Use formula: [µM] = [µg/mL] / MW(kDa)
Example: If you receive 200 µg of a 40 kDa protein in 100 µl, then the mass concentration is 2,000 µg/ml (2mg/ml). Therefore, your molar concentration is 2,000/40 = 50 µM (0.05 mM). Make sure to observe all the units of measurement.