Nitrous oxides (N2O) emissions contribute to climate change and stratospheric ozone depletion. Wastewater treatment is a significant source of N2O remissions but likely underestimated, as recent, long-term monitoring campaigns demonstrated. However, the available data are insufficient to representatively estimate countrywide emission given the duration of most monitoring campaigns. Here, we show that the emission estimates can be significantly improved using an advanced approach based on multiple continuous, long-term monitoring campaigns. In 14 full-scale wastewater treatment plants (WWTPs), we found a strong variability for the yearly emission factors (0.1 to 8% of the incoming nitrogen load), which exhibited a high correlation with effluent nitrite compared to the inflow nitrogen load and also good correlation with nitrogen removal efficiencies. But, countrywide data on nitrite effluent concentrations is very limited and not available for emission estimation in many countries. We propose to calculate a countrywide emission factor based the weighted emission factors of three nutrient removal WWTP categories (carbon removal: 0.1-8%, nitrification only: 1.8%, and full nitrogen removal: 0.9%). However, emission factors of carbon removal WWTPs are still highly uncertain given the expected performance variability. The newly developed approach allows representative, country-specific estimations of the N2O emissions from WWTP. Applied to the case of Switzerland, the estimations result in an average EF of 0.9-3.6% and total emissions of 410 to 1690 tN2O/year, which corresponds to 0.3-1.4% of the total Emission in Switzerland. Our results demonstrate that a better data availability and an improved understanding of long-term monitoring campaigns is crucial to improve current emissions estimations. Year-round denitrification, limiting nitrite accumulation, and a stringent control of sludge age in carbon removing plants are key measures to mitigate N2O emissions from wastewater treatment.