Quick-reference table

Dry batch sizes split 50/50, with water needed for a 65% target moisture.

Run any other batch size, or a different target moisture, through the masters mix calculator instead of redoing the arithmetic by hand.

Why 65% and not some other figure

65% sits in the commonly recommended range for masters mix, but it is a starting point, not a fixed law. The right figure depends on your ingredients' starting moisture, your container's drainage, and your species — see the substrate hydration guide for how to judge moisture by feel as a cross-check against the maths, and for the two different percentage conventions people quote.

Adjusting the ratio itself

Some growers move away from a strict 50/50 split once they have experience — a bit more soy hull for extra nitrogen and faster colonisation, or a bit more hardwood for a firmer, longer-lasting block. If you change the split, recalculate both the dry weights and the water figure, since a nitrogen-heavier mix can also hold moisture slightly differently.

Common mistakes

• Splitting by volume instead of weight. Soy hull pellets and hardwood pellets pack differently, so a 50/50 scoop-for-scoop split is not a 50/50 weight split.
• Adding all the water at once and judging moisture immediately. Pellets need time to fully hydrate and swell; a mix that looks too dry after five minutes may be correct after twenty.
• Forgetting that spawn ratio still applies on top of this. Once your masters mix is hydrated, you still need to work out spawn quantity — see the spawn ratio calculator.

FAQ

Does masters mix have to be exactly 50/50?

No, but 50/50 by dry weight is the standard starting point and a sensible default if you have no reason to deviate.

What if my pellets aren't completely dry to begin with?

Weigh them as received and treat that as your dry weight for the ratio calculation; the small residual moisture in "dry" bagged pellets is usually negligible against the amount of water you're adding.

Can I use fresh weight instead of dry weight for the 50/50 split?

Only if both ingredients have the same moisture content already, which is unusual. Using stated dry bag weights avoids the problem entirely.

My mix feels too wet even though the maths says 65%. What now?

Trust the squeeze test over the arithmetic if they disagree — the formula assumes bone-dry starting pellets, and real bags vary. Adjust down slightly and record what you actually used.

Next steps

• Run your own batch through the masters mix calculator
• Read the full recipe background in the masters mix substrate guide
• Work out spawn quantity next with the spawn ratio calculator