Mushroom Biological Efficiency Explained: How to Calculate BE %

What biological efficiency actually means

Biological efficiency — usually shortened to BE — is a yield ratio used in mushroom cultivation. It compares the fresh weight of mushrooms harvested with the dry weight of the substrate they grew on, expressed as a percentage.

It is a bookkeeping measure, not a biological law. It exists because growers needed one number that lets them compare a block against another block, a recipe against another recipe, or this month's run against last month's. It only does that job when everybody involved weighs the same things in the same way.

The formula

BE % = fresh harvest weight ÷ dry substrate weight × 100

Both weights must be in the same unit. Work in grams unless you have a reason not to; converting halfway through is where most arithmetic mistakes creep in.

Because fresh mushrooms are mostly water and dry substrate is not, BE figures above 100% are perfectly ordinary. A BE of 100% does not mean you turned every gram of substrate into mushroom — it means the fresh harvest weighed the same as the dry material the block started from.

Why the *dry* substrate mass is the denominator

This is the part that trips people up, and it is the single most common reason two growers' BE numbers cannot be compared.

A hydrated bulk substrate is typically somewhere around two-thirds water by mass. If you use the hydrated block weight as your denominator, your BE will look roughly a third of the value it would have using dry mass — same mushrooms, same block, wildly different headline number.

Dry mass is used because it is the closest simple proxy for the actual food available to the mycelium. Water is a medium, not a nutrient. Two blocks made from the same 1 kg of dry hardwood and soy hulls contain the same feedstock whether one was hydrated to 60% and the other to 68%.

The practical implication: weigh and record your dry ingredients before you add water. If you only ever record the finished block weight, you can still calculate a consistent internal number, but say so explicitly and never compare it with anyone else's BE.

Hydration is worth understanding properly on its own terms — see the substrate hydration guide for the two conventions people use and why they produce different-looking percentages.

Worked example 1 — a single bulk block

A block is made from 1,000 g of dry substrate (hardwood pellets plus soy hulls, weighed dry) and hydrated. Over the whole cycle it produces:

• Flush 1: 620 g fresh
• Flush 2: 310 g fresh
• Flush 3: 95 g fresh

Total fresh yield: 620 + 310 + 95 = 1,025 g

BE % = 1,025 ÷ 1,000 × 100 = 102.5%

Worked example 2 — a smaller block, one flush only

A 500 g dry-mass block produces a single flush of 340 g fresh before it is retired.

BE % = 340 ÷ 500 × 100 = 68%

Note what this second number does and does not tell you. It is a genuine measure of what that block returned. It is not evidence that the block was worse than the first one — it was retired after one flush, so it was never given the chance to produce more. Comparing a one-flush figure with a three-flush figure is comparing two different questions.

Use the total fresh yield for the cycle

BE is normally quoted for the whole productive life of the block, not per flush. That means you need to keep a running total.

• Weigh each flush at harvest, fresh, before any trimming, drying or cooking
• Add flushes together as they come in
• Decide in advance when the block is retired, and stick to it

You can also record a per-flush BE if you find it useful — first-flush BE is a reasonable early indicator — but label it clearly as first-flush BE so it never gets mixed in with cycle totals.

Two habits make this reliable: weigh at a consistent point (straight after picking, before the mushrooms lose moisture on the bench), and trim consistently. Whether you include the stem butt or not matters less than doing the same thing every single time.

Comparisons only work when conventions match

Before you compare any two BE figures, check that all of the following are the same on both sides:

• Dry mass measured before hydration, not the wet block weight
• Fresh weight, not dried weight
• The full cycle total, or the same flush number on both sides
• The same trimming convention
• The same scale, ideally calibrated

If any of those differ, the two numbers are measuring different things and the comparison tells you nothing. This is why published BE figures from different sources so rarely agree, and why your own consistently-recorded numbers are more useful to you than anyone else's.

Interpreting BE cautiously

It is tempting to treat BE as a score. Resist it. A short, deliberately non-committal way to read the numbers:

• Under 50% — modest for many gourmet species and worth investigating, though entirely normal for slow or demanding species and for blocks retired early
• 50–100% — a productive range for a lot of gourmet setups
• Above 100% — strong for many species and substrates

These are orientation bands, not standards. Whether a given BE is good depends on factors that sit outside the formula entirely:

• Species and strain. Some oyster strains are prolific on straightforward substrates; lion's mane and shiitake behave very differently, and shiitake in particular runs a long colonisation before it produces anything.
• Substrate and supplementation. More nutrition available can support more fruit body mass, but it also raises the stakes on sterility. Supplemented blocks are less forgiving of imperfect process.
• Moisture at the start. A block that was too dry, or so wet that it went anaerobic, is limited before it ever gets to fruiting.
• Spawn rate and colonisation. How the block was inoculated affects how quickly and evenly it colonises — see the spawn to substrate ratio guide.
• Environment. Temperature, humidity and fresh air exchange during fruiting shape both size and quantity.
• Harvest timing. Mushrooms gain a lot of mass in their final day or two. Picking later gives a heavier harvest and a higher BE, at some cost to shelf life and, for heavy sporulators, to your air quality. Two growers with identical blocks and different picking habits will report different BE.

No BE number, on its own, tells you which of those was the limiting factor. It tells you where to start looking.

What BE is genuinely good for

Used within one setup, over time, BE is one of the more useful numbers a small grower can keep. It lets you ask concrete questions:

• Did the substrate recipe change do anything measurable across the next five blocks?
• Does this strain out-produce that one under my conditions?
• Is my third flush worth the bench space and the time?
• Did the block I hydrated differently behave differently?

None of those need a published benchmark. They need your own records, taken the same way each time, over enough blocks that one unusual result does not dominate.

Recording it without the spreadsheet drift

The reason most growers stop tracking BE is admin. Dry weights end up on a scrap of paper, harvest weights in a phone note, and after four blocks nothing lines up.

Hyphae Fidelity keeps the dry substrate weight with the batch, then adds each harvest against the same record, so the cycle total and the BE come out of data you already had to write down. Every block carries a scannable label, so logging a flush is a scan and a weight rather than a hunt for the right row.

Start free: create a Hyphae Fidelity account and log your next harvest against its block.

Next steps

• Run your own figures in the biological efficiency calculator
• Get your inputs consistent with the spawn ratio and hydration calculators
• Browse the rest of the growing guides