Brew-in-a-bag puts the grain in a mesh bag in the kettle, mashes in the full volume of water, and lifts the bag out. A three-vessel system mashes in one pot, rinses the grain with fresh water in a second, and boils in a third. The argument between them is almost entirely about efficiency, and the numbers people quote range from "a couple of percent" to figures four times that. Both camps are right, about different bags.
Why sparging gets more sugar out at all
Nothing mysterious happens in a sparge. When conversion is done, the sugar is dissolved evenly through the liquid in the mash — and the wort clinging to the grain is exactly as sweet as the wort you drain off. So if you collect 27 L and the grain keeps 5 L, you have collected 27 ÷ 32 of the sugar and poured the rest onto the compost.
A batch sparge just does that dilution twice. Drain, add fresh water to the grain, stir, drain again — the second runoff rinses out most of what the first left behind. That is the entire mechanism, and it means the gap is arithmetic, not opinion.
So how big is the gap?
| How you treat the grain | Wort it keeps | Full-volume / BIAB | One batch sparge | Two sparges |
|---|---|---|---|---|
| Left to drip the usual figure for a grain bed | 5 L | 84.5% | 92.8% | 95.5% |
| Lightly squeezed pressed by hand in the bag | 4 L | 87.2% | 94.9% | 97.2% |
| Squeezed hard wrung out, or pressed between boards | 3 L | 90.1% | 96.8% | 98.5% |
Lauter efficiency — the share of dissolved sugar that reaches the kettle — for 5 kg of grain and this site's default 27 L pre-boil volume, computed from the engine. Absorption in L/kg: 1, 0.8, 0.6. This is lauter efficiency only; multiply by how well your mash converted to get the figure your brew day reports.
Read the first row and BIAB looks bad: 85% against 93%, a gap of 8.3 points. Read the last row and it nearly vanishes: a hard-squeezed bag gives up 90%, within 2.7 points of a batch sparge.
Squeezing takes the wort the grain retains from 5 L down to 3 L, and that alone is worth 5.6 points — two-thirds of the whole gap to a batch sparge. It is why the published claims disagree by a factor of four: a forum post saying BIAB costs "2%" and one saying it costs eight are both telling the truth about their own bag. Nothing else in this comparison moves the number as much, including the second sparge, which only buys 2.7 points.
And the practical version: if you never squeeze, you need about 0.5 kg (1.1 lb) more grain in this recipe to land on the same gravity as the sparger. At typical malt prices that is a couple of dollars a batch — real, but nothing like the difference in effort.
Can you taste which one you used?
Usually not, but this is the one mash-method experiment where people could. Brülosophy brewed a Vienna lager two ways, BIAB with a squeeze against a no-sparge mash, and finished at almost identical gravities — 1.046 against 1.045. In the triangle test 14 tasters picked the odd sample out where 13 were needed for significance (p = 0.02), which is a rare positive result from that series.
The author's own reading is worth quoting: he is "inclined to stick to my guns on my belief that a BIAB fabric filter is in no way appreciably different than any other lautering device, but that perhaps there's some element of truth to the old wisdom that squeezing the bag extracts something normally left behind in the spent grain" — and he describes the BIAB beer as having reduced hop character and a "sticky, almost cloying quality".
So the squeeze that buys back your efficiency may be the thing that costs you something else. One experiment is one experiment, and the effect is small enough that it took a panel to find it — but it is the only evidence anyone has, and it points the opposite way from "squeeze harder, it's free". A middle position is defensible: lift, let it drain properly, give it a gentle press, and accept a point or two.
What each system actually costs you
| BIAB | Three-vessel | |
|---|---|---|
| Vessels | One kettle | Mash tun, hot liquor tank, kettle |
| Extra gear | A bag, and a way to lift it | Manifold or false bottom, second burner, pumps or gravity |
| Brew day | Shortest — no transfer, no sparge | 60–90 minutes longer |
| Cleaning | One vessel | Three |
| Lauter efficiency | 85–90%, depending on the squeeze | 93–96% |
| Batch size limit | The kettle holds all the water and grain at once | Split across vessels, so bigger beers fit |
| Lifting | A wet bag of grain — heavy, and it drips | Nothing heavy, but more to move |
Efficiency figures from the table above. Time estimates as reported by the system vendors and the forum threads; the sparge itself is the bulk of it.
The volume constraint is the one that catches people out. A full-volume mash puts all the water in with the grain: for this 5 kg recipe that is 32 L of water plus the space the grain itself occupies, which is why BIAB brewers hit their kettle's ceiling on big beers while a three-vessel system just uses a bigger sparge. Our mash thickness calculator will tell you whether a given bill fits your vessel, and the water calculator splits the volumes for whichever system you set in your profile.
What a BIAB brew day actually looks like
It is shorter than the three-vessel version mostly because two steps disappear. Heat the full volume — 32 L for this recipe — to your strike temperature, drop the bag in and mash in. Hold the rest as usual, then lift the bag, let it drain over the kettle, and start the boil while it is still dripping. There is no transfer to a second vessel and no sparge, so the gap between "mash done" and "boil started" is about five minutes rather than an hour.
Two practical notes the comparisons skip. The bag is heavy — 5 kg of grain holding 5 L of hot wort is around 10 kg swinging over a pot of boiling liquid, which is why a pulley, a bag rack or a second pair of hands matters more than any efficiency point. And the strike temperature is different: a full-volume mash has far more water per kilo of grain, so the water needs to be only a little above the target rather than well above it. Our strike calculator handles that automatically once your profile says BIAB, because the thickness it uses is the real one.
Where all-in-one systems fit
The electric all-in-one systems that now dominate the market — the Grainfather, Anvil Foundry, BrewZilla and their relatives — are BIAB with a pump and a heating element. The grain sits in a perforated basket rather than a bag, the pump recirculates wort through it instead of you stirring, and the basket lifts on a handle rather than a pulley. The lautering arithmetic above applies to them unchanged: the basket holds wort at the same gravity as everything else, and lifting it and letting it drain is the same single dilution as a bag.
Which is why most of them ship with a sparge instruction anyway: pour a few liters of hot water over the raised basket and you have converted a no-sparge brew into something close to a batch sparge, recovering most of that 8-point gap without lifting anything heavier than a kettle. It is the best of both, and it is available to anyone brewing in a bag with a jug of hot water and somewhere to rest the bag.
Which should you choose?
BIAB if brew day competes with anything else in your life. One vessel, one cleaning job, an hour less, and — squeezed — an efficiency within three points of the alternative. The lost sugar costs a pound of malt, which is roughly what the extra propane for a longer brew day costs anyway. The all-in-one electric systems that dominate the market are BIAB with a pump and a heating element, which tells you where the hobby landed.
Three vessels if you brew big beers, or you like the process. Barleywines and imperial stouts are where full-volume mashing genuinely struggles, because the grain and the water have to share one kettle. And a three-vessel system rewards attention in a way that some people enjoy for its own sake — which is a perfectly good reason, and a better one than the efficiency argument that has been standing in for it.
Whichever you run, set it in your equipment profile and every calculator on this site will split the water accordingly. Then measure what you actually got with the efficiency calculator and use your own number, not a forum's.