How do you ensure accuracy in a spreadsheet that others depend on?

Spreadsheet errors are common and expensive, so a structured answer stands out.

• Separate inputs, calculations, and outputs. Ideally on different sheets, with a consistent colour convention — for example blue for hard-coded inputs, black for formulas. Anyone can then see immediately where a number comes from.
• Never hard-code a number inside a formula. A tax rate typed into forty formulas is impossible to update reliably. Put it in one labelled cell and reference it.
• Build in checks. A row that confirms a balance sheet balances, that percentages sum to 100, or that a total matches its source — flagged conditionally so it is impossible to miss.
• Use data validation on input cells to constrain what can be entered, and protect formula cells so they cannot be overwritten.
• Use named ranges and tables so formulas read meaningfully and expand automatically with new rows.
• Have someone else review it, and use Trace Precedents and Dependents when checking unfamiliar work.

Note: Version control is worth raising as a genuine weakness of Excel. Saying you kept the file in SharePoint or OneDrive with version history, and a dated changelog tab, shows you have felt that pain.