How this calculation works
Apply one month of the assumed return, then the end-of-month income and expense. Increase later expenses using the monthly equivalent of entered annual inflation. Stop at the first monthly bill that cannot be funded completely.
Closing reserve = opening reserve × (1+return/1200) + income − current expenseA worked example
A ₹1,20,000 reserve, ₹30,000 monthly expense and ₹10,000 monthly income fund six complete monthly bills at zero return and zero inflation.
Good to know
This is a scenario, not a recommended emergency-fund target or a promise of interest. Within-month timing, irregular emergencies, taxes and fees are omitted. The maximum horizon is 600 monthly periods; surviving that horizon does not prove permanent sustainability.
Reference: CFPB: emergency reserves and personal cash-flow needs (opens in a new tab) ↗
Count complete monthly bills
The monthly budget planner compares current take-home income allocations with entered spending and saving. It helps distinguish an actual saving contribution from money merely left below a spending target.
The default reserve is ₹1,20,000. With ₹30,000 monthly spending and ₹10,000 income, each zero-return month uses ₹20,000 of reserve. Six complete bills are funded; the seventh has a shortfall. This is a count of funded periods rather than an exact day of exhaustion.
The model credits one month of reserve return before receiving income and paying the bill at month end. Actual bill dates may create a cash shortage earlier within the month, even if a monthly total looks affordable.
The savings goal calculator estimates deposits toward a target. This reserve scenario instead asks how complete monthly bills draw on an existing balance.
Inflation and return have different conventions
The reserve return input is nominal annual and divided by twelve. The expense-inflation input is effective annual, converted to a monthly growth factor. The first bill is the amount you entered; inflation affects later bills.
The inflation calculator can explore a separate future-cost assumption. The savings goal calculator works in the opposite direction, finding deposits toward a future target instead of testing how an existing reserve supports spending.
A stable scenario is not permanent security
If all 600 modeled bills are funded, the result says at least 600 periods. It does not prove the reserve survives every future event or identify a recommended amount to save. Income, returns and expenses can all change unexpectedly.
The input income stays constant while expenses may rise. Irregular emergencies, taxes and fees are excluded. Use this as a transparent stress scenario and revise the assumptions when your actual circumstances differ.
A few questions, answered
Can a zero reserve fund a bill?
Yes, if the entered reliable income is enough under the stated month-end timing.
Why does the tool show no partial-month estimate?
Within-month payment timing is not modeled, so a fractional exhaustion date would imply more precision than the inputs support.
Is the default reserve a recommendation?
No. It is only a worked example for the cash-flow arithmetic.