We Tried Running a $100M Forecast in Excel—Here’s What Broke First

Running your entire FP&A process in Excel is like flying a commercial airline with Google Maps.

It’ll get you off the ground. But you’ll crash the minute conditions change.

Let’s get something straight:
This is not an anti-Excel piece. This is a pro-sanity one.

Excel is the most powerful modeling tool in finance. No question.

But somewhere along the way, we stopped using Excel to build thinking and started using it to run everything.

Forecasting. Budgeting. Hiring plans. Board decks. Strategic scenarios. All of it living inside one master file called:

Budget_FINAL_Q3_v22_ACTUALLY_FINAL_this_time.xlsx

It’s duct tape disguised as infrastructure.

And the entire finance org is one broken VLOOKUP away from disaster.

Excel isn’t the problem.

Our overreliance is.

Excel is fast. Flexible. Familiar. That’s why we love it.

You can build a headcount ramp in an hour. Pressure test sales comp. Model a five-year P&L in a weekend.

It’s our creative sandbox.

But when you scale that sandbox into a system, things get ugly.

You end up with:

Dozens of linked tabs
Inputs from 14 people
Versions no one can trace
A model so fragile even clicking the wrong tab feels dangerous

Then one day the file crashes before a board meeting, and no one knows if you’re at 9 months of runway or 6.

That’s not modeling. That’s operational roulette.

I worked with a company doing $100M ARR.

They were running all of FP&A off a single Excel file.

Forecasts. Hiring. Revenue pacing. Marketing spend. Product costs.

Everything fed off this one workbook. It was a masterpiece—until someone added a row to the compensation tab without updating the formulas downstream.

The payroll forecast was off by $800K.

No one caught it for three weeks.

It broke trust. Delayed decisions. And forced a re-forecast during the quarter.

Not because Excel failed.

Because the team scaled the process without changing the tool.

Here’s what Excel is brilliant for:

Modeling edge cases
Quick what-ifs
Scenario pivots
Dashboards for execs
Bridge analysis
Rapid iteration

But Excel is not:

A source of truth
A version-controlled system
A collaborative workflow
A secure approval platform
A place to store decisions

If five teams are updating tabs inside one file, you don’t have a plan—you have a spreadsheet with a nervous system disorder.

Want to know where most FP&A teams go wrong with Excel?

Here’s the hit list:

Everyone touches the same workbook.
No audit trail. No change history. No visibility on what’s live.
Manual assumptions fly in from Slack and email.
Somebody updated CAC because a director texted them to. That’s not planning. That’s finance by Post-it note.
No one documents the logic.
Nested IFs buried in column AY. One tab called “DON’T DELETE.” Every month, a new analyst gets thrown in and told to “figure it out.”
Files get renamed. Endlessly.
Forecast_Q2_v11,
Forecast_Q2_v11_FINAL,
Forecast_Q2_v11_REALLY_FINAL.
The result? Five people show up to the exec meeting with different numbers.
Inputs get overwritten. Accidentally or otherwise.
You build a model assuming 10 hires. Someone else changes it to 8, but doesn’t flag it. Welcome to 2 months of explaining headcount variances.

None of this is Excel’s fault.

It’s the result of trying to make Excel do a job it wasn’t designed for.

This doesn’t mean you stop using Excel.

It means you stop pretending Excel is your FP&A system.

Here’s how you use it right.

→ Model in Excel. Store data somewhere else.
Pull your inputs from a clean source—Google Sheets, SQL, planning software. Let Excel do the math, not the memory.

→ Use Excel for iteration, not consolidation.
If your close requires 12 workbooks stitched together by hand, your real bottleneck isn’t talent. It’s structure.

→ Protect your logic like code.
Lock cells. Add version notes. Use data validation. If engineers can commit code with comments, so can you.

→ Create input forms, not tab hell.
Give stakeholders a clean input sheet. Or better: automate intake. Don’t make them guess where to type.

→ Centralize assumptions.
Put them on one tab. Or even better, in a shared system. That way the 8.5% churn you agreed to doesn’t turn into 6.2% just because someone fat-fingered a cell.

→ Track versions like product.
Treat every major model change like a release. New tab. Date stamp. Notes. It’s not overkill—it’s survivability.

Let’s talk security.

Excel files get emailed. Shared. Uploaded to random drives. Passed around like candy.

Half the time, they include comp, strategy, hiring, and revenue targets.

Zero encryption. Zero access control. No logging.

That’s not just sloppy. It’s dangerous.

Want to keep using Excel?

Treat it like sensitive IP.

Because it is.

So what’s the answer?

It’s not ditching Excel.

It’s designing your process so Excel isn’t carrying the weight of your entire finance function.

Use Excel like a scalpel. Not like duct tape.

Let it power insights.

But stop asking it to be your database, your workflow engine, your system of record, your audit trail, and your presentation layer—all at once.

Excel is fast.

But fast breaks when it becomes the foundation.

When you scale FP&A without scaling your process, here’s what happens:

Your best people spend half their time on version control.
Your execs stop trusting the numbers.
You miss forecast pivots because the model can’t flex in time.
You burn cycles firefighting instead of driving insight.
You teach your team that maintenance is the job.

That’s how finance loses strategic leverage.

Not because they don’t know the numbers.

Because the numbers live in chaos.

So here’s the fix:

→ Use Excel for what it’s best at—agile modeling.
→ Build a simple system around it—clean inputs, clear rules.
→ Scale your process before your file size.
→ Invest in tools that support Excel, not replace it.
→ Make structure your superpower. Not your afterthought.

You don’t need to buy a $500K planning suite tomorrow.

You need to stop pretending Excel is bulletproof.

Because one broken formula buried in a legacy tab can derail your whole quarter.

And when that happens?

Nobody blames the model.

They blame the team.

Here’s the bottom line:

You can keep using Excel.

You should keep using Excel.

But if you want to run a fast, scalable, modern FP&A function, you can’t run it all in Excel.

Treat Excel like what it is:
A brilliant tool.
Not a system.
Not a strategy.
Not a planning platform.

Love Excel.
But build the muscle around it.