Mushroom Spawn Rate Calculator
Calculate grain or sawdust spawn amount from substrate dry weight and target spawn rate percent.
Oysters typically use 5-10%, shiitake 10-15%.
What is spawn rate?
Spawn rate (also called inoculation rate) is the ratio of spawn to substrate by dry weight. It determines how quickly mycelium colonizes the substrate. Higher spawn rates mean faster colonization but cost more. Lower rates save spawn but increase contamination risk due to slower colonization.
Spawn rate formula:
Spawn weight = Substrate dry weight × (Spawn rate % ÷ 100)
Recommended spawn rates by species and substrate:
| Species | Substrate | Recommended Rate | Colonization Time |
|---|---|---|---|
| Oyster (Pleurotus) | Straw | 5–10% | 10–14 days |
| Oyster | Sawdust | 10–15% | 14–21 days |
| Shiitake | Sawdust + bran | 10–15% | 60–90 days |
| Lion’s Mane | Sawdust + bran | 10–15% | 14–21 days |
| King Oyster | Sawdust + bran | 12–15% | 21–30 days |
| Button/Portobello | Compost (Phase II) | 2–3% of wet weight* | 14–17 days |
| Wine Cap | Straw/wood chips | 10–15% | 30–60 days |
| Reishi | Sawdust | 10–15% | 30–60 days |
*Button mushrooms are the exception and the calculator deliberately leaves them out. Their Phase II compost is never dried, so the 2–3% is a fraction of wet weight, and feeding a wet weight into a dry-weight calculator would understate the spawn by roughly three to one. Weigh the compost as it goes into the tray and take 2–3% of that instead.
Notice that oyster appears twice with different numbers. On straw it colonizes fast enough that 5–10% is plenty; on sawdust the same fungus is slower off the mark and wants 10–15%. The substrate matters as much as the species here, which is why the selector asks for both.
Worked example — oyster mushrooms on straw:
You have 2 kg of dry straw, targeting 7% spawn rate.
Spawn = 2,000 g × 0.07 = 140 g grain spawn
A standard grain spawn bag weighs 1–2 kg. So 140 g is a small fraction — one bag of spawn can inoculate about 14 kg of dry straw at this rate.
Grain spawn vs. sawdust spawn:
Grain spawn colonizes faster because each grain kernel is a separate inoculation point. Sawdust spawn is cheaper and has more inoculation points per volume but colonizes slower because each point has less stored energy.
| Spawn Type | Cost (relative) | Speed | Best For |
|---|---|---|---|
| Grain spawn | Higher | Fast | Most home growers, indoor bags |
| Sawdust spawn | Lower | Moderate | Outdoor beds, large-scale |
| Plug spawn | Highest per unit | Very slow | Log inoculation only |
| Liquid culture | Lowest | Requires transfer step | Making your own grain spawn |
The 10% rule of thumb: If you are unsure, use 10% spawn rate. It is the safest general-purpose rate that balances colonization speed, cost, and contamination resistance.
Scaling up: Commercial growers often use 3–5% spawn rate to reduce costs. They compensate with sterile technique, climate-controlled rooms, and faster-colonizing strains. Home growers should stick with 8–15% since contamination control is harder at home.
Temperature affects colonization speed: Most species colonize fastest at 24–27°C (75–80°F). Below 20°C, colonization slows dramatically. Above 30°C, mycelium stalls and bacteria thrive. Keep bags in a dark, warm area with minimal air movement during colonization.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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