Structural health monitoring techniques based on vibration measurements have been receiving large attention in the last decades, including techniques for estimating bolted joint tightness and detecting loosened bolts. Due to the exposure of bolted joints to external forces, the bolts may loosen and therefore affect healthy functioning of the bolted structure. In this work a technique is proposed to estimate the level of bolt tightness and to quantify the tension based on the measured natural frequencies of the bolt, in particular the first transverse natural frequency. An experiment is performed on two structure specimens each clamped with a bolt of different length. The bolts bending vibrations are excited by impacting the bolts head along the transverse direction. The excited transverse natural frequencies are then recorded as the bolts are gradually tightened. The measured frequencies trends are explained by modeling the bolt as a pre-stressed one dimensional beam with elastic supports at both ends. The experimental results are reproduced using an analytical function that expresses the boundaries stiffness in terms of the bolt tension. The sensitivity of the measured bolt first transverse natural frequency demonstrates the potential of this frequency-based technique in estimating bolt tightness.