HRA (House Rent Allowance) is the part of your salary structure most likely to have unused tax-saving potential, but the exemption formula isn't simply "whatever HRA I received." Section 10(13A) caps the exemption at the smallest of three separate amounts, and most people only find out which one binds them after the fact.
The first amount is straightforward: actual HRA received, exactly as shown on your payslip. The second is a percentage of your Basic salary, 50% if you live in one of four metro cities (Delhi, Mumbai, Kolkata, Chennai) or 40% everywhere else, including high-cost tech hubs like Bengaluru, Hyderabad, and Gurugram that don't qualify for the higher rate despite comparable rents. The third is rent paid for the year minus 10% of Basic, which means low rent relative to Basic can zero out your exemption even if HRA received and the city percentage are both generous.
Whichever of the three is lowest becomes your exempt amount; anything above that in your HRA received stays fully taxable. And critically, none of this matters at all under the New Regime, which disallows the exemption entirely, so the calculation only changes your actual tax bill if you file under the Old Regime with documented rent receipts.