Weighted average calculator
Compute a weighted mean from a list of values and weights, in weight or percentage mode, with automatic normalisation, an arithmetic-mean fallback and a per-item breakdown of weight and contribution shares.
Runs in your browserEvery computation happens in your browser — your data never leaves this device.
Input
Weights are normalised automatically: raw weights are scaled by their sum, and percentages are scaled too when they do not add up to 100. Without weights the tool falls back to the arithmetic mean.
Result
Per-item breakdown
| # | Value | Weight | Weight share | Contribution | Of the average |
|---|---|---|---|---|---|
| 1 | 90 | 3 | 75.00% | 67.5000 | 77.14% |
| 2 | 80 | 1 | 25.00% | 20.0000 | 22.86% |
What this tool does
- Compute a course grade: homework 30%, midterm 30%, final 40% — enter the three scores and the tool handles the weighting for you.
- Score a survey or a rubric where questions carry different weights; a weighted mean reflects the real level better than a plain average.
- Portfolio and cost accounting: weight average purchase prices or returns by position size or share count.
- Sanity-check the weights: the per-item table lists both the weight share and the contribution to the average, so you can see what is pulling the result up or down.
Example
Input
Values 90, 80; weights 3, 1 (raw weight mode)
Output
Weighted average 87.5000, total weight 4, normalised by the sum; breakdown: 90 (weight 3, weight share 75.00%, contribution 67.5000, 77.14% of the average), 80 (weight 1, weight share 25.00%, contribution 20.0000, 22.86% of the average)
Clear the weights and the result falls back to the arithmetic mean (90 + 80) / 2 = 85.0000. Switch to percentages with 30 and 70 (sum 100) and the result is no longer flagged as normalised.
Frequently asked questions
What is the difference between weight mode and percentage mode?
The maths is identical: weights are normalised by their sum before averaging. The modes only differ in the baseline used to decide whether normalisation happened — a sum of 1 for raw weights, 100 for percentages. Entering 3 and 1 gives the same result as 75 and 25.
What if my weights do not add up to 100 or 1?
Nothing breaks. The tool normalises by the sum and flags the result as normalised. For example weights 20 and 40 in percentage mode (sum 60) are treated as one third and two thirds rather than rejected.
What happens if I leave the weights empty?
The tool falls back to the arithmetic mean: every item gets a weight of 1. The result panel says so explicitly, which is a handy check that you did not forget to paste the weights.
Can weights be 0 or negative?
Zero is fine and simply excludes that item (its weight share is 0%). Negative weights are rejected: they break the "more important counts more" meaning of a weighted average and make the result hard to interpret.
How do "weight share" and "of the average" differ?
The weight share is the normalised weight itself (say 75%) and describes how much of the total weight this item carries. "Of the average" is that item contribution divided by the weighted average (say 77.14%) and describes how much of the result comes from it. The two diverge when an item sits well above or below the mean.
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