The BJCP Style Guidelines state the desired number of International Bittering Units (IBU) that are needed to obtain a balance between hopped bitterness and malt sweetness in a hopped beer style. Beers that don't taste sweet or bitter are considered to be balanced, although individual beer drinkers may perceive this differently.
The formula takes into consideration the beer's original gravity, actual attenuation and bittering level but does not take into account phenol, ester or other complexities. The beer drinker should use the desired IBUs as a reference point and decide for themselves what they consider to be balanced.
Screwy's Bitterness Balance Calculator
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On the BJCP balance scale a traditional Bock style beer is considered to be sweet as opposed to a Stout style beer which is considered to be bitter. The addition of bittering hops are used to bring a sweeter malty beer back in balance while the addition of fermentable sugars such as malts are used to bring a bitter beer back into balance.