When an NRI sells property in India, the very first question is how long it was held, because that single fact determines which tax regime applies: hold it for more than 24 months and the gain is classified as long-term (LTCG); hold it 24 months or less and it's short-term (STCG). The two aren't just taxed at different rates — they're calculated differently.
Long-term: flat rate, no indexation for NRIs
Following the Finance (No. 2) Act, 2024 changes to the capital gains regime, LTCG on property is generally taxed at a flat rate without the indexation benefit (which adjusts the purchase cost for inflation) that resident individuals and HUFs can still elect under a grandfathering option for pre-existing assets — that grandfathering choice does not extend to NRIs, which is a distinction worth double-checking against your own facts before assuming otherwise.
Short-term: taxed at your slab rate
STCG on property doesn't get a special flat rate — it's added to your other taxable income for the year and taxed at your applicable slab rate, which means the actual rupee tax figure depends on your total income for the year, not just the gain itself. That's also why an STCG calculator can classify the gain and compute it, but generally can't assert a single tax-rate figure the way an LTCG calculation can.
The LTCG/STCG classifier on the Real Estate Capital Gains page walks through the holding-period classification and the resulting gain calculation, including acquisition cost, improvement costs, and transfer expenses.