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Financial companies, engineering firms and even scientists create increasingly larger spreadsheets and spreadsheet programs. The creators of large spreadsheets make errors and must track them down. One common class of errors concerns unit errors, because spreadsheets often employ formulas with physical or monetary units. In this paper, we describe XeLda, our tool for unit checking Excel spreadsheets. The tool highlights cells if their formulas process values with incorrect units and if derived units clash with unit annotations. In addition, it draws arrows to the sources of the formulas for debugging. The tool is sensitive to many of the intricacies of Excel spreadsheets including tables, matrices, and even circular references. Using XeLda, we have detected errors in some published scientific spreadsheets.