Short answer: the exemption is the smallest of three numbers — HRA received, rent paid minus 10% of basic + DA, and 50% (four metro cities) or 40% of basic + DA. Enter the yearly figures and the page shows which test decided it.
Worked example
Basic + DA ₹6,00,000, HRA received ₹2,40,000, rent paid ₹2,64,000, non-metro. Test one is ₹2,40,000. Test two is ₹2,64,000 − ₹60,000 = ₹2,04,000. Test three is 40% of ₹6,00,000 = ₹2,40,000. The exemption is ₹2,04,000 and the remaining ₹36,000 of HRA is taxable.
What this page does not do
It does not compare the old and new regimes, does not apply the standard deduction or chapter VI-A limits, and does not file anything. Those belong to a tax calculator and to your return; this page answers one question precisely rather than five approximately.