Quick Answer
Use this formula: (Goal average x Total reviews) - Current total stars = Additional 5-star reviews needed (when the new reviews are all 5-star). Below is the plain-English version and a calculator table.
The Formula
Let:
- N = current number of reviews
- A = current average rating
- G = goal average rating
- X = number of new 5-star reviews needed
Then:
X = (G x N - A x N) / (5 - G)
Example 1: 4.2 to 4.5
You have 40 reviews averaging 4.2 and you want to hit 4.5:
- Current stars = 40 x 4.2 = 168
- New total needed = ((40 + X) x 4.5) = 180 + 4.5X
- Solve: 168 + 5X = 180 + 4.5X → 0.5X = 12 → X = 24 new 5-star reviews
Example 2: 3.8 to 4.5
You have 25 reviews averaging 3.8:
- Current stars = 25 x 3.8 = 95
- Solve: 95 + 5X = (25 + X) x 4.5 → 95 + 5X = 112.5 + 4.5X → 0.5X = 17.5 → X = 35 new 5-star reviews
Ready-Made Calculator Table
Assumes every new review is 5 stars. Read across for how many 5-star reviews you need to hit the goal rating.
| Current reviews | Current avg | Goal 4.3 | Goal 4.5 | Goal 4.7 |
|---|---|---|---|---|
| 20 | 3.8 | 14 | 28 | 60 |
| 20 | 4.0 | 9 | 20 | 47 |
| 50 | 4.0 | 22 | 50 | 117 |
| 50 | 4.2 | 8 | 30 | 83 |
| 100 | 4.2 | 15 | 60 | 167 |
| 100 | 4.4 | Already met | 20 | 100 |
Why the Rating Climb Slows Down
Every new review has less impact than the one before it. Going from 4.0 to 4.5 with 20 reviews is quick. Going from 4.4 to 4.5 with 500 reviews takes hundreds of new 5-stars. That is why building the review request habit early pays compounding returns.
Do Not Chase 5.0
Google will not display a perfect 5.0 to shoppers who filter to "4.5 stars and up," so the ROI stops at ~4.8. It also looks suspicious. 4.6 to 4.8 is the sweet spot for local service businesses.
Related Reading
Ask every customer for a review—without review gating.
SoTrustworthy sends a compliant, perfectly-timed review request to every customer after the job — no filtering, no gating, just more honest 5-star reviews on Google.
