Weighted Average Calculator
Average with custom weights (grades, portfolio)
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Weighted Average Calculator turns the numbers behind average with custom weights (grades, portfolio) into a result, step by step. Just enter Values (comma separated) and Weights (comma separated); Weighted Average updates the result as you type. Nothing in Weighted Average is a black box. That makes Weighted Average handy for quick estimates anywhere. Nothing typed into Weighted Average leaves your device or the tab. Weighted Average sits in the Math section next to related tools and weighted average calculator grades weights. Weighted Average is great for comparing scenarios. Change one value and watch. No sign-up or email — Weighted Average answers as you type.
What a weighted average does
Weighted average = Σ(value × weight) ÷ Σ(weight). With the defaults — 80, 90, 85 weighted 0.3, 0.4, 0.3 — the numerator is 85.5 and the weights sum to 1, so the answer is 85.5.
It is a plain average with one addition: each value is multiplied by how much it counts. A final exam worth 40% pulls the result harder than a quiz worth 10%.
The formula, term by term
Σ(value × weight) ÷ Σ(weight), where the denominator is the sum of the weights — not a count of the values.
Dividing by Σ(weight) is what makes the weights free-form. They can be 0.3/0.4/0.3, or 30/40/30, or 3/4/3, and the answer is identical, because the ratio between them is what carries the information.
Worked example
80 × 0.3 = 24
90 × 0.4 = 36
85 × 0.3 = 25.5
Σ(value × weight) = 85.5
Σ(weight) = 0.3 + 0.4 + 0.3 = 1
Weighted average = 85.5 ÷ 1 = 85.5.
Compare that with the unweighted average of the same three marks: (80 + 90 + 85) ÷ 3 = 85.0. The weighted figure is half a mark higher because the strongest score carries the largest weight.
Working with integer weights
Percentages and credit hours both work. Three modules worth 30, 40 and 30 credits with marks of 80, 90 and 85 give the same 85.5, because Σ(weight) is 100 and the division normalises it. Unnormalised weights such as 15, 20 and 15 behave identically for the same reason.
Where it goes wrong
- Mismatched lengths. Three values and four weights cannot be paired; the calculator needs one weight per value.
- Weights as percentages written with a % sign. Enter 30, not 30% — the field reads numbers.
- Assuming the weights must sum to 100. They do not. Only their ratio matters, and the formula divides by their sum.
- Averaging portfolio returns with weights that drift. Weights must be the weights at the same point in time as the values, or the result describes a portfolio that never existed.
- Commas in the wrong place. A decimal comma turns 0.3 into two entries.
When a weighted average is the wrong tool
If the quantities compound rather than add — a series of annual returns, for instance — the correct figure is the geometric mean, not a weighted arithmetic average. The arithmetic version overstates a volatile series, and the gap grows with the volatility.
Frequently Asked Questions
Why is 80, 90, 85 weighted 0.3/0.4/0.3 equal to 85.5?
Each mark is multiplied by its weight: 24 + 36 + 25.5 = 85.5, and the weights sum to 1 so there is nothing to divide by. The unweighted average of the same marks is 85.0.
Do the weights have to add up to 100%?
No. The formula divides by the sum of the weights, so 3/4/3 gives the same answer as 30/40/30. Only the ratio between the weights changes the result.
Can I use credit hours as weights?
Yes — that is one of the most common uses. A 40-credit module pulls the average four times as hard as a 10-credit one, and the normalising division keeps the result on the same scale as your marks.
What is the difference from a normal average?
A normal average treats every value equally. A weighted average multiplies each by its importance first, so a heavily weighted item moves the result far more than a lightly weighted one.
Should I use this for investment returns?
Not for a series of years — those compound, so a geometric mean is correct. Weighted averages are right when you are combining holdings within one period, using each holding’s share of the portfolio as its weight.
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