Hydration Calculator
Work out exactly how much water to add to dry substrate to reach field capacity — or how to rescue a mix that came out too wet or too dry.
Pick your species and the target moisture loads automatically: straw holds more water than sawdust, so each gets its own target. There's a second mode for correcting a mix you've already wetted. Whatever the number says, the squeeze test has the final word — a firm squeeze of finished substrate should yield a few drops, not a stream.
The squeeze test is the final word: a firm squeeze of finished substrate should yield a few drops, not a stream. Straw holds more water than sawdust — the species target reflects that.
How to use it
- Pick a mode. Hydrate dry substrate from scratch, or fix a mix that's already wet.
- Pick your species. It sets the target moisture — straw around 68%, sawdust around 60%.
- Enter the weights. Dry weight for a fresh hydrate, or current weight plus current moisture to correct a mix.
- Read the result. Water to add (or drive off), the final wet weight, and a squeeze-test reminder.
Questions
What moisture should mushroom substrate be?
About 60% for sawdust mixes and up to 68% for straw. The squeeze test is the final check: a firm squeeze yields a few drops, not a stream.
How do I know if my substrate is too wet?
If a squeeze produces a steady stream, it's too wet. Above about 72% moisture the mix goes waterlogged and anaerobic, and mycelium struggles to breathe.
Can I fix substrate that's too wet?
You can't easily un-add water. Spread the mix and air-dry it, or add dry substrate to soak up the excess.
What is field capacity?
The most water a substrate can hold without free water pooling at the bottom — the target for a healthy, breathable mix.