Although the determination of calcium ions (Ca2+) is of high importance to monitor water hardness, currently available devices for on-site analysis suffer from a lack of user-friendliness and sensitivity. This work demonstrates fully inkjet-printed and low-cost microfluidic paper-based analytical devices (μPADs) for the simple naked-eye colorimetric determination of calcium ions (Ca2+) in drinking and tap water samples. The quantification of Ca2+ relies on visual readout of the length of a colour-changed detection channel modified with ionophore-doped ion-selective optode nanospheres (nano-optodes), eliminating the requirement of a scanner or a camera. All fabrication steps for deposition of assay reagents have been performed by means of a simple desktop thermal inkjet printer, which is expected to contribute to highly batch-to-batch reproducible device preparation. The detectable Ca2+ concentrations between 0.05 mmol L-1 and 5 mmol L-1 cover the range recommended by the International Organization for Standardization (0.05-2.5 mmol L-1) and the World Health Organization (WHO) guideline for Ca2+ quantification in drinking water (less than 5 mmol L-1). The lowest concentration of Ca2+ detectable by the naked eye was found to be 0.05 mmol L-1, which is below the value achieved with previously reported paper-based devices. μPAD quantified Ca2+ concentrations in tap or drinking waters were within 15% error of the results obtained with a classical complexometric titration. Hence, distance-based μPADs relying on nano-optodes are sensitive and reproducible tools for equipment-free on-site assaying of Ca2+ in real samples.