The filter paper technique consists of obtaining the equilibrium water content of a filter paper that is either in direct contact with a soil sample or inside an airtight container together with the sample but not in direct contact with it. After the final water content of the filter paper is determined, the suction in the soil is estimated from a previously established calibration curve relating the filter paper water content and suction. The ASTM D5298-10 calibration curve is routinely used for indirect suction estimation from Whatman Grade 42 filter paper water content measurements. This note identifies limitations in the calibration curve in ASTM D5298-10 that lead to inaccuracies in the estimation of suction values, particularly for very low filter paper water contents. The paper proposes new equations not subject to these limitations for the calibration curve for Whatman Grade 42 filter paper using the same data used to construct the calibration curve in ASTM D5298-10.