How to scale a beer recipe to your batch size
Divide your batch by the recipe's and multiply everything by the result — that's every scaler on the first page, and it's right for hops, salts, sugar and extract. It's wrong for the grain by exactly the difference between the recipe's efficiency and yours, and it says nothing about the kettle. Here is a 5-gallon, 70 % recipe brought to the default profile (5.5 gal at 72 %), the naive way and this page's way:
| Line | × 1.10 only | This page | Why |
|---|---|---|---|
| Pale malt, 10 lb | 11 lb | 10.69 lb | volume × efficiency |
| Crystal, 1 lb | 1.1 lb | 1.07 lb | mashed — same correction |
| DME, 1 lb (if it were extract) | 1.1 lb | 1.1 lb | dissolves at 100 % — volume only |
| Hops, 1 oz at 60 | 1.1 lb | 1.1 lb | volume only |
Factor 1.100 = 5.5 ÷ 5; efficiency correction 70 ÷ 72 = 0.972. Computed by this page's engine.
A recipe written at 70 % and brewed on a 72 % system wants 2.8% less grain, or it comes out 2.8% stronger than the author meant. Two of the ranking pages — Homebrew Answers and the Mad Fermentationist — give this correction as hand math; the calculators skip it. This one reads your efficiency from your profile, applies it to the mashed lines, and leaves the extract alone, because extract dissolves at 100 % no matter whose kettle it's in.
Why a 1-gallon batch isn't a fifth of a 5-gallon one
Your kettle boils off liters per hour, not a percentage of what's in it. Put a fifth of the wort in the same kettle for the same hour and it loses the same 1.25 gallons — which is now most of the batch:
| Batch into the fermenter | Pre-boil, hot | Boils off in 60 min | Share of the wort |
|---|---|---|---|
| 1 gal | 2.81 gal | 1.25 gal | 44% |
| 2.5 gal | 4.37 gal | 1.25 gal | 29% |
| 5 gal | 6.98 gal | 1.25 gal | 18% |
| 5.5 gal | 7.5 gal | 1.25 gal | 17% |
| 10 gal | 12.19 gal | 1.25 gal | 10% |
Default profile's kettle: 1.25 gal/hr, 0.5 gal left behind, 4 % shrinkage. Computed by this page's engine.
So the scaled recipe's pre-boil volume can't be the recipe's times the factor. This page takes it from the water ledger for the target batch in your kettle — and the kettle has a second, bigger effect that no scaler mentions. The half gallon of hopped wort your kettle keeps is the same half gallon whether the batch is 5.5 gallons or 1; in the small batch it's a third of the boil. Scale the hops by the batch factor and you've put a fifth of the hops into a quarter of the boiling wort: the 1-gallon version comes out 27% less bitter, by Tinseth at the real boil volume. The IBU line above is computed that way, not assumed — which is honest, and a larger effect at homebrew sizes than the 10–20 % utilization guess CraftJacks offers. Handy Chefdom's scaler asks you to type the boil-off rate in; yours is already in the profile, measured on the Brew Day log if you've saved one.
An ounce of hops scaled to a gallon is 5.7 g. On the ±1 g kitchen scale most of us own that's an 18 % error on the bittering charge before the recipe's own rounding. The calculator flags every hop line under 7 g (a quarter ounce) and every grain line under 50 g, and suggests the honest fix — a 0.1 g scale, or a lower-alpha hop so the weight comes up. Yeast has the same problem the other way: a 1-gallon ale at 1.050 needs 35 billion cells, 0.35 of a liquid pack; pitch the whole pack (harmless — an over-pitch) or 4 g of dry yeast, which is the tidy answer. The 5-gallon version needs 176 billion and 2 packs.
Scaling up: what changes, and what the forums argue about
Going from 5 gallons to 10 in the same kettle is the easy direction — the boil-off share falls and the gravity lands closer to plan — until the mash tun runs out of room, which is why the calculator prints the mash volume against the profile's tun. What doesn't change at homebrew scale is hop utilization: BYO's Mr Wizard scales hops straight by volume for 20 → 60 L, and the BeerSmith figures of "125 %, 150 %" utilization are for multi-barrel commercial kettles, where a deep, vigorous boil extracts more. Below about 20 gallons the effect is inside the error of the alpha-acid number on the packet, so this page doesn't invent a factor for it. If your hops' alpha acid differs from the recipe's, that's a real correction and there's a field for it — the hop-substitution rule, applied after the volume scaling.
Brewhouse efficiency: the share of the grain's potential sugar that ends up in the fermenter. A recipe's stated efficiency is the author's system; yours is in your profile, and the Brew Day log measures it every batch. If you don't know the recipe's, most are written at 70–75 %; use 72 and the correction is small. If you don't know yours, brew this recipe as scaled, log the OG, and the next scaling will be right.
The recipe scaled, then handed to Brew Day
The scaled bill and hops go to Brew Day with one click, where they become the strike water, sparge, pre-boil target and the printable sheet — the numbers the recipe author never had, because they didn't know your kettle. For the worked example above, that means 7.5 gal in the kettle at a pre-boil of about 1.047, an OG of 1.057 and 29 IBU from the scaled ounce of 10 % hops. The honest summary: scaling is arithmetic, and the arithmetic is easy; what a scaler can add is knowing which lines the arithmetic doesn't apply to, and the kettle the beer is going into. That's what the profile is for.