This is frequently seen expressed as BU/GU. That is Bitterness Units/Gravity Units. This is used as a comparator and for checking balance (or lack thereof) in a recipe. The Gravity Units are the decimal portion of the original gravity. A 1.060 OG wort would have a GU of 60. If this recipe called for 30 IBUs, then the ratio would be 30/60 or 0.5
It is with this method that a brewer can estimate whether bitterness or malt will dominate the flavor profile.