How much should you invest monthly? Working backwards from FIRE
By the Fire Finance team · Updated September 25, 2026 · 6 min read
Key takeaway
Divide the journey, don’t guess: a ₹4.8 crore target in 20 years at 12% needs roughly ₹48,000/month flat — or a ₹30,000 SIP stepped up 10% yearly. The step-up is the working person’s cheat code.
“How much should I invest?” has a lazy answer (“as much as you can”) and a useful one: exactly enough to hit your FIRE number on your timeline. That is a solvable equation, and solving it turns an anxious question into a monthly instruction.
The backwards method
- Get your FIRE number — say ₹4.8 crore in 20 years.
- Subtract what your current savings will grow into (₹10 lakh today at 12% ≈ ₹96 lakh in 20 years — already a fifth of the target).
- The SIP covers the rest. The FIRE calculator solves this in one step.
Required flat SIP by target and timeline
Monthly SIP needed to build ₹1 crore (12% expected return, from zero):
| Timeline | Required monthly SIP | Total invested |
|---|---|---|
| 10 years | ≈ ₹43,500 | ≈ ₹52 lakh |
| 15 years | ≈ ₹20,000 | ≈ ₹36 lakh |
| 20 years | ≈ ₹10,100 | ≈ ₹24 lakh |
| 25 years | ≈ ₹5,300 | ≈ ₹16 lakh |
Read that last column twice: for the same ₹1 crore, the 25-year investor puts in a third of what the 10-year investor does. Starting early is not a platitude — it is a 3× discount.
The step-up SIP effect
A step-up SIP rises every year — typically 10%, tracking salary growth. Over 20 years at 12%:
| Strategy | Starting SIP | Corpus after 20 years |
|---|---|---|
| Flat SIP | ₹30,000 | ≈ ₹3.0 crore |
| 10% yearly step-up | ₹30,000 | ≈ ₹4.9 crore |
| Flat SIP matching that corpus | ≈ ₹49,000 | ≈ ₹4.9 crore |
The step-up investor starts ₹19,000/month lighter and lands in the same place — because the increases arrive when their salary can afford them. If today's required SIP looks impossible, this is the lever to reach for. Model your own numbers in the SIP calculator's step-up mode.
Rules of thumb worth keeping
- Automate the SIP for the day after payday.
- Step it up every appraisal, before lifestyle claims the raise.
- Revisit the required-SIP math once a year — not once a week.
Frequently asked questions
What percentage of salary should go into SIPs?
FIRE-oriented savers typically invest 30–50% of take-home pay. But the honest answer runs backwards from your goal: pick your FIRE number and timeline, and the required SIP falls out of the math — then negotiate with your budget.
What if I can’t afford the required SIP today?
Start with what you can and commit to a yearly step-up. Because contributions rise with your salary, a step-up SIP reaches targets that look impossible from your current income. Alternatively, extend the timeline — two extra years often cuts the required SIP by 20%+.
Should I pause SIPs when markets are high?
No. Missing the market’s best stretches costs far more than riding out the worst ones, and SIPs already average your purchase price. The plan works because it is boring and uninterrupted.
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Educational content, not financial advice. Projections use the assumptions shown and are estimates — actual investment returns vary. Consult a registered advisor for personal decisions.