The Bruun rule is a formula for estimating the magnitude of the retreat of the shoreline of a sandy shore in response to changes in sea level. Originally published in 1962 by Per Bruun,[1] the Bruun rule was the first to give a relationship between sea level rise and shoreline recession.[2] The rule is a simple, two dimensional mass conversion, and remains in common use to estimate shoreline recession in response to sea level rise, despite criticism and modification, and the availability of more complex alternate models.[3]