How to calculate substrate moisture content
The squeeze test tells you close; the scale tells you exactly.
The squeeze test is fast, but if you want a repeatable substrate — or you're scaling recipes — you need the actual moisture content by weight. It's a two-weighing measurement and one short formula.
The moisture-content formula
Moisture content is the weight of water as a fraction of the total wet weight:
Moisture % = (wet weight − dry weight) ÷ wet weight × 100
Weigh a small sample wet, dry it completely (a low oven or dehydrator until the weight stops falling), then weigh it dry. Example: a sample weighs 100 g wet and 40 g dry → 60 g of water → 60% moisture. Do this once per substrate type and you know how your process behaves.
How much water to hit a target
Going the other way — you have dry substrate and want a target moisture — the water to add is:
Water = dry weight × m ÷ (1 − m), where m is the target moisture fraction.
| Dry substrate | Target | Water to add | Wet total |
|---|---|---|---|
| 400 g | 60% | 600 g | 1000 g |
| 400 g | 65% | 743 g | 1143 g |
| 1000 g | 60% | 1500 g | 2500 g |
Water weighs 1 g per mL, so grams of water and mL are interchangeable here. Account for any water the substrate already holds — subtract it from the amount you add.
That's the common mistake. 60% moisture means water is 60% of the wet total, which is 1.5× the dry weight of water — not 0.6×. Using the wrong version leaves substrate far too dry.
FAQ
How do you calculate substrate moisture content?
Moisture % = (wet weight − dry weight) ÷ wet weight × 100. Weigh a sample wet, dry it fully, weigh it dry; the loss is the water. 100 g wet → 40 g dry = 60% moisture.
How much water do I add to hit 60% moisture?
Water = dry weight × m ÷ (1 − m). For 400 g dry at 60%: 400 × 0.6 ÷ 0.4 = 600 g of water.
What moisture percentage should substrate be?
About 60–65% for most fruiting substrates (straw nearer 70%) — the field-capacity range.
Gear this guide uses
Moisture math is only as good as the scale under it.
- Weigh wet and dry samples on a gram-accurate digital scale.
- Dry samples down in a dehydrator or low oven.
- Hydrate a predictable base substrate to the number you calculated.
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Sources
Targets are typical ranges; tune to your species, substrate, and container.