Author: Taylor Warfield, Former Bain Manager and interviewer

Last Updated: July 17, 2026

 

A weighted average case interview question asks you to combine numbers of different sizes into a single average, like a blended profit margin or an average price across channels. This guide gives you the formula, four worked examples, a fast mental shortcut, and practice problems so you never confuse a weighted average with a simple one again.

 

Before reading on:

 

Most candidates struggle to land interviews and even fewer turn them into offers. Watch my free training to learn how to triple your chances of landing interviews and increase your chances of receiving an offer by 8x.

 

👉 Watch for free

 

Key Takeaways

 

A weighted average accounts for the different sizes of each group, so you multiply each value by its weight, add the results, and divide by the total weight.

 

• Use a weighted average whenever the groups you are combining are different sizes, like product lines with different revenues

 

• The formula is the sum of each value times its weight, divided by the total of the weights

 

• When weights are percentages that add to 100%, you can skip the final division

 

• The answer always lands between your two values, closer to the one with the bigger weight

 

• The most common mistake is taking a simple average when a weighted one is needed, which skews the result

 

What Is a Weighted Average in a Case Interview?

 

A weighted average is an average that accounts for the relative size, or weight, of each value instead of treating them equally. You calculate it by multiplying each value by its weight, adding the results, then dividing by the total weight. In case interviews, it most often appears as a blended margin, price, or growth rate.

 

Here is the formula: weighted average equals (Value 1 times Weight 1 plus Value 2 times Weight 2 and so on) divided by (Weight 1 plus Weight 2 and so on).

 

In my years interviewing candidates at Bain, weighted averages came up constantly, usually hidden inside a profitability or market sizing question rather than announced directly. Interviewers rarely say the words weighted average. They hand you two or three segments of different sizes and expect you to blend them correctly.

 

Weighted averages are one of the most useful tools in case interview math because real businesses are almost never made of equal parts.

 

Top consulting firms care because real client work is full of blended numbers. McKinsey's problem-solving interview asks you to perform quick calculations under pressure. BCG's case interviews score you on the same skill.

 

Weighted Average vs. Simple Average: When Do You Use Each?

 

Use a simple average when every item carries equal weight, and a weighted average when the items differ in size. A simple average of a 10% margin and a 30% margin is 20%, but that is only correct if both products sell the same amount. Once one product is bigger, the true blended margin shifts toward it.

 

Question to ask	Simple average	Weighted average
Are the groups the same size?	Yes, so you can treat them equally	No, each group carries a different weight
What is the formula?	Sum of values divided by the count	Sum of value times weight, divided by total weight
Typical case use	Averaging a few equal data points	Blended margin, price, or growth rate

 

Recognizing which average to use is a quick decision, but it is the difference between a clean answer and one that is off by several points, which is why your mental math matters less than picking the right method.

 

How Do You Calculate a Weighted Average?

 

Calculating a weighted average takes three steps. Multiply each value by its weight, sum those products, then divide by the total weight.

 

1. Identify each value and its weight: the value is the number you are averaging, and the weight is how big that group is
   
   
2. Multiply each value by its weight: keep your zeros lined up to avoid order-of-magnitude errors
   
   
3. Add the products and divide by the total weight: if your weights are percentages that sum to 100%, the division is already done

 

Example: let's say a company has two product lines. Product A brings in $60M in revenue at a 30% margin, and Product B brings in $40M at a 10% margin.

 

Multiply each margin by its revenue weight: ($60M x 30%) plus ($40M x 10%) gives $18M plus $4M, or $22M in total profit. Divide by the $100M in total revenue, and the blended margin is 22%.

 

A simple average would have given you 20%, understating profitability by two full points. On a real client recommendation, that gap can change the answer.

 

Blended margins show up constantly in profitability cases, where you compare product lines or customer segments to find what is dragging results down.

 

If you want to learn case math methods like this quickly, my case interview course walks through every formula with timed practice and worked cases.

 

What Types of Weighted Average Questions Appear in Case Interviews?

 

Weighted averages appear most often as a blended figure across segments of different sizes. The four you will see most are a blended margin, a blended price, a segmented market sizing estimate, and a blended growth rate.

 

How do you find a blended price across channels?

 

Example: a company sells 10,000 units online at $15 each and 30,000 units in stores at $9 each. The blended price is ($15 x 10,000 plus $9 x 30,000) divided by 40,000 total units, which is $420,000 divided by 40,000, or $10.50 per unit.

 

Blended price questions are common in pricing case interviews, especially when a product sells through more than one channel at different prices.

 

How do weighted averages work in market sizing?

 

Example: you are estimating coffee demand in a city. Say 40% of adults are heavy drinkers at 2 cups a day and 60% are light drinkers at 0.5 cups a day, so the weighted average is (0.4 x 2) plus (0.6 x 0.5), which equals 1.1 cups per person per day.

 

Segmenting the population and taking a weighted average is the core of almost every market sizing question, because not everyone in a population behaves the same way.

 

How do you calculate a blended growth rate?

 

Example: a business has two divisions. Division A is 70% of revenue and growing at 5%, while Division B is 30% of revenue and growing at 15%, giving a blended growth rate of (0.7 x 5%) plus (0.3 x 15%), or 8%.

 

Blended growth rates matter in growth strategy cases, where one fast-growing division can mask a slow-growing core that needs attention.

 

What Is the Fastest Way to Estimate a Weighted Average?

