How much SIP should I invest is the question every SIP calculator is really built to answer, and it has no single number — it has a method. The goal-first method inverts the future-value formula: pick a target, a time period and an assumed return, and the monthly amount follows. Want $100,000 in 15 years at an assumed 10%? The answer is $239.28 a month. The budget-first method starts at the other end: work out what you can spare every month without straining, and see what that buys over your horizon. $100 a month at 12% for 20 years buys $99,914.79. This guide works both methods with tables, shows what starting late costs, explains why a step-up is often the bridge between a small start and a large goal, and gives an affordability test you can apply in five minutes. All returns are illustrative assumptions, and no figure here is a recommendation for any specific fund or amount.

Method 1: goal-first — solve for the monthly amount

The goal-first method is the calculator’s goal mode, and its formula is the future-value formula solved for M: M = Target × i ÷ (((1+i)ⁿ − 1) × (1+i)), where i is the monthly rate and n is the number of months. Work it for a $100,000 target in 15 years at an assumed 10%: the monthly rate is 0.10 ÷ 12 = 0.008333, the months are 180, and the arithmetic gives $239.28 a month. Notice what the formula does not ask: it does not ask what you earn, what you spend, or whether $239.28 is comfortable. It answers a maths question, and the budget check that follows is where the planning happens. Run the goal mode on the SIP Calculator page by switching to “Reach a goal” and typing the target instead of the monthly amount; the page solves the same equation and also shows the year-by-year schedule for the resulting plan.

Two warnings belong with every goal-first figure. First, state the target in the right price basis: $100,000 in today’s money in 15 years is a $155,796.74 nominal target at 3% inflation, needing $372.79 a month at 10%, not $239.28. The inflation guide explains the conversion in full, but the rule fits in one sentence — compare nominal with nominal or real with real, never one of each. Second, treat the assumed return as a scenario, not a fact. A goal that only works at 12% is fragile; a goal that works at 8% has margin. Compute the monthly amount at 8%, 10% and 12% and look at the spread before committing to any of them: for the $100,000-in-15-years goal, the amounts are $287.07 at 8%, $239.28 at 10%, and $198.19 at 12%. Planning around the 8% figure costs about $48 more a month than planning around the 10% figure, and buys a plan that survives a disappointing decade.

Monthly amounts for common targets

Read the table for the pattern, not just your row. Doubling the horizon roughly halves the monthly amount for the same target, because compounding does more of the work — $100,000 needs $239.28 a month over 15 years at 10% but only $100.09 over 20 years at 12%, and the difference is mostly time, not rate. That is why the most powerful answer to “how much should I invest” is often “for how long can you keep investing,” and why parents starting a plan for a young child can reach large targets with surprisingly small monthly amounts. Time is the input nobody can buy later, at any price.

Method 2: budget-first — what can you actually spare?

The budget-first method starts from take-home pay. A common frame is 50/30/20: about 50% of take-home pay on needs such as housing, food and transport, 30% on wants, and 20% on saving and debt repayment combined, with the SIP inside the 20% slice. On $4,000 a month take-home pay, the saving slice is $800, and a SIP of $300–$500 sits comfortably inside it alongside an emergency fund and any debt repayment. On $2,000 a month, the slice is $400, and a $100–$200 SIP is the realistic range. These are frames, not rules: a household with high rent may have a smaller slice, a household with no debt may have a larger one, and the right test is always the affordability test below, not a percentage copied from a guide. What the frame prevents is the most common budgeting error — choosing a SIP amount from a goal table without checking whether the money exists every month, including in the months when the car breaks down.

Budget-first also answers the question people actually ask when they say “how much SIP should I do”: how little is still worth doing? The answer is that almost any sustained amount is worth doing, because the maths rewards duration so heavily. $100 a month at 12% for 20 years is $99,914.79 on $24,000 invested; $200 a month over the same period is $199,829.58 on $48,000 invested. Even $50 a month for 30 years at 12% reaches $176,495.69 on $18,000 invested. Small is not pointless — small is how most large plans actually start, and the investors who end with large pots are usually the ones who started small and never stopped, not the ones who started large and quit.

The cost of delay, in one table

The delay table is the most persuasive page in SIP planning, and it deserves a plain reading. Starting a $500 monthly plan at 12% and running it for 30 years gives $1,764,956.89; starting the identical plan 10 years later, so it runs for only 20 years, gives $499,573.96; starting 20 years later, so it runs for 10 years, gives $116,169.54. The late starter does not lose a little — the 20-year-late starter ends with less than one-fifteenth of the early starter’s pot, despite investing one-third as much money. The missing money is not contributions, it is compounding that never happened. Every year of delay costs more than the year before it, because each lost year is a year of growth on an ever-larger pot. If you take one action from this guide, make it starting this month, even at an amount you intend to raise later.

Starting small, and growing with a step-up

A step-up is the bridge between a small start and a large goal, and it works because it matches how incomes actually grow. A flat $500 plan at 12% for 10 years reaches $116,169.54 on $60,000 invested. The same plan with a 10% annual step-up — $500 in year one, $550 in year two, $605 in year three, and so on — reaches $168,716.31 on $95,624.55 invested. The step-up plan invests about 59% more money and ends about 45% higher, but its real virtue is behavioural: the increase arrives with a pay rise, so it rarely feels like a sacrifice, and the plan grows without a fresh decision every year. This guide keeps the step-up brief deliberately: fixed-dollar step-ups, the flat-vs-step-up comparison, and the year-by-year schedule are owned by the dedicated Step-Up SIP Calculator, which is the right tool once you have decided a step-up suits you. One caution: a step-up is a commitment that grows, so set it at a percentage your income can plausibly sustain — 5–10% a year is the usual range, and a step-up you abandon after two years helps less than a flat plan you keep.

A five-minute affordability test

Before locking in any amount, run this five-minute test. One: can you pay it for six months without touching it, including a month with an unexpected bill? If not, halve it. Two: is your emergency fund — typically three to six months of essential spending — funded first, or being funded alongside? A SIP is not an emergency fund, and selling investments in a panic to cover a bill destroys exactly the compounding this guide celebrates. Three: does the amount survive a 20% income drop? Plans that only work at peak income get cancelled at the first setback. Four: have you run the plan at a return two points below your hope, and does the goal still roughly work? If the answer to all four is yes, the amount is affordable; if any answer is no, adjust the amount, not the assumption. Five: write down the review date — one year from now — and the trigger for raising the amount, such as a pay rise. A plan with a review date gets raised; a plan without one quietly stays small forever.

Reviewing the amount once a year

Review the amount once a year, at the same time you review anything else financial, and ask three questions. Has your income risen? If so, raise the SIP before lifestyle spending absorbs the rise — the step-up option automates this, but a manual annual raise works just as well. Has the goal changed? A nearer goal may need a higher amount or a shorter horizon; a goal that moved further away may allow a lower one. Has the plan’s own progress changed your thinking? A pot that grew faster than expected is not a reason to stop — it is evidence the habit works, and stopping converts a good decade into a single good decade instead of a good lifetime of them. The investors who reach their targets are rarely the ones who chose the perfect amount on day one; they are the ones who chose a survivable amount and raised it, year after year, without drama.

Sources: Future-value and goal-solving (PMT-equivalent) mathematics; worked figures computed for this guide from the SIP formula; CalcFolk SIP Calculator (growth and goal modes). All return and inflation rates are illustrative assumptions.