“First runnings” are the first liquids to drain out of your mash tun when you’re doing an all-grain batch. The first runnings have the most sugar. Subsequent sparging extracts less and less sugar from the grain. I suppose it’s analogous to using a tea bag a second time.
The first runnings are tempting because they are so thick and rich. Sometimes you want to make a really strong beer just out of the first runnings! But that’s not the only temptation. A buddy of mine and I have tried “hot scotchies” on brew day. You mix some of your first runnings with scotch. It’s a particularly nice treat on a cold brew day because the first runnings are pleasantly warm.
My buddy decided to make a batch exclusively for hot scotchies. He wrote as follows.
I am making a batch of wort just for drinking on brew day. I am shooting for 1 gallon of first runnings. How’s this look? 4.5 lbs of grain, 6.75 quarts of water.
That means he’s going to mash 4.5 lbs of grain with 6.75 quarts of water. The question is how much of that 6.75 quarts will come out with the first runnings — because the grain will absorb some of the water.
His ratios assume a grain absorption of about 0.15 gallons per pound. I.e., 4.5 lbs x 0.15 gallons per pound = 0.675 gallons, which is 2.7 quarts. So he should get just a touch over a gallon of first runnings.
This page says “Average grain absorption is 0.15 gallons per pound. It can vary between 0.1 to 0.2.”
But this page says you should assume 0.2.
This page has a more complicated equation that includes liquid remaining in the bottom of the mash tun and in the lines or pump. My friend doesn’t use a pump, but there’s always the chance of some liquid will stay in the mash tun — below the level of the spigot.
I suspect that absorption also varies by the type of grain and by how well it’s milled, so I don’t think it’s possible to get the figure exactly right without some experimentation. Everybody’s system will differ a little.
My recommendation was to go with a little more water.
Also, it’s a shame to leave that extra sugar on the grains. He could sparge a couple times and use that for a mild.