How to Calculate Your Retirement Corpus (And Why Most People Get It Wrong)

Ask most people how much they need to retire, and you'll get an answer based on today's expenses — something like "I spend 50,000 a month now, so I'll need enough to cover 50,000 a month in retirement." It's a completely understandable way to think about it. It's also wrong, often by a factor of three or four, because it silently assumes inflation stops the day you retire.

Here's the actual math, done properly, and why skipping a step or two can leave you decades short.

Mistake #1: Using Today's Expenses as the Target

If you're 30 and plan to retire at 60, your expenses in 30 years will not resemble your expenses today — not because your lifestyle will change, but because prices will have risen for three decades straight.

The correct first step is inflating your current expenses forward using the standard compound growth formula:

Future Annual Expense = Current Annual Expense × (1 + inflation)years to retirement

At a conservative 6% inflation assumption, 6,00,000 in annual expenses today becomes:

6,00,000 × (1.06)30 ≈ 34,46,000 per year

That's not a typo — three decades of 6% inflation multiplies your expenses by roughly 5.7x. Skip this step, and every subsequent calculation you do will be built on a number that's a fraction of what you'll actually need.

Mistake #2: Picking an Arbitrary Corpus Number

Once you know your future annual expense, the next question is: how large a corpus generates that income indefinitely (or for your expected retirement length)?

This is where the safe withdrawal rate comes in — the percentage of your total corpus you can withdraw each year, adjusted for inflation, without running out of money over a long retirement. The commonly cited figure is 4%, derived from historical market studies (the "Trinity Study"), though it's a rule of thumb, not a law of physics — more conservative retirees, especially those retiring early, often use 3–3.5%.

The corpus formula follows directly from the withdrawal rate:

Required Corpus = Future Annual Expense / Safe Withdrawal Rate

Using our example, at a 4% withdrawal rate:

34,46,000 / 0.04 ≈ 8,61,50,000

That's roughly 8,61,50,000 — a very different number from the 6,00,000-a-year lens most people start with. This is the step where retirement planning either becomes real or stays comfortably vague.

Mistake #3: Not Converting the Corpus Into a Monthly Savings Number

A corpus of 8,61,50,000 sounds abstract and possibly discouraging in isolation. The useful next step is converting it into a monthly savings figure you can actually act on — a SIP target.

This uses the future value of an annuity formula, solved for the monthly contribution:

FV = P × [((1 + r)n − 1) / r]

where FV is your target corpus, P is the monthly contribution being solved for, r is your expected monthly rate of return, and n is the number of months until retirement.

At a 10% expected annual return (≈0.83% monthly) over 30 years (360 months), reaching 8,61,50,000 requires a monthly SIP of roughly 34,000–35,000 — assuming you're starting from zero. If you already have some retirement savings, that existing amount is grown forward at the same rate and subtracted from the target first, reducing the monthly figure needed.

Putting It Together: A Worked Example

Let's run the full sequence for a 30-year-old:

1. Current annual expense: 6,00,000
2. Years to retirement: 30
3. Inflation assumption: 6%
4. Future annual expense: 6,00,000 × (1.06)³⁰ ≈ 34,46,000
5. Safe withdrawal rate: 4%
6. Required corpus: 34,46,000 / 0.04 ≈ 8,61,50,000
7. Expected return: 10% annually
8. Required monthly SIP (from zero): ≈ 34,000–35,000/month

Each of those inputs meaningfully changes the final answer — which is exactly why two people with identical current expenses can have wildly different "correct" retirement numbers, depending on their assumptions about inflation, returns, and retirement age.

Why the Assumptions Matter So Much

Inflation assumption: Moving from 5% to 7% inflation over a 30-year horizon changes your future expense figure by roughly 60% — a huge swing from what looks like a small input change. When in doubt, err toward a higher inflation assumption; being wrong in the conservative direction costs you a slightly larger SIP today, while being wrong in the optimistic direction costs you running out of money decades from now.

Safe withdrawal rate: Dropping from 4% to 3.5% increases your required corpus by roughly 14%. This is a meaningful lever, especially for early retirees whose money needs to last 40–50 years rather than the 25–30 year retirement the original 4% studies were based on — the scenario Financial Independence models directly. (Our Portfolio Longevity calculator lets you stress-test how long a given corpus actually lasts at different withdrawal rates.)

Return assumption: This is the input people most often get wrong in the optimistic direction — using historical nominal equity returns (12–15%) without adjusting for the fact that some of that is just inflation, not real growth in purchasing power. A more defensible planning assumption uses expected real returns, then separately accounts for inflation in the expense side, rather than mixing nominal returns with inflated future expenses (which double-counts inflation's effect in your favor).

Common Mistakes Recap

• Using today's expenses as the retirement target, ignoring 20–40 years of compounding inflation
• Picking a safe withdrawal rate without considering retirement length — a 35-year-old retiring early needs a lower withdrawal rate than someone retiring at 60, simply because the money needs to last longer
• Using optimistic nominal returns while also inflating expenses, which can understate the required corpus by mixing real and nominal figures inconsistently
• Treating the final number as fixed rather than revisiting it every few years as your actual expenses, inflation experience, and portfolio performance become clearer

Retirement planning isn't a one-time calculation you do at 30 and forget about — it's a number worth recalculating every few years as your real expenses and returns diverge from your original assumptions. To run your own numbers with these adjustments built in, use our Retirement Planning calculator.