Google review calculator
Enter where you are now and where you want to be. The answer is how many more five-star reviews it takes, and how long that is at your asking rate.
Enter where you are now and where you want to be. The answer is how many more five-star reviews it takes, and how long that is at your asking rate.
A realistic number, not an ambitious one.

Your rating is the average of every review. To move the average you add reviews above it, so the maths is the number of five-star reviews that pulls the mean up to your target. It is one sum, and the honest part is what it implies: the closer you already are to five, the more reviews each extra tenth of a point costs.
The number it gives you is a number of asks, and asks are the thing most businesses never get round to. Happy customers leave without saying anything unless somebody puts the question in front of them while they are still there.
ReviewMix gives every business a direct review link and a QR code that opens Google's review box in one scan, on the free plan. Branded cards and printed kits are on Pro.