We introduce a linear measurement-based method for interference estimation in wireless mesh networks. The approach is based on measuring the channel occupancy and requires only O(n) measurements for a network with n nodes, thus making it suitable for large network deployments. We show, how we utilize the carrier sensing mechanism of commodity IEEE 802.11 hardware to measure the channel occupancy. Based on the measurements, we develop the Channel Occupancy Interference Model (COIM). The model is validated in the DES-Testbed, a wireless mesh network with 128 multi-radio network nodes at the Freie Universität Berlin. The validation shows, that the model is capable of predicting interfering nodes in the communication and also in the interference range. Additionally, our evaluation shows that the often used 2-hop interference model is too pessimistic for indoor network deployments, about 60% of the 2-hop neighbors are not interferers in our set-up.