CTC (Cost to Company) is the total the employer spends on you every year, and it isn't the same thing as gross salary. Before you get to gross pay, two costs get carved out: employer EPF contribution (12% of Basic) and gratuity accrual (4.81% of Basic, the standard 15-days-per-year-of-service formula spread across 12 months). Neither reaches your bank account monthly, they're real employer costs, provisioned for retirement savings and a long-service benefit respectively.
What's left after that carve-out is gross salary, split into Basic, HRA (House Rent Allowance), and Special Allowance, a balancing figure that soaks up whatever CTC has left over. Under the Old Regime, and only if you pay rent, part of your HRA can be sheltered from tax: the exemption is the smallest of three amounts (actual HRA received, 50% of Basic in a metro or 40% in a non-metro, or rent paid minus 10% of Basic), a genuine statutory formula under Section 10(13A), not an approximation.
From there, this calculator hands the resulting taxable income to the same India income tax engine used across this site (New vs Old Regime, Section 87A rebate with marginal relief, surcharge, and 4% cess), then subtracts your own employee EPF contribution (12% of Basic, mirroring the employer's) and a representative professional tax estimate to land on final net in-hand pay, shown both monthly and annually.