Inflation Calculator

One factor, two readings

Every result on this page comes from a single number, the growth factor (1 + rate ÷ 100)^years. The engine turns the assumed annual rate into a multiplier, raises it to the power of the number of years entered, and then applies that factor in both directions. Multiplying today's amount by the factor gives the future cost: what the same purchase costs once the assumed inflation has run for the whole horizon. Dividing today's amount by the factor gives the purchasing power: what that many of today's dollars buy at the end of the same period. The third line, the total price increase, is the factor minus one, stated as a percentage.

The reference example, locked by the engine's tests, is $100.00 at an assumed 3% for 20 years. The factor is 1.8061, so the future cost is $180.61 and the purchasing power is $55.37; prices in that scenario rise 80.6%. Every figure on this page is computed at build time by the same engine that runs the calculator above — none is typed in, so the explanation cannot drift from the tool.

Why the two directions are reciprocals

The future-cost and purchasing-power lines look like two different facts about inflation, but they are one fact read from opposite ends. If a basket that costs $100.00 today will cost $180.61 after 20 years at an assumed 3%, then $100.00 handed over at the end of those 20 years buys only $100.00 ÷ 1.8061 = $55.37 of that basket. The first statement scales the price up; the second scales the money down; the multiplier is the same factor either way. Dividing each result by the starting amount makes the symmetry visible: 1.8061 in one direction, 0.5537 in the other, and their product is 1.0000.

One consequence is easy to misread. Prices rising 80.6% does not mean purchasing power falls 80.6%. Because the second reading is a division, the loss is 1 − 1 ÷ factor: in the reference example, today's dollars lose 44.6% of what they buy. The two percentages are never equal for any rate above zero, and the gap between them widens as the factor grows. The calculator reports the price increase as a percentage; the purchasing-power line already has the division built in, which is why it is shown as a dollar amount rather than as a second percentage.

The starting amount cancels out of both proportions. The factor depends only on the rate and the years, so $100.00 and $1,000,000.00 undergo exactly the same proportional change over the same period — the amount field only sets the scale of the dollar figures. That is why the tables below use $100.00: the results double as percentages.

Compounding a rate over years

The factor is an exponent, not a multiplication. Twenty years at an assumed 3% does not add twenty lots of 3% to prices; each year's increase applies to the already-raised level of the year before, so the total climbs faster than the years count. Holding the assumed 3% fixed and changing only the horizon:

Each additional decade does not add a fixed amount to the price increase; it multiplies the factor by another 1.3439, the ten-year factor at 3%. From 10 to 20 years the increase moves from 34.4% to 80.6%; from 30 to 40 years it moves from 142.7% to 226.2% — a larger jump in percentage points for the same ten years. This is the mechanism retirement arithmetic keeps running into: a horizon measured in decades is long enough for the exponent to dominate, and the later years of a long horizon do more of the compounding than the earlier ones.

A concrete way to feel the exponent is to ask when the factor passes 2 — the year in which the modeled price level has doubled. Rather than quoting a rule of thumb, the build steps the engine one whole year at a time and records the first year in which the future cost of $100.00 reaches $200.00:

The doubling year is not a forecast, and it is reported as a whole year: at an assumed 3% the factor is 1.9736 after 23 years and 2.0328 after 24, so the crossing lands on year 24. The shape is the point. Because the rate sits inside the exponent, halving the assumed rate roughly doubles the time to the same factor, and the same relationship runs in reverse when the assumption is raised.

Sensitivity to the assumed rate

The rate is the input nobody knows in advance — which is why every rate on this page is labelled as an assumption rather than a forecast. When the rate field is left blank the form fills in 3% so that it can produce a result; that default is a placeholder for the arithmetic, not an estimate of anything. Holding the 20-year horizon fixed and changing only the assumed rate:

Reading down the table: moving the assumption from 2% to 4% — a two-point change in the input — takes the 20-year price increase from 48.6% to 119.1%, more than doubling it, and cuts what today's $100.00 buys from $67.30 to $45.64. At an assumed 6% the same twenty years more than triple prices, leaving $100.00 with the purchasing power of $31.18. A small difference in the assumed annual rate is not a small difference in the result, because the difference is compounded twenty times over. Running the same amount and horizon at several assumed rates therefore describes the model's uncertainty better than any single run does: the spread between the runs is the honest output.

Nominal dollars, real dollars, and where this page fits

The growth calculators on this site follow the convention documented on the methodology page: balances are shown in future, nominal dollars, and entering a real (after-inflation) return shows purchasing power in today's dollars instead. This page is the bridge between those two views. A nominal balance divided by the inflation factor for the same period is that balance restated in today's dollars — the purchasing-power direction of this calculator, applied to a projection rather than to a price.

