The new regime has lower rates and a higher tax-free threshold, but it removes almost every deduction — no 80C, no HRA, no home loan interest. The old regime has higher rates but lets you subtract all of that first.
As a rough rule of thumb: if your total deductions (80C + 80D + HRA + home loan interest, added together) comfortably exceed about ₹4-5 lakh, the old regime often still wins. If they're modest — say, under ₹2-3 lakh — the new regime usually comes out ahead, especially now that income up to ₹12 lakh is effectively tax-free under it.
The only way to know for certain is to run your actual numbers through both, which is exactly what the Salary Tax calculator on this site does — enter your 80C and other deductions once, and it tells you which regime wins and by how much, instead of asking you to guess from slab tables.
One thing worth remembering: you can choose a different regime every year if you're a salaried employee with no business income, so it's worth re-checking this whenever your deductions change — a new home loan, a jump in HRA, or maxing out 80C for the first time can all flip which regime is better for you.