We propose a microelectromechanical systems-based tunable asymmetric Fabry-Perot cavity for the high-precision weighing of macro samples. The device is based on an in-plane design and is structured in a silicon-on-insulator substrate. The cavity length of the optical resonator is tuned under the action of an external force. The force can be determined from the resulting spectral shift of the optical resonance. Measurements can be done under static conditions and are immune to electromagnetic interferences. Various designs have been simulated, fabricated and characterized. We report the experimental performances of four devices that have been tested under loads up to 98 mN (10 g). Sensitivities ranging from 0.51 to 67.69 nm/mN and absolute resolution ranging from 0.15 to 19.61 μN are reported. The maximum relative resolution of the sensor is below 100 ppm.