The Brewers Association (US) offers the following style parameters:
"Robust porters are black in color and have a roast malt flavor but no roast barley flavor. These porters have a sharp bitterness of black malt without a highly burnt/charcoal flavor. Robust porters range from medium to full in body and have a malty sweetness. Hop bitterness is medium to high, with hop aroma and flavor ranging from negligible to medium. Fruity esters should be evident, balanced with roast malt and hop bitterness."
Original Gravity (ºPlato): 1.045-1.060 (11-15 ºPlato)
Apparent Extract/Final Gravity (ºPlato): 1.008-1.016 (2-4 ºPlato)
Alcohol by Weight (Volume): 4.0-5.2% (5.0-6.5%)
Bitterness (IBU): 25-40
"Brown porters are mid to dark brown (may have red tint) in color. No roast barley or strong burnt/black malt character should be perceived. Low to medium malt sweetness is acceptable along with medium hop bitterness. This is a light- to medium-bodied beer. Fruity esters are acceptable. Hop flavor and aroma may vary from being negligible to medium in character."
Original Gravity (ºPlato): 1.040-1.050 (10-12.5 ºPlato)
Apparent Extract/Final Gravity (ºPlato): 1.006-1.014 (1.5-3.5 ºPlato)
Alcohol by Weight (Volume): 3.5-4.7% (4.5-6.0%)
Bitterness (IBU): 20-30
Color SRM (EBC): 20-35 (40-70 EBC)
The BJCP offers the following style parameters:
12A. Brown Porter
Aroma: "Malt aroma with mild roastiness should be evident, and may have a chocolaty quality. May also show some non-roasted malt character in support (caramelly, grainy, bready, nutty, toffee-like and/or sweet). English hop aroma moderate to none. Fruity esters moderate to none. Diacetyl low to none."
Appearance: "Light brown to dark brown in color, often with ruby highlights when held up to light. Good clarity, although may approach being opaque. Moderate off-white to light tan head with good to fair retention."
Flavor: "Malt flavor includes a mild to moderate roastiness (frequently with a chocolate character) and often a significant caramel, nutty, and/or toffee character. May have other secondary flavors such as coffee, licorice, biscuits or toast in support."
Mouthfeel: "Medium-light to medium body. Moderately low to moderately high carbonation."
Overall Impression: "A fairly substantial English dark ale with restrained roasty characteristics."
History: "Originating in England, porter evolved from a blend of beers or gyles known as "Entire." A precursor to stout. Said to have been favored by porters and other physical laborers."
Ingredients: "English ingredients are most common. May contain several malts, including chocolate and/or other dark roasted malts and caramel-type malts. Historical versions would use a significant amount of brown malt. Usually does not contain large amounts of black patent malt or roasted barley. English hops are most common, but are usually subdued. London or Dublin-type water (moderate carbonate hardness) is traditional. English or Irish ale yeast, or occasionally lager yeast, is used. May contain a moderate amount of adjuncts (sugars, maize, molasses, treacle, etc.)."
OG: 1.040 – 1.052
IBUs: 18 – 35
FG: 1.008 – 1.014
SRM: 20 – 30
ABV: 4 – 5.4%
12B. Robust Porter
Aroma: "Roasty aroma (often with a lightly burnt, black malt character) should be noticeable and may be moderately strong. Optionally may also show some additional malt character in support (grainy, bready, toffee-like, caramelly, chocolate, coffee, rich, and/or sweet). Hop aroma low to high (US or UK varieties). Some American versions may be dry-hopped. Fruity esters are moderate to none. Diacetyl low to none."
Appearance: "Medium brown to very dark brown, often with ruby- or garnet-like highlights. Can approach black in color. Clarity may be difficult to discern in such a dark beer, but when not opaque will be clear (particularly when held up to the light). Full, tan-colored head with moderately good head retention."
Flavor: "Moderately strong malt flavor usually features a lightly burnt, black malt character (and sometimes chocolate and/or coffee flavors) with a bit of roasty dryness in the finish. Overall flavor may finish from dry to medium-sweet, depending on grist composition, hop bittering level, and attenuation."
Mouthfeel: "Medium to medium-full body. Moderately low to moderately high carbonation. Stronger versions may have a slight alcohol warmth. May have a slight astringency from roasted grains, although this character should not be strong."
Overall Impression: "A substantial, malty dark ale with a complex and flavorful roasty character."
History: "Stronger, hoppier and/or roastier version of porter designed as either a historical throwback or an American interpretation of the style. Traditional versions will have a more subtle hop character (often English), while modern versions may be considerably more aggressive. Both types are equally valid."
Ingredients: "May contain several malts, prominently dark roasted malts and grains, which often include black patent malt (chocolate malt and/or roasted barley may also be used in some versions). Hops are used for bittering, flavor and/or aroma, and are frequently UK or US varieties. Water with moderate to high carbonate hardness is typical. Ale yeast can either be clean US versions or characterful English varieties."
OG: 1.048 – 1.065
IBUs: 25 – 50+
FG: 1.012 – 1.016
SRM: 22 – 35+
ABV: 4.8 – 6%