How to Use an Excel Formula with an Unknown Variable?

Have you ever needed to create an Excel formula that includes an unknown variable? Maybe you’re building a spreadsheet model where certain inputs will change, but you want the formulas to flexibly handle any value. Or perhaps you need to solve an equation in Excel where one part remains variable. Luckily, it’s possible to work with unknown variables in Excel formulas. In this article, we’ll show you exactly how to write Excel formulas with an unknown variable.

Understanding Variables in Excel

Before we dive into using an unknown variable in an Excel formula, let’s clarify what a variable is in the context of spreadsheets:

• A variable represents a changeable value in a formula
• Variables are symbolized by a single letter (e.g. x, y, z)
• The variable stands in for any possible value that might be inserted there
• Using a variable allows your formula to flexibly accept different inputs and calculate accordingly

So in Excel, an unknown variable in a formula means that part of the formula will change depending on what value is entered there. The formula will still calculate, but the result depends on the specific value substituted for the variable.

Syntax for an Excel Formula with an Unknown Variable

To write an Excel formula with an unknown variable, follow this general syntax:

= known parts of formula & variable

For example, let’s say you want to write a formula that multiplies some input value by 5. The formula with an unknown variable would be:

= 5 * x

Here’s how it breaks down:

• = starts the formula
• 5 * is the known part (multiplying by 5)
• x is the variable (representing the unknown input value)

To use this formula, insert it in a cell and substitute a real value for x in another cell that you reference. For instance, if you put this formula in cell B2, you could enter a value for x in cell A2. Then in B2 refer to the variable as A2 instead of x:

= 5 * A2

Now the formula will calculate based on whatever value you enter in A2. If A2 contains 4, the formula output will be 20. If you change A2 to 10, the formula result will automatically update to 50. The variable allows the formula to work with any input.

How to Calculate a Formula with Multiple Unknown Variables

Excel formulas can include more than one variable. This allows you to write flexible equations where multiple inputs remain unknown.

The syntax for multiple unknown variables works just like a single variable:

= known parts & variable1 & known parts & variable2 & known parts…

Here’s an example formula with two variables:

= x * 5 + y * 2

In this case:

• x and y are the two unknown variables
• The known parts are multiplying x by 5 and y by 2, then adding the results

To use this formula in Excel, replace x and y with cell references where you’ll enter the two input values.

You can include as many variables as you need in a formula. The key is clearly denoting each one with a different letter and referencing cells where the actual values will be entered.

Tips for Working with Unknown Variables in Excel

Here are a few tips to keep in mind as you work with unknown variables in Excel formulas:

• Use clear variable names: While you can use any letter to represent a variable, it’s often helpful to choose letters that clearly relate to what the variable represents. For instance, use q for quantity or p for price. This makes complex formulas easier to decipher.
• Keep track of your variables: When using multiple variables in a formula, it can be easy to lose track of what each one represents. Consider adding comments to your formulas to note what each variable corresponds to.
• Double-check cell references: Since variables rely on referencing other cells for their actual values, always double-check that your formulas are pointing to the correct inputs. A single mistyped reference can throw off the whole formula.
• Test with different input values: To ensure your variable formulas are working correctly, test them out with a variety of input values. Try edge cases and verify the results make sense.

Example of an Excel Formula Using an Unknown Variable

Let’s walk through a concrete example of using an unknown variable in Excel. We’ll create a simple formula to calculate the revenue generated from selling a certain quantity of an item at a set price.

The revenue formula is:
Revenue = Quantity x Price

In Excel, we want to write this formula using an unknown variable for both quantity and price, so the formula can automatically adjust the revenue if either input changes.

Here are the steps:

1. Set up your spreadsheet with a cell for quantity and a cell for price. We’ll use A2 for quantity and B2 for price.
2. In another cell, enter the revenue formula using variables:
   = A2 * B2
3. Enter some initial values for quantity and price. For example:

• In A2 enter 100 for the quantity
• In B2 enter 9.99 for the price

1. The revenue formula will calculate accordingly. With the example inputs, it would show a revenue of 999.
2. Now change the quantity and price inputs to any values. The formula automatically recalculates revenue based on the new inputs.

For instance, if you update the quantity in A2 to 250 and the price in B2 to 7.50, the revenue formula updates to display 1875.

This example demonstrates the power of using unknown variables. No matter what quantity and price you enter, the revenue formula calculates accordingly without having to be rewritten.

How to Solve an Equation with Unknown Variables in Excel

Besides writing flexible formulas, unknown variables also let you solve equations in Excel. These are mathematical equations where certain parts are represented by variables, and you need to find the value of the variables that makes the equation true.

Excel can’t directly solve for variables, but you can set up the equation using cell references and then use trial-and-error or Goal Seek to home in on the right value for the unknown variable.

Here’s the general approach:

1. Represent the equation using cell references
2. Enter a guess for the unknown variable
3. See if the equation balances
4. If not, adjust the value of the variable until it does

Optionally, you can use Excel’s built-in Goal Seek tool to find the variable value:

1. On the Data tab, click What-If Analysis
2. Choose Goal Seek
3. Set the cell containing your equation as the “Set cell”
4. Enter the value your equation should equal in “To value”
5. Select the cell containing your variable guess as the “By changing cell”
6. Click OK and Excel finds the variable value that makes the equation true

Goal Seek is especially helpful for solving more complex equations with unknown variables.

Summary

Unknown variables are a powerful tool in Excel. They allow you to write flexible formulas that adjust results based on changing inputs. You can use a single variable or multiple variables in a formula, referencing other cells that contain the actual values.

Variables also come in handy for solving equations in Excel. While spreadsheets can’t directly solve for variables, you can set up the equation using cell references and then adjust the variable value manually or use Goal Seek to find the solution.

As you work with unknown variables in Excel, remember to:

• Use clear variable names for readability
• Keep track of what each variable represents
• Double-check cell references to ensure accuracy
• Test variable formulas with different inputs to verify results

FAQs

Vaishvi Desai

Vaishvi Desai is the founder of Excelsamurai and a passionate Excel enthusiast with years of experience in data analysis and spreadsheet management. With a mission to help others harness the power of Excel, Vaishvi shares her expertise through concise, easy-to-follow tutorials on shortcuts, formulas, Pivot Tables, and VBA.