Developer shares input validation lessons from building 77 browser calculators

A developer building 77 free browser-based calculators for a platform called Efficienco found that handling user inputs correctly was far harder than writing the underlying formulas. Key lessons included distinguishing between empty fields and invalid entries, rather than treating all falsy values as missing. Domain-specific range checks, unit conversion to a single base unit before calculation, and preserving full numeric precision until the final display step were identified as critical patterns. The developer also noted that hidden assumptions — such as waste factors or efficiency rates — should be surfaced and made editable to avoid giving users a false sense of precision. The article focuses entirely on pre- and post-formula logic, offering reusable validation patterns applicable across varied calculator types.