Every SIP calculator headline is a nominal figure: the dollars you would hold at the end of the plan, with no adjustment for rising prices. That figure flatters every long plan, because a dollar in twenty years’ time will not buy what a dollar buys today. The correction is one division — real value equals nominal future value divided by (1 + inflation rate) raised to the number of years — and it changes decisions. A $500 monthly plan at an assumed 12% for 10 years reaches $116,169.54 nominally, but that is $86,441.05 in today’s money at 3% inflation, a gap of $29,728.49 that is neither a fee nor a loss but the quiet work of rising prices. This guide works through the nominal-to-real conversion with tables at 2%, 3% and 5% inflation, explains real returns and why they are not simply nominal minus inflation, and shows how to compare a future goal with a projection without mixing nominal and real figures — the single most common error in long-term SIP planning. All rates here are illustrative assumptions, never forecasts.

Nominal vs real: two prices for the same pot

Inflation is the general rise in prices over time, and its effect on a savings plan is easiest to see with a single purchase. If a basket of goods costs $100 today and inflation runs at 3% a year, the same basket costs $103 next year, $106.09 the year after, and $134.39 after 10 years — each year’s increase applies to the previous year’s higher price, so the growth compounds exactly like investment returns do, only working against you. A SIP projection that ignores this tells you how many dollars you will hold, which is a fact about arithmetic, while what you actually want to know is how much of life those dollars will buy, which is a fact about purchasing power. Both numbers are legitimate; the error lies in reading the first as if it were the second.

The distinction matters more the longer the plan runs, because inflation compounds too. Over 10 years at 3%, prices rise by a factor of 1.03¹⁰ = 1.3439, so each future dollar is worth about 74 cents in today’s money. Over 20 years the factor is 1.8061, so each future dollar is worth about 55 cents. Over 30 years it is 2.4273, so each future dollar is worth about 41 cents. A 30-year projection that looks enormous in nominal dollars can therefore be merely adequate in real terms — $200 a month at 12% for 30 years gives $705,982.75 nominally, but $290,855.55 in today’s money at 3% inflation. Neither figure is wrong; they answer different questions, and a plan should be judged by the second one unless the goal itself is stated in future prices.

The adjustment, step by step

Take the headline example from the SIP Calculator: $500 a month, 12% assumed, 10 years, nominal future value $116,169.54. Step one: pick an inflation assumption — use 3% here, a round illustrative figure, not a forecast. Step two: compute the price factor, 1.03¹⁰ = 1.343916. Step three: divide, $116,169.54 ÷ 1.343916 = $86,441.05. That is the value of the pot measured in today’s dollars. Step four, the sanity check: the amount invested was $60,000 in money spread across the decade, so a real value of $86,441.05 means the plan genuinely grew purchasing power by about $26,441.05 in today’s terms, not by the $56,169.54 the nominal gains column suggests. The nominal gains column is not lying — it counts dollars — but it counts dollars of different years as if they were equal, and they are not.

A useful shortcut for quick comparisons: to convert any nominal result to today’s money at 3% inflation, multiply by 0.744 for 10 years, 0.554 for 20 years, and 0.412 for 30 years. At 5% inflation the multipliers fall to 0.614, 0.377 and 0.231. Those three numbers explain why long-horizon goals set in nominal terms keep surprising people: a plan that “reaches $500,000” in 30 years at 5% inflation reaches only $115,688.72 in today’s purchasing power. Always ask, of any large future figure, “in which year’s dollars?”

How inflation eats a 20-year plan

Read the table across a row to feel inflation’s weight, and down a column to feel duration’s. At 2% inflation the 20-year $500 plan keeps about two-thirds of its nominal value in real terms; at 5% it keeps barely more than a third. The 30-year plan is hit hardest in percentage terms even though its nominal figure is the largest — duration multiplies both compounding and inflation, and the two forces fight over the same decades. That is why the common advice to “just invest longer” is only half right: longer helps real wealth only if the assumed return stays above inflation, which is the subject of the next section. Note also that the 2% and 5% columns for the second and third rows are computed the same way — nominal value divided by (1 + inflation) raised to the years — so you can reproduce any cell with the formula in this guide and the nominal figures from the calculator.

