Four FMTs (Fineness and Maturity Tester) were upgraded at SRRC and calibrated. The calibration compares data gathered from a dozen cottons, averaged across several independent reference methods, to the FMT data. Two of the reference methods are the British Standard Methods (BSM) C used in the original calibration C and image analysis. The FMTs were calibrated at the standard airflows of 4 and 1 L/min with the sample cell volume varied to produce the desired pressure drops through the cotton. A variety of relationships exist between the FMT and reference data sets, from polynomial to linear with designated slopes and intercepts to a constant offset. This paper will demonstrate the relationships and show that the constant offset is the easiest and most direct route to accurate FMT calibration.