Mycelocity
Operations

Costing a Block: Spawn Rates, Substrate and COGS

What belongs in the cost of a single fruiting block — substrate, spawn ratio, packaging and sterilisation — and how spawn rate trades cost against speed.

Before you can talk about margin, you need an honest cost for one fruiting block. “Cost of goods sold” (COGS) per block is the all-in variable cost to produce one block — the costs that scale with each block you make, kept separate from the fixed overhead the business carries regardless.

The four components

The COGS per Block calculator builds the number up from its parts so you can see what actually drives it:

  • Substrate — cost per kg × kg per block, plus a waste allowance for spillage and trim.
  • Spawn — driven by your spawn ratio (below).
  • Packaging — bag, filter patch, label, per block.
  • Sterilisation — the energy and labour of a sterilisation/pasteurisation run, divided across the blocks in that batch.

That last point matters: sterilisation is a batch cost, not a per-block cost. Running a half-empty autoclave makes every block in it more expensive. Filling the batch is one of the easiest ways to bring COGS down.

Spawn rate: the central trade-off

Spawn rate is the ratio of spawn to substrate, written as 1:N — 1 part spawn to N parts substrate. It is the lever that ties this guide to contamination:

  • More spawn (e.g. 1:5) → faster colonisation, less time for competitors to establish, lower contamination risk — but higher spawn cost per block.
  • Less spawn (e.g. 1:10) → cheaper per block, but slower colonisation and higher contamination risk.

So the cheapest-looking spawn rate is not always the cheapest outcome. A point saved on spawn can be lost several times over to contamination — model both the COGS here and the loss rate in Contamination Loss before settling on a ratio.

Costing feeds capacity

COGS does not stand alone. The Capacity & Chambers Needed calculator tells you how many blocks you can physically run per cycle and per year; multiplying that volume by COGS gives the variable cost of running at scale. A small per-block cost difference is trivial on ten blocks and decisive on ten thousand.

Getting an accurate number

  1. Cost each component from real invoices, not estimates.
  2. Set a realistic substrate waste percentage.
  3. Use your actual blocks-per-batch for the sterilisation split.
  4. Revisit COGS whenever input prices or batch sizes change — it is the foundation every economics calculator sits on.