How much should I invest every month to retire early?

"How much should I invest every month?" is the question that finally makes FIRE feel real. The target corpus is abstract and far away; the monthly SIP is a decision you make this payday. The good news is that the monthly number is not something you pluck from the air or copy from a friend. It falls out of three things you can actually pin down: where you are now, where you want to end up, and how many years you have to get there.

Think of the monthly SIP as a bridge. On one bank is what you already have invested. On the far bank is your FIRE number. The SIP is the span that connects them, and compounding is the material it is built from. Work out the target, subtract the head start you already have, and the monthly investment is whatever it takes to cover the rest at a realistic return.

Why when you start beats how much you invest

Here is the part that feels almost unfair. A rupee invested in your 20s does far more work than a rupee invested in your 30s, because it compounds for a decade longer. Meet Neha and Sameer. Neha invests ₹20,000 a month from age 25 and stops at 35, putting in ₹24 lakh total. Sameer starts at 35 and invests ₹20,000 a month all the way to 55, putting in ₹48 lakh, twice as much. At a steady 11%, Neha often ends up with a comparable or larger pot, despite investing half the money, simply because her early rupees had more time.

That single idea is the engine behind Coast FIRE: invest hard and early, then let compounding coast you the rest of the way. The lesson for your monthly number is blunt. The best time to start a SIP was years ago. The second best time is this month, even if the amount is smaller than you would like.

Skip the algebra. Open the FIRE Planner, enter your current corpus, a monthly SIP and an expected return, and nudge the SIP up or down until the final corpus lands on your target. Two minutes beats a spreadsheet.

Find a number you can actually sustain

A calculator will happily tell you to invest ₹1.5 lakh a month. If your take-home is ₹1.8 lakh, that is not a plan, it is a fantasy you will abandon by March. The number has to be one you can keep feeding month after month, through birthdays, breakdowns and bad moods, because the magic of a SIP is entirely in its consistency.

That sustainable figure is really your savings rate in disguise, the share of income you can genuinely part with. A smart move is to start at a rate you can hold, then use step-up SIPs: raise the amount by 5 or 10% each year, ideally timed to your appraisal, so the increase comes out of new income you never got used to spending. That way the SIP grows with you instead of squeezing you.

And if, like most Indian investors, your plan is basically a stack of SIPs, it is worth pressure-testing whether that alone can carry you to early retirement. That is exactly the question in can you retire early with SIPs alone.