Extract and all-grain brewing get discussed as though they're two entirely different hobbies, and in terms of equipment and brew-day time they genuinely are — but in terms of the actual arithmetic behind a recipe, the difference is much narrower than the equipment gap suggests. It's worth being precise about exactly what changes and what doesn't, because the answer surprises people who assume extract brewing is somehow "simplified math," when in fact the arithmetic downstream of the kettle is completely identical between the two, and only the pre-fermentation stage genuinely differs.
What doesn't change at all
ABV comes entirely from OG and FG — the ABV calculator has no idea, and no need to know, whether that OG came from mashing grain or dissolving liquid or dry extract into water. A 1.052 OG extract batch fermenting to 1.012 computes the identical ABV as an all-grain batch hitting the same two numbers. IBU comes entirely from hop weight, alpha acid, boil time and boil gravity — the IBU calculator doesn't care where the wort's sugar came from either, only what's actually happening to the hops during the boil itself. SRM colour comes from grain weight and Lovibond rating for whatever's actually contributing colour — and this is genuinely relevant to extract brewers, because most extract recipes still steep specialty grains (crystal, roasted malts) for colour and flavour even without a full mash. 0.5 kg of a 40°L crystal malt steeped in an otherwise-extract 19 L batch contributes roughly 6.6 SRM through exactly the same SRM calculator an all-grain brewer would use for the same addition. Check it here — steeped colour contribution works identically whether the specialty grain sits in a full mash or a mesh bag in a kettle by itself.
What disappears entirely
The strike-water calculation simply doesn't apply to an extract batch with no grain to mash — there's no grain temperature, no target mash temperature, no water:grain ratio, because there's no starch conversion happening on brew day at all. The malt extract producer already did that conversion at the maltery, delivering a syrup or dried powder that's already fermentable. This is the single biggest reason extract brewing is faster and requires less equipment: an entire calculation, and the equipment and time that go with executing it (a mash tun, sparge water, a lauter step), simply isn't part of the process.
Diastatic power — a malt's own enzyme content, relevant to whether it can convert its own starch (and potentially some unmalted adjunct alongside it) — stops mattering too, for the same reason. Extract brewers steeping specialty grains for colour and flavour are using those grains for their non-diastatic contribution only (colour, flavour, a small amount of unfermented body-building sugar that leaches out during steeping) — they aren't relying on any enzyme activity from those grains, since there's no mash for enzymes to act in.
Mash efficiency: the concept that doesn't translate at all
All-grain brewers track mash efficiency — the percentage of a grain bill's theoretical maximum extract that actually ends up in the kettle, typically 65-78% depending on equipment and technique — because that efficiency figure genuinely varies system to system and batch to batch, and getting it wrong is one of the most common reasons an all-grain OG misses target. Extract brewing sidesteps this concern almost entirely: the extract itself already represents converted, concentrated sugar at a known, manufacturer-published extract potential, so hitting a target OG from extract is close to simple arithmetic (weight of extract, its published potential, and batch volume) rather than an estimate subject to your own system's mash and lauter efficiency. This is part of why extract recipes are often more forgiving for a beginner's first few batches — one major source of OG variance simply isn't in play.
Where the two approaches genuinely reconverge
Once wort is in the fermenter, extract and all-grain batches are running through completely identical downstream math — fermentation, attenuation, ABV, real vs. apparent gravity readings, priming, carbonation. See why is my final gravity stuck at 1.020 or how much priming sugar for 5 gallons — neither of those pages, or any of this site's fermentation-and-packaging content, needs to know or care how the wort was made. The split between the two brewing methods is real, but it's front-loaded entirely into the pre-fermentation stage of brew day; everything downstream of "wort in the fermenter" is one shared body of arithmetic regardless of which path got you there.
Partial mash: a genuine hybrid, not a compromise
Partial mash brewing — mashing a portion of the grain bill (often specialty and a modest amount of base malt) while relying on extract for the bulk of fermentable sugar — sits between the two approaches described above, and it inherits arithmetic from both sides rather than needing an entirely separate framework. The mashed portion still needs a real strike-water calculation, scaled to whatever smaller grain weight is actually being mashed rather than a full grain bill; the extract portion still contributes fermentable sugar without any mash-efficiency tracking of its own. A brewer doing a partial mash is, in the math sense, running two of the calculations described above simultaneously on two different fractions of the same batch, then combining the results into one OG reading — a genuinely useful stepping stone for a brewer building toward full all-grain, since it introduces the strike-water calculation and real mash technique at a smaller, more forgiving scale before committing to mashing an entire grain bill, with a smaller strike-water volume and correspondingly lower stakes if the temperature target is missed on a first attempt.
A boil-timing wrinkle worth knowing about, specific to extract
Extract brewers sometimes add a portion of their extract late in the boil rather than all of it upfront — a technique aimed at reducing wort colour darkening and improving hop utilization, since a lower boil gravity earlier in the boil means hops extract bitterness somewhat more efficiently than they would in a denser, higher-gravity boil from the start. This means an extract brewer's actual boil gravity, and therefore actual IBU, can differ meaningfully from what a recipe calculated assuming all extract went in at the start — see why do my IBUs differ between calculators for the broader point about boil gravity assumptions, which applies here in a form specific to extract technique. If you're using a late-extract-addition technique, enter your actual early-boil gravity (not your final OG) into the IBU calculator's boil gravity field for an accurate result, since that's the gravity your bittering hops were actually extracting into — using the final, post-addition OG instead will understate the true utilization and, therefore, the true IBU your bittering charge actually delivered.
A practical takeaway for a brewer switching between the two
If you're moving from extract to all-grain (the most common direction, as brewers gain equipment and confidence), the calculators and concepts you already know for ABV, IBU, SRM, attenuation, priming and carbonation all carry over completely unchanged — what you're adding is the strike-water calculation, mash efficiency tracking, and the process steps (mashing, lautering, sparging) that go with them. It's genuinely one new skill layered onto a foundation you already have, not a wholesale relearning of how the numbers work — the fermentation science, the packaging calculations, and the style targets you've already learned to hit stay exactly as relevant on your first all-grain batch as they were on your last extract one.