Real return: why it is not nominal minus inflation

It is tempting to say the real return is the nominal return minus inflation — 12% minus 3% equals 9% — but that is an approximation, and over long periods the error is worth correcting. The exact relationship divides growth factors: real return = (1 + nominal) ÷ (1 + inflation) − 1. At 12% nominal and 3% inflation that gives 1.12 ÷ 1.03 − 1 = 8.74%, not 9%. At 10% and 3% it gives 6.80%; at 8% and 3% it gives 4.85%; at 12% and 5% it gives 6.67%. The subtraction shortcut overstates the real return every time, because it ignores that inflation also erodes the return itself, not just the principal. For a one-year comparison the difference is small enough to ignore; for a 20-year projection it is not, because the small annual gap compounds into thousands of dollars.

The real-return formula also draws the line that matters most: a plan only grows purchasing power if its return beats inflation. A “safe” return of 4% while inflation runs at 5% loses about 0.95% of purchasing power a year, every year, however steady it feels. That is the risk that conservative savers underestimate, and it is the mirror image of the risk that aggressive savers underestimate when they chase the highest nominal return without asking what it costs in volatility. The calculator’s expense-ratio field interacts here too: a 1% expense ratio on a 12% assumption does not just cut the nominal result from $116,169.54 to $109,493.64 over 10 years, it cuts the real return from 8.74% to about 7.77% at 3% inflation, because the fee comes out before inflation does its work. Fees, inflation and return must be read together, never one at a time.

What the return assumption does to the real figure

The sensitivity table makes the planning point brutally clear. Dropping the assumption from 12% to 8% cuts the 20-year nominal result by about 41%, and the real result by the same proportion — $276,601.99 becomes $164,150.25 in today’s money. That is why this guide, like the calculator page, insists on running the low scenario first: a goal that only works at 12% is a hope, while a goal that works at 8% is a plan. If the low scenario falls short, the honest responses are to invest more, invest longer, or lower the goal — not to raise the assumption until the shortfall disappears. The assumption is a dial on a model, not a lever on reality.

Setting goals in real terms

Goals should be stated in one price basis and kept there. If the goal is “a $100,000 house deposit in today’s money in 15 years,” then the target for the calculator is not $100,000: it is $100,000 grown at inflation for 15 years, which at 3% is $155,797. That is the nominal target the SIP must reach, and goal mode on the calculator can solve for it — at an assumed 10%, the monthly amount for $155,797 in 15 years is $372.79, against $239.28 for a flat $100,000 target. Alternatively, keep everything in today’s money: deflate the projection, as this guide does, and compare $100,000 with the real value. Both routes give the same answer if done consistently; the expensive mistake is to compare a nominal projection with a today’s-money goal and declare victory years too early. Write the price basis next to every goal figure you set, in the same sentence, and the mistake becomes almost impossible to make.

What inflation adjustment cannot tell you

Inflation adjustment has limits that deserve equal honesty. First, your personal inflation rate is not the headline rate: if your goal is education or housing, the prices that matter to you may rise faster than the general index, and no single percentage captures that. Second, the adjustment assumes one steady inflation rate for decades, which is itself a simplification — real inflation varies, sometimes sharply. Third, the calculation says nothing about whether the assumed investment return will materialise; it only restates whatever nominal figure you feed it. The adjustment is a lens, not a forecast: it makes a projection honest about purchasing power, and it leaves every other uncertainty exactly where it was. Use it on every long-term figure you take seriously, and distrust any long-term figure presented without it.

Sources: Standard real-value and real-return mathematics (Fisher relationship); worked figures computed for this guide from the SIP future-value formula; CalcFolk SIP Calculator nominal outputs. All return and inflation rates are illustrative assumptions.