A scanning force microscope (SFM) has been used to measure the force interaction between a pyramidal silicon nitride tip and a dextran hydrogel immersed in PBS solutions with various concentrations. The dextran gel was attached to a gold surface via alkane chains. Samples with only alkane chains attached to the gold surface served as a reference and gave force-distance curves consistent with a double-layer interaction. The obtained force-distance curves depend on the buffer concentration and can be used to estimate the gel thickness. It was found that the gel was around 190 nm thick for buffer concentrations lower than physiological and about 140 nm for physiologic buffer. A tentative explanation is given to this observation.