I Inherited a 100-Sheet Workbook: A Survival Guide

Week 1: Don't Touch Anything

Seriously. Your first instinct will be to start “cleaning up.” Resist it.

Before making any changes:

• Make a backup copy with today's date
• Document the current state — how many sheets, what seems to be the output
• Identify who uses this workbook and for what
• Find out when it runs (daily? monthly? quarterly?)

You need to understand the workbook before you can improve it. Changes made in ignorance will break things.

Map the Architecture

Large workbooks usually follow a pattern. Look for:

Input sheets: Where does data come from? Look for sheets with external connections, paste areas, or “Data” in the name.

Calculation sheets: Where are formulas doing heavy lifting? These often have names like “Calc”, “Working”, or “Engine.”

Output sheets: What do stakeholders actually look at? Usually formatted nicely, might have “Report”, “Dashboard”, or “Summary” in the name.

Reference sheets: Lookup tables, configuration values, mappings. Often hidden.

Follow the Formula Trail

Pick an important output cell (like a final total) and trace its precedents. Use Ctrl+[ to jump to cells it references, or use Formulas tab → Trace Precedents.

Work backwards through the chain. This reveals the actual data flow, which is often very different from what sheet names suggest.

Document what you find. Even rough notes like “Summary pulls from Calc_Q1 which pulls from RawData” will help.

Find the Orphan Sheets

Many large workbooks accumulate sheets that are no longer used. They were created for one-time analysis, or they're old versions kept “just in case.”

To identify orphans:

• Search for the sheet name in formulas (Ctrl+F, search in Formulas, look in Workbook)
• If no formulas reference the sheet, it might be orphaned
• Check if the sheet has any formulas pointing outward

Don't delete orphans immediately. Move them to the end and add “UNUSED_” prefix. If nothing breaks after a few cycles, then consider removal.

Create a Table of Contents

Add a new sheet at the beginning called “TOC” or “Index.” List every sheet with a brief description of its purpose.

This documentation doesn't exist? Create it as you learn. Future you (and your successor) will be grateful.

Include: Sheet name, purpose, where data comes from, where data goes to, last updated date if relevant.

Establish Naming Conventions

When you eventually do start cleaning up, use consistent prefixes:

• DATA_ — Raw input data
• CALC_ — Calculation/transformation sheets
• OUT_ — Reports and outputs
• REF_ — Reference/lookup tables
• OLD_ — Deprecated sheets pending deletion

Rename gradually, testing after each change. Sheet name changes break formulas that use the old name.

Official Resources

• Insert or delete a worksheet — basic sheet management
• Rename a worksheet — sheet naming guidance