Additive manufacturing (AM) enables the fabrication of complex internal cooling channels for high-temperatureaerospace applications, but the rough as-built surfaces strongly influence flow. Understanding and predicting theseeffects is critical for accurate thermal–hydraulic design of AM-based cooling components. This study investigates therole of surface roughness on pressure drop and friction factor in an AM rectangular channel fabricated from Haynes230, a nickel-based superalloy. The cooling channel, with a hydraulic diameter of 1.995 mm, was tested in a closed-loop nitrogen flow facility at different mass flow rates, corresponding to Reynolds numbers in the range of 104 - 105.The test facility was developed under the MERiT+ project funded by Swedish National Space Agency at KTH Royal Institute of Technology, with industry partners from Siemens Energy and GKN Aerospace. Pressure drops wererecorded using multiple taps located along axial direction, and the Darcy friction factor was determined from localpressure drops. Complementary optical measurements of cross-sections at different streamwise locations wereperformed to assess variations in hydraulic diameter, while white light interferometry was employed to quantify surface roughness parameters, including Sa, Sq ,Ssk, and Sku. Numerical simulations were carried out in ANSYS Fluent usingthe pressure-based steady solver. Simulations were conducted with equivalent sand-grain roughness height, ks, derivedfrom characteristic parameters. Comparison between experimental and numerical results demonstrated consistenttrends in pressure drop and validated the approach for implementing hydraulic roughness in CFD. The data was furtheranalyzed to establish a predictive correlation between measured roughness parameters and equivalent sand-grainroughness. Several log-linear model forms were evaluated, and fitting was performed by minimizing errors in predictedfriction factors using the Haaland equation. The final correlation expresses (ks/Dh) as a function of normalized RMS roughness, and skewness, providing a compact, physically interpretable model. This work demonstrates a combinedexperimental, metrological, and numerical methodology for quantifying and predicting friction factor in AM channels.The proposed correlation, calibrated for Haynes 230 LPBF channels in the turbulent regime, bridges advanced surface characterization with classical fluid mechanics and offers a foundation for more reliable hydraulic design of AM-based cooling systems.