This study deals with determination of the effect of the ratio of tank diameter D to tank length L on the thermal performance of horizontal mantled hot water tanks. The D/L ratio has a great impact on the thermal performance of hot water tanks because the storage volume and heat transfer surface area of a tank change with the D/L ratio. In this study, the effect of different D/L ratios on thermal performance for a horizontal mantled hot water tank was investigated numerically by considering a constant storage volume and heat transfer surface area separately. The D/L was taken equal to 0.35, 0.50, and 0.65. The FLUENT 17.1 code was used in numerical analyses. The numerical model was validated by experimental data. The results were evaluated in terms of energy and exergy efficiencies. The results showed that a shorter tank length caused higher inlet water jet effects. So, lower D/L ratios yield higher thermal performance for a horizontal mantled hot water tank. The highest exergy efficiencies were seen at D/L = 0.35 for a constant heat transfer surface area and a constant storage volume. Mantle flow and main flow rates had no significant effects on energy and exergy efficiencies. However, they significantly changed the stored water temperature. An increasing mantle inlet temperature decreased the exergy efficiency, an increasing main inlet temperature increased the exergy efficiency. However, since the main objective of a hot water tank is to supply high temperature hot water, the mantle inlet temperature should be higher. Energy efficiency was nearly 98% and almost same for all cases. Exergy efficiency changed from 10% to 60% among different cases.