The simple average (arithmetic mean) is the first statistical tool most people learn. But it has a fundamental limitation: it treats every data point as equally important. When some values should count more than others — different course credit hours, different investment position sizes, different survey sample sizes — a weighted average is the correct tool.

The Formula

Weighted Average = Sum of (Value x Weight) / Sum of Weights

Example: You have three courses with grades of 92, 78, and 85. Course A is 3 credits, B is 4 credits, C is 2 credits:

Weighted GPA = (92×3 + 78×4 + 85×2) / (3+4+2) = (276 + 312 + 170) / 9 = 758 / 9 = 84.2

Simple average of grades: (92+78+85) / 3 = 85. The 4-credit course with 78 pulls the weighted average down because it has more weight.

Applications in Finance

Weighted Average Cost of Capital (WACC)

The blended cost of a company’s debt and equity financing, weighted by their proportions. A company with 60% equity at 12% return and 40% debt at 5% after-tax has a WACC of 0.60 x 12% + 0.40 x 5% = 9.2%.

Portfolio Return

If you hold a portfolio where Apple is 40% of your holdings and returned 25%, and cash is 60% at 4%, your portfolio return is 0.40 x 25% + 0.60 x 4% = 10% + 2.4% = 12.4%.

Average Purchase Price (Dollar-Cost Averaging)

If you bought 10 shares at $50 and 20 shares at $40, your average cost basis is not ($50+$40)/2 = $45. It is (10x$50 + 20x$40) / 30 = $1,300 / 30 = $43.33 per share. This weighted average cost basis is what determines your gain or loss when you sell.

Applications in Business

Inventory Valuation (WAC Method)

The weighted average cost method values inventory by the weighted average cost of all units available for sale. This is one of the three accepted inventory accounting methods (with FIFO and LIFO).

Customer Satisfaction Scores

If survey responses come from different-sized customer segments, weighting responses by segment size gives a more accurate overall picture than treating each response equally.

When NOT to Use Weighted Average

When all items genuinely are equally important, a simple average is correct — overcomplicating it adds noise. And when you are measuring central tendency in a skewed distribution (like income), the median is often more informative than any average.