Complete guide
Reviewed July 2026The most important number in personal finance is your retirement corpus - the lump sum that must fund the decades after your salary stops. Save too little and you outlive your money; obsess over an impossibly large figure and you sacrifice the present needlessly. Getting this number right, and knowing the monthly saving to reach it, is the foundation of a secure retirement.
This calculator works out the corpus you'll need and the monthly SIP to build it, accounting for the one factor that wrecks naive plans: inflation. Your expenses won't stay at today's level - they'll roughly double every 12 years at 6% inflation - so the corpus must be sized against future costs, not present ones.
Below: how the corpus is estimated, the 25x and 4% rules, worked examples, why starting early changes everything, and the mistakes that leave people short.
How the retirement number is built
The calculation runs in three stages: project today's expenses forward to retirement using inflation, multiply by a factor that covers your retired years, then work out the monthly SIP that grows to that corpus by your retirement age.
The formulas
Future monthly expense = today's expense x (1 + inflation)^years Corpus needed = future annual expense x 25 (the 25x / 4% rule) Required SIP = corpus x r / ((1 + r)^n - 1) (r = monthly return, n = months to retirement)
The 25x rule and the 4% rule are two sides of the same coin: if you need 25 times your annual expenses, you can withdraw about 4% of the corpus each year (1/25 = 4%), a rate widely studied as sustainable over a long retirement. Multiply your first-year retired expenses by 25 to get the target corpus.
Worked example
- Age 30, retire at 60 (30 years). Today's monthly expense Rs 50,000; inflation 6%.
- Future monthly expense at 60 = 50,000 x (1.06)^30 = about Rs 2,87,175; annual = Rs 34.46 lakh.
- Corpus needed = 34.46 lakh x 25 = about Rs 8.6 crore.
- Required SIP at 12% return over 360 months: roughly Rs 24,500/month builds that corpus.
- Start 10 years later at 40 with 20 years left, and the required SIP jumps to about Rs 86,000/month - the price of delay.
Why starting early matters, and the rules of thumb
| Start age | Years to 60 | Required monthly SIP |
|---|---|---|
| 25 | 35 | ~Rs 13,400 |
| 30 | 30 | ~Rs 24,500 |
| 35 | 25 | ~Rs 45,800 |
| 40 | 20 | ~Rs 86,000 |
| 45 | 15 | ~Rs 1,72,000 |
Useful rules of thumb
- 25x rule: aim for a corpus of 25 times your annual retirement expenses.
- 4% rule: withdraw about 4% of the corpus in year one, adjusting for inflation after - it targets a ~30-year retirement.
- For a longer retirement (early retirement, high longevity), use 30-33x (a 3-3.3% withdrawal) for extra safety.
- Factor a buffer for healthcare, which inflates faster than general prices, and for one-off costs.
Using this calculator and avoiding mistakes
- Enter your current age, retirement age, current monthly expenses, expected inflation and investment return.
- Read the corpus needed and the required monthly SIP.
- If the SIP looks unaffordable, extend the timeline (start now), moderate expenses, or plan a partial income in retirement.
- Revisit annually - salary, expenses and markets change, and small course corrections early are painless.
Common mistakes
- Planning with today's expenses instead of inflation-adjusted future expenses - the biggest under-estimation.
- Assuming a short retirement; longer lifespans need a bigger corpus or a lower withdrawal rate.
- Using an over-optimistic return and under-saving as a result.
- Ignoring healthcare inflation, which runs well above general inflation.
- Delaying the start 'until I earn more' - the delay multiplies the required SIP far faster than income grows.
Frequently asked questions
Glossary
- Retirement corpus
- The lump sum needed to fund expenses after your salary stops.
- 25x rule
- Target a corpus of 25 times your annual retirement expenses.
- 4% rule
- Withdraw ~4% of the corpus in year one, adjusting for inflation - a ~30-year guideline.
- Inflation
- Rising prices that make future expenses far higher than today's.
- Required SIP
- The monthly investment needed to reach the corpus by retirement.
- Withdrawal rate
- Annual withdrawal as a share of the corpus; lower lasts longer.
- Glide path
- Shifting from aggressive to conservative assets as retirement nears.
- Sequence-of-returns risk
- The danger of poor early-retirement returns while withdrawing.
Key takeaways
Your retirement corpus is today's expenses inflated to retirement, times about 25 (the 25x/4% rule), and the required monthly SIP is whatever grows to that by your retirement age. Inflation is the make-or-break factor - plan on future, not present, expenses. And time is everything: every five-year delay roughly doubles the SIP needed, so starting now beats every other optimisation. Use conservative assumptions, add a healthcare buffer, and review the plan each year.
Enter your age, expenses, inflation and return above for your retirement corpus and required SIP; then move your start age earlier and watch the monthly figure fall.