<p>In industry, water hardness causes scaling in industrial equipment like boilers and cooling towers. This also results in the release of heavy metals into the water. Therefore, water hardness should be monitored continuously to avoid costly breakdowns. This work reports a titrimetric colorimetric based sensor using nanocellulose (NC) for determination of water hardness. Nanocellulose (NC) was synthesized with acidic hydrolysis method and it was employed for preparation of the titrimetric sensor. The synthesized NC was characterized by scanning electron microscopy (SEM) and Fourier transform-infrared spectroscopy (FT-IR). In the prepared titrimetric sensor, on the glass slides, spots of the mixtures of NC and eriochrome black T (ECBT) with increasing amounts of ethylene diamine tetra-acetic acid (EDTA) were added and dried at room temperature. After addition of water samples to the spots, the end point is where the color of the glass slide changes to blue. The proposed titrimetric sensor can be utilized for rapid and on-site determination of the water hardness. It was demonstrated that the results of the estimation of water hardness by the titrimetric sensor are accurate. The analysis has no need to any solution and can be performed on site. The method has no need to any equipment and analytical tools.</p>