The concept
A spreadsheet model is a set of linked calculations that turns inputs (assumptions you can change) into outputs (numbers you make decisions on). A list of last month’s expenses is data, not a model. A sheet where you change the price and the profit updates automatically is a model. The difference is that a model answers “what if” questions.
Why it matters
Operators use models to test decisions before committing money: hiring a second employee, raising prices, taking on a lease, or launching a product. A well-built model lets you change one assumption and see the knock-on effect everywhere. A badly built one hides hard-coded numbers inside formulas, so nobody trusts the result and nobody can update it.
The three-layer structure
Every reliable model separates work into three layers, usually on three sheets or three clearly labeled blocks:
- Inputs – every assumption lives here, once. Price, unit cost, growth rate, headcount, tax rate. Nothing is calculated on this sheet.
- Calculations – formulas that reference the inputs. No typed numbers anywhere in this layer.
- Outputs – summary tables and charts that pull from the calculations. This is what you show a partner, lender or your own team.
Worked example
Suppose you sell a $40 product and want monthly profit. The weak version types =40*120-2800 into one cell. It works today and is useless tomorrow. The model version puts Price (40), Units (120) and Fixed Costs (2800) in three labeled input cells, then calculates Revenue = Price × Units, and Profit = Revenue − Fixed Costs. Change Price to 45 and Profit updates instantly. Add a second month and you copy the formulas instead of retyping.
| Input | Value |
| Price | 40 |
| Units | 120 |
| Fixed Costs | 2,800 |
| Output | Formula |
| Revenue | =Price*Units → 4,800 |
| Profit | =Revenue−Fixed → 2,000 |
Common mistakes
- Typing a number inside a formula (“hard-coding”) so later you cannot find where 0.08 came from.
- Mixing inputs, calculations and outputs on one sheet in no particular order.
- Building for one period and then copying the whole block for each new month, which breaks the links.
- Skipping labels. Unlabeled cells are unreadable in a week, even to you.
Action Step
Open a blank workbook and create three sheets named Inputs, Calc and Output. On Inputs, enter five assumptions for a product or service you actually run: price, units per month, unit cost, fixed monthly costs and a monthly growth rate. Label every cell. Do not calculate anything yet. You will build on this file through the rest of the course.