Disclosed are various embodiments of an auto-injector device that automatically and dynamically adjusts the depth that a needle will penetrate into a target area of the body. In one embodiment, the auto-injector device includes the needle being moveably disposed within a needle housing. The auto-injector device further includes a needle depth estimator having a needle depth limiter that extends into an inner portion of the needle housing and is designed to engage with a needle stop of the needle and restrict downward movement of the needle. The needle depth estimator can automatically adjust a position of the needle depth limiter within the needle housing according to movement of a compression mechanism surrounding a lower portion of the needle housing.