 

The fastest way is to remember that the answer always sits between your two values, closer to the heavier weight. Instead of grinding through the full formula, start at the value with the bigger weight and nudge toward the smaller one.

 

Here is the shortcut. Take the difference between the two values, multiply it by the weight of the value you are moving toward, and add that to your starting value.

 

Example: your margins are 10% and 30%, and the 30% product carries 60% of the revenue. Start at 10%, then add 60% of the 20-point gap, which is 12 points, landing you at 22% without long division.

 

This also doubles as a sense check. If you ever calculate a blended margin of 35% from a 10% and a 30% product, you know instantly it is wrong, because the answer cannot fall outside the two values.

 

This interpolation trick is one of a handful of case interview formulas worth committing to memory before your interview.

 

What Mistakes Should You Avoid With Weighted Averages?

 

The biggest mistake is using a simple average when the groups are different sizes, which quietly skews your answer. A few others trip up candidates under time pressure.

 

• Taking a simple average of percentages when the underlying groups differ in size, which is the single most common error

 

• Forgetting to divide by the total weight when your weights are raw numbers rather than percentages

 

• Mixing up the value and the weight, for example weighting margins by units instead of by revenue

 

• Dropping or adding a zero during multiplication, which throws off the order of magnitude

 

• Calculating the number correctly but never explaining what it means for the client

 

The last one is the one I cared about most as an interviewer. A correct 22% blended margin means nothing until you say whether it is healthy, who is dragging it down, and what to do about it.

 

Weighted Average Practice Problems

 

Work through these three problems before your interview, and time yourself at 60 to 90 seconds each. Try to solve each one before reading the solution.

 

Problem 1: A company has two segments. Segment A has $80M in revenue at a 25% margin, and Segment B has $20M at a 5% margin. What is the blended margin?

 

Solution: ($80M x 25%) plus ($20M x 5%) is $20M plus $1M, or $21M. Divide by $100M in revenue, and the blended margin is 21%.

 

Problem 2: A retailer sells 5,000 units online at $40 and 15,000 units in stores at $20. What is the average selling price?

 

Solution: ($40 x 5,000) plus ($20 x 15,000) is $200,000 plus $300,000, or $500,000. Divide by 20,000 total units, and the average price is $25.

 

Problem 3: A market has three customer segments. Half spend $100 a year, 30% spend $250, and 20% spend $500. What is the average annual spend?

 

Solution: (0.5 x $100) plus (0.3 x $250) plus (0.2 x $500) is $50 plus $75 plus $100, or $225 per customer.

 

If you want feedback on how you talk through math like this under pressure, one-on-one coaching with a former interviewer is the fastest way to find your blind spots.

 

The weighted average case interview question is not hard once you stop reaching for the simple average and start weighting each value by its size. Drill the formula, memorize the shortcut, and run a few timed problems until blending segments feels automatic.

 

Frequently Asked Questions

 

What is the weighted average formula?

 

The weighted average formula is the sum of each value times its weight, divided by the total of the weights. For example, with values of 10% and 30% weighted 60% and 40%, you calculate (10% x 0.6) plus (30% x 0.4), which equals 18%. When your weights already add up to 100%, you can skip the final division.

 

How is a weighted average different from a regular average?

 

A regular, or simple, average treats every value equally by adding them up and dividing by the count. A weighted average gives more importance to larger groups by multiplying each value by its size first. The two only match when every group is exactly the same size.

 

When should you use a weighted average in a case interview?

 

Use a weighted average any time you combine groups of different sizes, such as product lines with different revenues or customer segments with different spending. Blended margins, blended prices, segmented market sizing, and blended growth rates are the most common cases. If the groups were equal, a simple average would do.

 

How do you calculate a weighted average quickly without a calculator?

 

Start at the value with the bigger weight, then shift toward the other value by the smaller weight's share of the gap. For values of 10% and 30% where the 30% group holds 60% of the weight, add 60% of the 20-point gap to 10%, giving 22%. This avoids long division and works in a few seconds.

 

Do you always divide by the total weight?

 

You divide by the total weight whenever your weights are raw numbers, like revenue in dollars or units sold. If your weights are percentages that already add up to 100%, the division is built in, so you can stop after summing the products. Forgetting this step is a frequent source of errors.

 

Are weighted averages common in McKinsey, BCG, and Bain case interviews?

 

Yes, weighted averages appear regularly across McKinsey, BCG, and Bain case interviews, usually inside profitability, pricing, or market sizing questions. Interviewers rarely use the term directly, so you have to spot when groups of different sizes need to be blended. Recognizing the pattern quickly is what separates strong quantitative candidates.

 

Everything You Need to Land a Consulting Offer

 

Need help passing your interviews?

• Case Interview Course: Become a top 10% case interview candidate in 7 days while saving yourself 100+ hours
  
  
• Fit Interview Course: Master 98% of consulting fit interview questions in a few hours
  
  
• Interview Coaching: Accelerate your prep with 1-on-1 coaching with Taylor Warfield, former Bain interviewer and best-selling author

 

Need help landing interviews?

• Resume Review & Editing: Craft the perfect resume with unlimited revisions and 24-hour turnaround

 

Need help with everything?

• Consulting Offer Program: Go from zero to offer-ready with a complete system

 

Not sure where to start?

• Free 40-Minute Training: Triple your chances of landing consulting interviews and 8x your chances of passing them