Spawn to Substrate Ratio Calculator for Monotubs and Bags
Pick a spawn-to-substrate ratio and enter how much substrate you have, or give the tub's length, width and fill depth. You get the spawn to mix in. The result also shows what share of the finished batch that spawn makes, because the same ratio reads two ways.
Result
2 L of spawn
for 6 L of substrate at 1:3
- Substrate
- 6 L
- Spawn
- 2 L
- Mixed total
- 8 L
- Spawn share
- 33% of the substrate, 25% of the mix
How this compares
- Your rate, spawn as a share of the substrate 33%
- UF/IFAS oyster on straw 5-10%
- Lion's mane grow guide, grain spawn by substrate weight 10-20%
Each bar runs from 0 to 100% of the substrate. Published rates are measured by weight or volume as each source states.
How it works
What 1:3 Means in Litres of Spawn
At 1:3 you mix one part spawn into every three parts substrate, so the calculator divides your substrate by three. Six litres of substrate at 1:3 takes 2 L of spawn and makes 8 L of mix. If you measure a tub, the calculator multiplies length, width and the depth you plan to fill, then divides by 1,000 to turn cubic centimetres into litres.
Most confusion starts because the same ratio has two readings. At 1:3 the spawn equals a third of the substrate, yet it makes up only a quarter of the finished mix. The result shows both shares, so you can match whichever one your guide uses. UF/IFAS gives its oyster spawn rate as a percentage without naming which of the two it counts against.
Your spawn and substrate also need the same unit. Grain spawn is far denser than fluffy coir or straw, so 1:3 by volume and 1:3 by weight describe different batches. When a guide prints a bare ratio, it leaves that question open, and the bulk substrate guide's section on spawn rate works through both readings.
Your numbers, worked through
spawn = substrate / N, for a ratio of 1 part spawn to N parts substrate
- 6 L / 3 = 2 L of spawn
- 6 + 2 = 8 L of mixed substrate
- Spawn is 33% of the substrate and 25% of the whole mix
Limits
What this tool cannot do
- It cannot tell you which ratio suits your strain. It only shows what a chosen ratio means in spawn.
- Many guides report faster colonization at higher rates, but a higher rate never guarantees a clean tub.
- It does not check that your spawn and substrate use the same unit.
Questions
How much grain spawn goes into a monotub?
It depends on the tub and the ratio you choose. If you fill a tub with 6 L of substrate, 1:3 calls for 2 L of spawn and 1:5 calls for 1.2 L. Measure the depth you actually plan to fill, since a half-filled tub needs half the spawn.
Is 1:3 the same as 33%?
It equals 33% only when you measure the percentage against the substrate. Against the whole mix, 1:3 is 25%, because the spawn is one of four equal parts. Before you copy a percentage from a guide, check which of the two it means.
Does more spawn stop contamination?
More spawn does not stop contamination on its own. It gives the mycelium more starting points and can shorten colonization, and Cornell Small Farms notes that heavy grain inoculation of straw can raise yields. Contamination still depends on how you prepared and handled the substrate and the spawn.
Sources2
- UF/IFAS, Grow Your Own Oyster Mushrooms at Home (SS662) Lists a 5% to 10% spawn rate by weight or volume for its oyster straw method without naming the denominator. Figure as summarized in our straw substrate guide, checked 2026-10-20.
- Cornell Small Farms, Seven Stages of Cultivation Notes that heavy grain inoculation of straw can raise yields, so spawn rate changes more than speed. As summarized in our straw substrate guide, checked 2026-10-20.