How to Understand an Inherited Excel Model

Someone leaves, and their model lands on your desk with no documentation and no handover. You now own numbers you did not build and cannot yet explain. The way out is not to read every cell but to reverse-engineer the model methodically, from structure down to the assumptions that drive it.

Map the Sheets Before Touching Formulas

Resist the urge to dive into cells. First, walk the tabs and build a mental map: which sheet holds inputs, which does the heavy calculation, which presents results.

Note the flow. Most models move from assumptions to a calculation engine to outputs, and recognizing that shape orients everything that follows.

1. List every sheet and a one-line guess at its purpose
2. Identify where the headline outputs are presented
3. Find the sheet that holds the assumptions
4. Note any sheets that look like scratch work or old versions

Trace the Headline Number Backwards

Pick the most important output, the one the model exists to produce, and walk back through its precedents. This is the fastest way to learn what actually drives the model.

Excel's Trace Precedents draws arrows, but on an unfamiliar dense sheet they overlap into noise. Stepping back one layer at a time, following each reference deliberately, builds real understanding.

1. Select the model's key output cell
2. Trace its direct precedents and note which sheets they live on
3. Step into each precedent and trace its inputs
4. Keep going until you reach the raw assumptions

Find the Assumptions, Including the Hidden Ones

Once you reach the inputs, separate the real assumptions from the buried ones. Some will sit in labeled cells. Others will be hardcoded inside formulas where the previous owner never pulled them out.

Cataloguing every hardcode is essential. Each one is an undocumented assumption you now own, and you cannot flex what you cannot find.

• Labeled input cells: the documented assumptions
• Constants typed inside formulas: the hidden ones
• External links: assumptions that live in another file

Find What Reaches Outside the Workbook

Inherited models often pull from other files: a master assumptions workbook, last quarter's actuals, a shared rates file. If those links break or go stale, your model quietly goes wrong.

Identify every cell that references another sheet or workbook so you know exactly what the model depends on beyond its own walls.

Document As You Learn

As you trace and discover, write it down. Even a short notes sheet listing the key drivers, the hidden hardcodes, and the external dependencies turns a black box into something you can defend.

The next person, possibly future you, will thank you. An inherited model becomes truly yours only once you can explain it without re-deriving it every time.