How to Calculate a Retirement Corpus and Monthly SIP

Work back from the spending gap

Start with the monthly spending you expect the modeled portfolio to fund. Estimate the balance required at retirement, then determine what remains to be funded by monthly contributions.

The two phases of the model

A retirement SIP model first values a user-supplied monthly spending gap across retirement, then estimates the fixed nominal contribution needed to build that corpus before retirement.

Existing savings and new contributions grow before retirement. After retirement the remaining balance supports a series of price-adjusted withdrawals.

Test a longer retirement horizon

Hold the monthly spending gap and other inputs fixed, then extend the retirement horizon. More scheduled withdrawals increase the required corpus. Compare different assumptions without treating the scenarios as probabilities.

Corpus and monthly funding formulas

Convert effective annual assumptions to monthly factors. Let p be the price factor, f the retirement growth factor, n months until retirement and m retirement months. A gap G in opening purchasing power gives a withdrawal G × p^(n+k) at retirement month k. Discount by f^k for k from 1 to m and sum. Fund the resulting corpus with projected existing savings and fixed monthly contributions.

Corpus = Σ [G × p^(n+k) ÷ f^k], for k = 1 through m

Reconcile a small two-phase example

With a monthly spending gap of 100, three months of retirement and all rates zero, the required corpus is 300. Existing savings of 100 leave a gap of 200. Over three accumulation months, the mathematical contribution is 200/3. A payment of 66.67 ends at 300.01. A payment of 66.66 falls short at 299.98.

This small scenario illustrates the mechanics. It does not estimate a typical retirement budget.

1. Enter a spending gap of 100, existing savings of 100 and three months in each phase.
2. Set accumulation return, retirement return and inflation to zero.
3. Check the corpus of 300 and the verified monthly payment of 66.67.

Read the money-year labels

The required corpus is in retirement-date money. The first withdrawal occurs one month later and uses that checkpoint’s money. The rounded-payment surplus is an accumulation result and does not increase the spending schedule.

If retirement return and inflation match, the post-retirement real rate is zero. Negative real rates raise the discounted value of later withdrawals relative to that equal-rate case.

Supply the two horizons explicitly

Enter the gap in opening purchasing power, existing savings, months before and during retirement, and separate effective annual accumulation, retirement and inflation rates. Choose beginning or end accumulation payments. Retirement withdrawals are at month end.

No age or life expectancy default is applied. A zero accumulation horizon leaves no payment opportunity.

Choose the matching planning question

Use SIP Goal when you already know the terminal target. Use Inflation-Adjusted SIP to express a fixed saving plan in opening purchasing power. Use SWP to test withdrawals against an available balance.

Place withdrawals at month end

Use whole equal months and a constant price assumption. Withdrawals begin after one retirement month and the corpus targets zero terminal estate. Accumulation contributions are fixed nominal amounts. Zero retirement months means no withdrawals.

At positive inflation the first withdrawal exceeds the gap measured at retirement prices. The monthly payment before retirement does not rise automatically.

Understand bounds and uncertainty

Each horizon is from 0 to 1,200 months. Input amounts are 0 through 10^12 and rates are above −100% through 100%. Decimal inputs support 12 fractional places. The model rejects unsupported payments, monetary results above 10^30 and unresolved funding thresholds.

The corpus does not account for variable investment returns, longer-than-modeled retirement, changing spending, eligibility or a planned bequest.

Avoid double-counting income and inflation

Keep opening purchasing power, retirement-date money and withdrawal-date money distinct. Use the gap you want funded after any income you considered. Avoid applying inflation twice or assuming that returns are constant outside the model.

Use consistent assumptions for what is covered by the gap. The calculator does not subtract income again or adjust the result for tax.

Use consistent currency and a supplied gap

India often uses SIP and corpus. UK pension income, Canadian retirement income, Australian drawdown and US retirement savings are useful local terms. Choose USD, GBP, CAD, AUD or INR. The tool does not apply local account or benefit rules.

Educational scope

Educational constant-rate projection. It excludes dates, lifespan, pensions, tax, fees, FX, account rules and variable returns. It does not determine sustainable income or provide personalized investment advice.

Frequently asked questions

Sources and review

Sources reviewed Nov 10, 2026. Consult current local rules before relying on time-sensitive information.

• HDFC Mutual Fund retirement calculator
• Investor.gov savings goal calculator
• MoneyHelper pension calculator
• Canadian Retirement Income Calculator
• ASIC account-based pension calculator