The Intergovernmental Panel on Climate Change (IPCC) 1996 Revised Guidelines for National Greenhouse Gas Inventories proposes two methodologies for estimating methane emissions from solid waste disposal sites (SWDS): (1) the mass-balance method; and (2) the first-order kinetics method. This first method is the default methodology and is the easiest method to apply for estimating country-specific methane emissions and requires the least amount of data. Alternatively, the second method is more complex and requires more information than the first method. As many countries do not have detailed information on solid waste disposal practices, it is anticipated that most countries use and will continue to use the mass-balance approach for estimating time-series of methane emissions. The mass-balance approach uses an assumption regarding annual waste disposal that can overestimate methane emissions. In this paper a correction factor is presented for adjusting the mass-balance approach to account for non-steady state conditions in annual waste disposal. Use of such a correction factor results in estimates that approach those generated by more complex methods. In summary, the analysis performed indicates that the modified approach typically results in more than a 20% reduction in the methane emissions inventory and methane emissions that are within the range of estimates based on the more complex first-order kinetics approach.