Weighted Average: 5 Mistakes to Avoid
Calculates averages where items count differently, like weighted exam components. The maths is not complicated, but a handful of errors come up again and again in this and related math & geometry calculations. Here is what to watch for.
Calculators are best used to check working rather than replace it, especially for exams where method marks count. Work the problem by hand first, then confirm with the calculator.
Each formula has assumptions built in. Area formulas assume flat shapes, volume formulas assume regular solids and statistical measures assume the data you enter is the data you want to describe. Knowing the assumptions helps you pick the right tool.
1. Mixing units
Measurements must all be in the same unit before applying the formula.
2. Using the wrong percentage base
A percentage change is measured against the original value, not the new one.
3. Confusing radius and diameter
Many circle formulas use the radius, which is half the diameter.
4. Population versus sample formulas
Sample standard deviation divides by n − 1; population divides by n.
5. Forgetting the order of operations
Powers and brackets come before multiplication and addition.
The correct method
Weighted mean = Σ(value × weight) ÷ Σweight.
For example, with these inputs:
- Values: 80, 72, 90
- Weights: 30, 30, 40
the calculator returns:
- Weighted average: 81.6
Related calculations
- Area of a Trapezium Calculator: Calculates the area of a trapezium from the measurements you enter.
- Area of a Parallelogram Calculator: Calculates the area of a parallelogram from the measurements you enter.
- Area of an Ellipse Calculator: Calculates the area of an ellipse from the measurements you enter.
- Area of a Rhombus Calculator: Calculates the area of a rhombus from the measurements you enter.
The Weighted Average Calculator applies the formula consistently, so it is a quick way to double-check your own working.
Run your own numbers: Weighted Average Calculator