Ecosystem nutrient budgets often report values for pools and fluxes without any indication of uncertainty, which makes it difficult to evaluate the significance of findings or make comparisons across systems. We present an example of a simple Monte-Carlo approach to estimating error in calculating the N content of vegetation at Hubbard Brook, using Excel spreadsheets. Participants should bring laptop computers and ecosystem calculations in need of uncertainty analysis (you can use ours if you don't have your own). You will need to know (or guess) the uncertainty in all the parameters used in the calculations. At the end of the workshop, some participants will have found the coefficient of variation in their ecosystem result. All participants will understand the principles of Monte-Carlo sampling and will have tools for testing and debugging spreadsheet calculations.