Efficiency is about the amount of extract you get from the malt compared to the amount available, expresses as a percentage.

Here is a snip from an old Weyermann COA, full document at end.

We can see the potential is 81.1% that is the FGDB (fine grind dry basis) fine grind is 0.2mm, so way finer than we or even a commercial brewer would use.

The moisture content is given as 4.5% and there is one other factor to take into account. The Coarse/Fine difference, the difference between a sample crushed at 0.2mm and 0.7mm and subjected to the Congress Mash (a world wide agreed standard test)

What is left is called the CGAI (coarse grind as is) potential.

Take the FGDB (81.1) subtract the moisture content (4.5) leaves 76.6%. There is still the C/F difference to take into account, generally it is less than 2% for a well modified malt, less than 1% can indicate problems with the malt, lets call it 1%, so the CGAI would be 75.6%.

If you mashed 1kg of the above malt and got 756g of extract (it doesn't matter how much water that is dissolved in) you would have achieved 100% Efficiency! It isn't going to happen.

Lets say we want to make reasonably strong beer and at full volume no sparge...mash at 6:1

The amount of water retained by the grist is between 0.65 and 0.8L/kg, depending on the fineness of the grind. lets call it 0.7 for this case.

Using the standard equation to determine first running's (in this case all the running's)

oP Wort = 0.7569/(6+0.7569) = 0.7569/6.7569 = 0.112, or 11.2oP

We put in 6L/kg and got back 5.3L at 11.2oP or an SG of 1.0448 (I doubled checked this time)

The mass of wort recovered is 5.3*1.0448 = 5.53744kg

At 11.2oP we have 5.53744*0.112 = 0.620kg of extract (solids)

The malt had a potential of 0.756kg so we recovered 0.620/0.756 = 0.82 or 82%, the rest is still in the grist and you need to sparge to get more of it out.

Having a quick play with the L:G (it takes ~2:1) to cover the grist in water. Is pretty clear that the more water you use the more efficient your extraction is. Naturally we pretty quickly run into the problem of having a very thin wort and needing to boil for a long time to get back to the OG we are looking for.

Pretty easy to rearrange the equation so you can predict what L:G you need to hit a target OG.

Interesting that back in the old days before sparging brewers made three beers from a grain bill. The first being a Strong Beer, then the grain bed was refilled with hot water, allowed to rest, drained to give Table Beer, and again to give Small Beer. Overall Efficiency would have been pretty high.

Full Volume mashing can be very easy, there is going to be a cost in efficiency but if you wanted to do a Party Gyle that can be mostly recovered. If you wanted to make a low ABV Farm House Saison or the like it would be the way to go.

100% extraction/efficiency is never going to happen.

Mark