A static gas flowmeter for sensing a flow of gas includes an outer housing, a flow tube coupled to an outlet or an inlet, ≥1 sensor positioned for sensing gas flowing through the flow tube, and a controller including a memory including a flow calibration and flow measurement algorithm coupled to a sensor receiving a sensing signal. An integrated gas valve is connected to the inlet or outlet and/or there is an additional external valve positioned outside the housing coupled to the inlet to provide the gas flowmeter at least two state operation. Using the valve(s), the flow calibration algorithm triggers a zero-flow condition, then obtains a measured gas flow, and from the measured gas flow a flow offset is determined. If the flow offset is outside a predetermined limit, in-line calibrating is performed by adding the flow offset reflected as a correction factor to the flow measurement algorithm.