This paper proposes a new algorithm for real-time leak detection in a water distribution network using real-time pressure measurements only. In particular, we newly introduce a Head Loss Ratio (HLR), a ratio of two differences of pressure measurements, as an indicator for burst leaks in a water distribution network. A detection algorithm based on HLRs is then developed. A theoretical justification is provided for a limited class of water distribution networks for the proposed algorithm. Simulation results are also given to demonstrate that the proposed algorithm can be successfully applied to a wider class of water distribution networks.