An estimation method includes: an acquisition step (S101) of acquiring deflection stress information that indicates a relationship between a deflection and a tensile stress of a specimen (200) of a reinforced concrete structure; and a derivation step (S102) of deriving an estimation formula for estimating a depth of a crack generated in the reinforced concrete structure when a deflection of the reinforced concrete structure is no less than a first deflection of the specimen at a start of a generation of the crack, and is no greater than a second deflection of the specimen at an end of the generation of the crack.