Can someone guide me on how to calculate monthly gross salary? Does it include basic, allowances or it includes 1/12 of bonuses too?

Assume you are referring to the income ceiling for BTO flats since theres no cap for resale HDB. Assessment is based on the average gross monthly income excluding bonuses and OT, including additional allowances. Detailed explanation can be found at HDB website https://www.hdb.gov.sg/cs/infoweb/residential/b...

Note that Notice of assessment from IRAS is not acceptable

For salaried workers, HBD will base on past monthly CPF contributions.

yes, so it would include basic, overtime, allowances, bonuses, incentives.​​​