As global oil and gas exploration advances into deep reservoirs, reaming-while-drilling (RWD) tools (hereinafter referred to as the reamer) are widely used to enlarge wellbores for unconventional well structures, prevent stuck pipe caused by formation shrinkage, and improve cementing quality. The flow distribution and pressure loss of reamer directly determine their operational performance, and thus, affect the success rate of reaming operations and construction quality. However, limited by intellectual property protection of core technologies and commercial barriers, no general hydraulic calculation method for reamers is publicly available. This paper presents theoretical calculations of flow distribution and pressure loss for reamers and verifies their accuracy against numerical simulations and lab tests. The results show that at a field flow rate of 40 L/s, the relative error between theoretical and experimental pressure loss is only 2.93%. Flow distribution between the bit and reamer depends solely on equivalent nozzle diameter, which dominates bottom hole assembly (BHA) pressure loss and directly governs blades pushing force. Extra flow outlets during activation cause negligible pressure loss reduction, yet pressure change at pin failure remains the key status-switching criterion. The results of this paper can serve as a theoretical reference for the research and development and field deployment of reamer.