Worked through with the methodology page's own reference accumulation: $10,000 plus $500 a month at an assumed 7% return for 30 years grows to $691,150.47 in nominal dollars. Deflating that balance at an assumed 3% inflation over the same 30 years — the factor is 2.4273 — restates it as $284,744.84 of today's purchasing power. The nominal figure is not wrong; it is denominated in year-30 dollars, and the deflated figure is the same balance denominated in today's. Comparing either one against a spending figure only works when the spending figure is in the same dollars.

The shortcut of entering a real return gives a related but different answer, and the difference is instructive. Subtracting the assumed 3% inflation from the assumed 7% return and running the same inputs at 4% produces $380,159.68 — noticeably more than the deflated figure. Two things separate them. The larger is what happens to the contributions: the nominal run puts in a fixed $500 every month for 30 years, so in today's dollars each later contribution is worth less than the one before it, while the real-return run treats every $500 as today's dollars — equivalent to a contribution that rises with inflation each year. The two runs model different saving patterns and answer different questions; neither is the wrong one. The smaller is that subtraction is an approximation: the real rate implied by a nominal rate and an inflation rate is (1 + nominal) ÷ (1 + inflation) − 1, a little less than the simple difference, which is one reason the shortcut is an approximation rather than an exact conversion.

One further convention: this page compounds once per year, raising the annual factor to the number of years exactly as entered, while the growth calculators compound monthly at the annual rate ÷ 12. The deflation in the example above applies an annual inflation factor to a monthly-compounded balance, which is how the two schedules are meant to combine, and stating it here keeps them from being mistaken for each other.

Two retirement-sized readings

The reference example uses $100.00 so that the results read as percentages; the same arithmetic on retirement-sized figures shows the two directions doing different jobs. Priced forward: an annual spending figure of $50,000 in today's dollars, at an assumed 3% over 25 years, becomes $104,688.90 — the nominal amount required in year 25 to buy what $50,000 buys now. That is the escalation the Retirement Calculator applies to its modeled withdrawal each year, and it is the "adjusted for inflation" step in the withdrawal convention laid out in The 4% Rule guide, where the first year's dollar amount is raised for inflation annually and the portfolio balance is never consulted again.

Deflated back: a $1,000,000 balance projected 25 years out, at the same assumed 3%, has the purchasing power of $477,605.57 today. Both readings use the same factor, 2.0938, and both carry the same 109.4% price increase — the spending figure is multiplied by it and the balance is divided by it. Stating a future balance and a future spending need in the same dollars, either both nominal or both today's, is what makes them comparable; setting a nominal balance against a today's-dollars spending figure overstates how far the balance stretches by exactly this factor. The Retirement Withdrawal Calculator's savings-runway mode avoids the conversion by taking spending in today's dollars together with a real return, so that purchasing power is held constant inside its formula; this page performs the conversion explicitly when the figures on hand are nominal.

Where the model stops

This is a steady-rate projection, and its limits follow from that. It applies one constant rate for the whole horizon; real inflation varies year to year, and the model does not simulate that variation. It applies the same rate to everything — one factor for the whole amount — and does not distinguish between kinds of spending. It holds no historical price index, so it cannot say what a past year's dollars are worth today; that is a lookup against the published index for those years, not a projection at an assumed rate. It models prices only: no investment return, no taxes, no income and no account rules enter the arithmetic, which is why it pairs with the growth and drawdown calculators rather than replacing them. And at a 0% rate it is inert by design — $100.00 for 10 years returns $100.00 as the future cost and $100.00 as the purchasing power — a boundary the engine's tests check.

The inputs are bounded. The rate accepts 0% to 50% and the horizon more than zero and up to 100 years, and the exponent is the number of years exactly as entered. Outside those ranges the calculator reports an input error rather than a number, so an out-of-range assumption cannot quietly produce a result.

Reading the three lines

The calculator reports three lines. The first — $180.61 to buy what $100.00 buys today, in the reference case — is the amount multiplied by the factor: a future price in future dollars. The second, what the amount will be worth in today's dollars, is the amount divided by the factor: a future sum restated in present purchasing power. The third is the factor minus one, as a percentage. Three misreadings recur. Treating the price increase as the purchasing-power loss — they differ, as shown above. Treating the increase as rate × years — it compounds, so it is larger than that for any horizon past one year. And reading the purchasing-power line as a prediction — it is the arithmetic consequence of the assumed rate and nothing more; change the assumption and the line changes with it.

Frequently asked questions

Not financial advice: a steady-rate projection — real inflation varies year to year and across spending categories. Values are processed locally in your browser and never transmitted. See the methodology page.