How low can a beer's final gravity be?

I've had a Brut IPA that utilized White Labs ULTRA-FERM(liquid amyloglucosidase) and some champagne yeast(after primary fermentation) finish at 0.996. That means it had more alcohol volume than residual sugar volume I suppose? The sample smelled and tasted amazing, currently waiting on the bottles to condition to have the final product. I also used a small amount of the enzyme on another IPA and had it finish at 1.002. Also a very good finished product.

It's feasible to have a "negative" gravity reading; alcohol is less dense than water and if all of the fermentables are gone there's nothing to push the density higher.

I am currently fermenting a Brut IPA, made from all barley and wheat (no adjuncts whatsoever). I used WLP001 and glucoamylase enzymes, and the current gravity reading according to both my refractometer and my Tilt hydrometer is 0.998, or -0.45 °P. I mashed at the low end for sure, never going above 148 and most of the conversion probably happened around 145. I have no reason to suspect an infection (and it's not my first negative gravity beer).

According to the math for adjusting a refractometer reading in the presence of alcohol, you could in practice see an FG as low as 0.990, assuming a very dry, very strong beer (10% ABV) with OG of 1.068. But that's extreme brewing.

Also check malt type. My Pils frequently heads for very low f.g's in the 1000 - 1002 range. Have to be careful if you want to try to keep the abv below 5% (which I do, generally). If anything, the malt ferments too cleanly (I will be heading up the temp range for future mashes).

Edit / update - this stuff was on S-04 (not S-05) apologies. 55% Pale Ale, 35% Pils, 10% others (caramel malts), finished on 1.004 on this same yeast.

What yeast were you using? Some, such as the 3711 mentioned earlier, are notorious for fermenting out very dry. Many of these are identified as var, diastaticus. These strains can convert longer chain sugars into shorter, more fermentable ones, which leaves very little in the wort that the yeast can't eat. My batches with 3711, regardless of OG, always end up around .5 degrees plato, which is about 1.002.

I manage to get FG on a Wyeast Labs 3724 French Saison down to 1000. Taste was great too, no infection. In the same batch we also fermented WLP565 Beligian Saison and ended up on 1004. Same base wort, time and temprature as 3724.

I also just a Belgian Dubbel fermented with 1762 and aged on mission figs to drop to 0.993, down from 1.016 after primary. Doesn't taste infected but drinking early just in case. It is quite alcoholic

Theoretically, a beer could finish below 1.000 due to the alcohol in it. You'd need to have a very fermentable wort and a very attenuative yeast (or bacteria), but in theory it's possible.

If you add enough simple sugars, you can probably get it below 1.000.

I had a saison that finished at 1.002 that was fermented with Wyeast 3711 French Saison at about 84F. The attenuation rate of the yeast strain along with the use of fully fermentable sugars can bring your final gravity quite low. Your mash temperature will also dictate how fermentable your wort will be, a lower beta amalyes rest will produce more simple sugars.

Lacotbacillus and brettanomyces bacteria will also consume more complex sugars than regular yeast strains dropping the FG below normal levels.