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Screensize: 640 x 480 800 x 600 1024 x 768 1280 x 1024 UserName: Password: Anti-Spam Code: Format Mode: Basic  Help  Prompt  Format: Font Andale Mono Arial Arial Black Book Antiqua Century Gothic Comic Sans MS Courier New Georgia Impact Lucida Console Script MT Bold Stencil Tahoma Times New Roman Trebuchet MS Verdana   Size 1 2 3 4 5 6   Color Black Red Yellow Pink Green Orange Purple Blue Beige Brown Teal Navy Maroon LimeGreen Message: * HTML is OFF * Forum Code is ON Smilies Hello, Could you please look at the 'Parameters' section on this page? https://docs.originlab.com/origin-help/nlfit-theory/ Pay close attention to equations (8) and (9) on this page. Equation (9) is similar to your Excel calculation. Equation (8) defines how Origin's variance-covariance matrix is calculated. Origin also accounts for how sensitive the fitted curve is to parameter uncertainty at different x-values. In a given sample, usually there are more data points near the center of the dataset, so the fit is more reliable there and the CI band envelopes are narrower. Near the edges, small changes in the fitted parameters can shift the curve much more, so the CI band envelopes widen. You can see this in the image you shared. Please let me know if this answers your question. Thanks, Nadeesha   Check here to subscribe to this topic.