In an IPO what is the formula of share allotment?

After the subscription is closed, you can determine your probability of allotment.

E.g. XYZ ltd. issues IPO lot size 60, so let's assume retail subscription was 5x after closure.

It is clear that those who apply for at least 5 lots will get atleast 1. However, for those who applied to less than 5, it will be on a lottery basis. This means that if you apply for 1 lot, then you will get at least 1. For example, those who applied for 1 lot out of 5 will receive allotment.

Please refer to the Basis of allotment document from recent IPO's for more information about shares allotment formula.