How Much Grain Spawn per Kilogram of Substrate?

The quick answer

For most gourmet species on a typical bulk substrate, use 200 g to 500 g of grain spawn per kilogram of bulk substrate, measured by mass, before hydration is counted separately. That range covers the common ratios growers quote as 1:2 through 1:5 (spawn : substrate).

If you want the arithmetic and the reasoning behind why a ratio changes anything, read the spawn to substrate ratio guide — this article is the short, lookup-table version for when you already know roughly what you're doing and just need the numbers for today's batch.

The formula

spawn (g) = bulk substrate (g) ÷ X, where X is the second number in the ratio.

So for 1 kg (1,000 g) of bulk substrate:

• 1:2 → 1,000 ÷ 2 = 500 g spawn
• 1:3 → 1,000 ÷ 3 = 333 g spawn
• 1:4 → 1,000 ÷ 4 = 250 g spawn
• 1:5 → 1,000 ÷ 5 = 200 g spawn

Lookup table: 1 kg to 10 kg of substrate

All figures are grams of grain spawn, rounded to the nearest 5 g.

For anything outside this table, or to save the figure against a batch, use the spawn ratio calculator — it also rounds to whole bags if you tell it your spawn bag size.

Which ratio to pick

• 1:2 — fastest colonisation, most resistant to contamination, highest spawn cost per kilogram of finished substrate. A reasonable choice for a first home-made batch or a substrate you don't fully trust.
• 1:3 — a common middle ground for oyster and similar fast colonisers on straightforward substrates such as masters mix.
• 1:4 — stretches spawn further, colonises more slowly, and leaves a longer window where the substrate is vulnerable before mycelium fully occupies it.
• 1:5 — mainly used by growers with very reliable sterile technique who are optimising cost over speed. Not a beginner setting.

Worked example

You have prepared 3.6 kg of hydrated bulk substrate and want a 1:3 ratio.

1. Spawn = 3,600 ÷ 3 = 1,200 g
2. If your spawn bags are 500 g each, that's 2.4 bags — round up to 2.5 bags (two full bags plus half of a third) or round down to two bags and accept a slightly leaner ratio of roughly 1:3.6.
3. Weigh the spawn out before breaking it up, so you know exactly what went into the block if you need to compare batches later.

Common mistakes

• Using hydrated substrate weight when your ratio convention assumes dry weight. Check which convention your source is using — see the substrate hydration guide for the difference.
• Eyeballing spawn quantity by volume rather than weighing it. Grain spawn density varies by grain type and how much it has consolidated, so volume is not a reliable stand-in for mass.
• Assuming more spawn always means a better outcome. Beyond a certain point extra spawn buys you speed, not extra mushrooms — the bulk substrate is still what feeds the fruiting bodies.

FAQ

Do I need to weigh spawn and substrate separately, or can I just estimate?

Weigh both. A kitchen scale accurate to a few grams is enough, and it removes the guesswork that causes most beginner ratio mistakes.

What if my spawn bag doesn't divide evenly into the ratio I want?

Round to the nearest practical amount and note what you actually used. A ratio of 1:3.4 instead of 1:3 makes very little practical difference; what matters is knowing the real figure for next time.

Does the ratio change between species?

The convention doesn't, but the sensible range shifts. Faster, more aggressive colonisers such as oyster tolerate leaner ratios well; slower species like shiitake are often run richer to compensate for a longer colonisation window.

Can I use too much spawn?

Yes, in the sense that you waste spawn and money without a proportionate yield benefit past a certain density. It rarely causes a bad outcome on its own, unlike too little spawn on a marginal substrate.

Next steps

• Get an exact figure for your batch in the spawn ratio calculator
• Read the full reasoning in the spawn to substrate ratio guide
• Check your substrate choice with the substrate selector