A step-up (or top-up) SIP increases your monthly instalment at a fixed interval — usually once a year — by a fixed percentage or amount. The idea is to let your investing keep pace with a rising income instead of staying at the amount you could afford when you started.

The calculator invests the current instalment at the start of each month and applies the monthly return (annual rate ÷ 12). After every 12 instalments, the instalment is raised by your step-up percentage. The final corpus is the sum of all instalments plus the growth on them.

With monthly rate i = annual rate ÷ 12 ÷ 100, the instalment in year k (starting at k = 0) is Ak = A0 × (1 + s)k, where s is the annual step-up. Each year's 12 instalments form a short annuity-due whose value at the end of that year is Ak × [(1 + i)12 − 1] ÷ i × (1 + i); that block then compounds at (1 + i)12 × (remaining years) until the end of the plan. The corpus is the sum of all these blocks. The calculator evaluates this month by month, which gives the same result as the closed form while also producing the year-by-year table.

Assume an illustrative 12% annual return (actual returns may be higher or lower).

The 10% step-up more than doubles what you invest and roughly 1.7× the corpus — most of the extra money goes in during the later, higher-income years and has less time to grow. For comparison, a flat SIP of ₹15,000 (invested ₹27 lakh) reaches about ₹75.7 lakh over the same 15 years: starting bigger beats stepping up when you can afford it, but stepping up is the realistic path when you cannot.

Years invested, then the return assumption, then the step-up rate. A step-up has its biggest effect on total invested and a smaller proportional effect on the corpus, because the increments arrive late. Check the step-up against realistic income growth — a 10% raise every year for 15 years is a strong assumption — and compare the result with a simple SIP at a higher fixed amount.

With instalments rising every year, more of your money is invested for a short time. Dividing the final gain by total invested, or applying CAGR to the lump total, understates the true annualised return. Use the XIRR calculator with your actual dated instalments to measure what a step-up plan really earned.

Because each increase applies to every future year, a modest step-up compounds twice: the extra money itself grows, and so does the base that later step-ups are calculated on. Compare with the flat-SIP figure shown to see the gap.