The shortcut: final year's rent × number of years
A quick way to estimate 10 years of rent might be to escalate today's rent to what it'll be in the final year, then multiply by 10 — treating the whole period as if every year cost the same as the last one.
The real final-year rent, from the escalation formula
Starting from ₹25,000/month (₹3,00,000/year) escalating 5% annually, the rent actually paid in the 10th and final year of a 10-year lease (9 escalations applied, since the first year uses the starting rent) comes to ₹4,65,398.46/year. Multiplying that by 10 years gives a naive estimate of ₹46,53,984.65 in total rent.
The real total, from the rent vs buy calculator
The rent vs buy calculator sums each year's actual escalating rent — ₹3,00,000 in year one, rising 5% each year through ₹4,65,398.46 in year ten — for a real total of ₹37,73,367.76.
The naive estimate overstates by ₹8,80,616.89
₹46,53,984.65 (naive) − ₹37,73,367.76 (actual) = ₹8,80,616.89 — nearly a quarter of the real total rent paid, all because the naive shortcut assumes every year cost as much as the most expensive (last) one, when in fact the earlier years were meaningfully cheaper.
Why the year-by-year total is the one that matters
Any decision comparing total rent paid against a lump sum (like a home's appreciated value, or an invested down payment) needs the real cumulative total, not a stand-in based on one year's figure — using the naive shortcut would make renting look meaningfully more expensive than it actually was over the period.