Average Rating Calculator (star rating from review counts)

Understanding the average rating calculator — concept and principle

This calculator works out the average star rating from the number of 1- to 5-star reviews, the share of each star level, the standard deviation and how many more 5-star ratings you would need to reach a target average. Sellers, app developers and shoppers use it to understand review scores.

Weighted mean of stars: with the default inputs the result is 4.26 (94 more 5★ to reach 4.5).

How it is calculated

Average = (5·n₅ + 4·n₄ + 3·n₃ + 2·n₂ + 1·n₁) ÷ total ratings

Share of each level = count ÷ total × 100

Standard deviation = √(Σ count × (stars − average)² ÷ total)

More 5-star ratings to reach target T: k = ⌈(T × n − sum) ÷ (5 − T)⌉

The principle

The average is a weighted mean where each star value is weighted by how many people chose it. The standard deviation shows how much opinions differ: 0 means everyone gave the same score. To raise the average to T, each new 5-star rating adds 5 to the sum and 1 to the count, so solving (sum + 5k) ÷ (n + k) ≥ T for k gives the formula.

Worked example

Default inputs: 120 five-star, 45 four-star, 15 three-star, 8 two-star, 12 one-star; target 4.5.

Total = 200 ratings; sum = 600 + 180 + 45 + 16 + 12 = 853.
Average = 853 ÷ 200 = 4.265, shown as 4.26 (4.3 with one decimal).
Shares: 5-star 60%, 4-star 23%, 3-star 7.5%, 2-star 4%, 1-star 6%; positive (4–5) 82.5%.
Standard deviation = 1.14 stars; to reach 4.5: k = ⌈(900 − 853) ÷ 0.5⌉ = 94 more 5-star ratings.

Each point is the calculator re-run with only “Number of 5-star ratings” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Number of 5-star ratings	Average rating (out of 5)
30	3.66
60	3.95
90	4.14
120	4.26
150	4.36
180	4.43
240	4.54

How to read the result

A high average with a high standard deviation means opinions are split; reading the low reviews may reveal specific problems. With few ratings, the average is unstable. Platforms may weight recent reviews or hide some, so their displayed score can differ from this simple mean.

Common mistakes

Averaging percentages instead of counts.
Rounding the average before using it in further calculations.
Assuming the platform shows a simple mean.

Keep in mind: This calculator does arithmetic only. Ratings are opinions and can be affected by fake or incentivised reviews.

Frequently asked questions

How do I calculate an average star rating?

Multiply each star value by its count, add them and divide by the total number of ratings.

Why is it so hard to raise a high average?

Each new 5-star rating moves the average only a little when there are many ratings, and less as the target nears 5.

What does a high standard deviation mean?

Reviewers disagree strongly; the product suits some people but not others.

Sources: NIST/SEMATECH e-Handbook of Statistical Methods, §1.3.5.1 "Measures of Location" and §1.3.5.6 "Measures of Scale"

Checked 9 Nov 2026. Weighted arithmetic mean and population standard deviation as defined in the NIST/SEMATECH e-Handbook of Statistical Methods, §1.3.5.1 “Measures of Location” and §1.3.5.6 “Measures of Scale”.

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