We propose a numerical method to estimate the local wave height and period of a tsunami from the distributions of boulders. The method was applied to boulders (<23 t) at Pakarang Cape, Thailand that were displaced from the reef slope onto the tidal bench by the 2004 Indian Ocean tsunami. They were deposited characteristically below the high-tide line, irrespective of size and no boulders were deposited on land. These features were used as the constraint of the calculation. We conducted over 10,000 cross-sectional calculations to satisfy above mentioned constraints, showing that the wave height and period at the shoreline were calculated to be 4–6 m and 18–37 min, respectively, which well concur with observed values. The input parameters for this calculation are the sizes and initial positions of boulders with seaward/landward limits, which are obtainable through the geological survey for historical and pre-historical tsunamis. This method is useful to estimate the local wave height and period of historical and pre-historical tsunamis from boulders reported throughout the world.