“Which retirement calculator is most accurate?” is one of the most-asked questions in this corner of the internet, and the honest answer is not a calculator. The question is a number in disguise. A calculator that does not show its assumptions is a box that takes your inputs and hands back a slogan, and a slogan cannot be checked. The number is only as good as the assumption behind it, and an assumption that is not shown cannot be dated, and an assumption that is not dated cannot be updated. This site’s retirement calculator shows all three: every input, the dated 2026 assumption behind it, and the formula that turns the input into the number. This article is the inside of the calculator, shown step by step.
The Inputs
The retirement calculator on this site takes seven inputs. The table is in 2026 dollars, and that is the first dated fact: inflation ran 3.4 percent over the twelve months through August 2026, per the Bureau of Labor Statistics.
| # | Input | What it sets |
|---|---|---|
| 1 | Annual spending in retirement, in today’s dollars | What the plan must pay every year |
| 2 | Retirement age, the first year of withdrawals | When the plan starts |
| 3 | End age | The horizon of the plan |
| 4 | Inflation rate | How the withdrawal is indexed each year |
| 5 | Portfolio growth | How savings compound to the start date |
| 6 | Withdrawal rate | The first-year drawdown the portfolio can sustain |
| 7 | Social Security benefit | What the portfolio does not have to pay |
The first input is the one the other calculators quietly assume away. The spending figure is yours. It is not a percentage of salary, not a median, not a survey number. The number is built from the life you want to afford, not from a savings target, and that is the frame the site runs on, worked out in How Much Do You Need to Retire?.
The other six inputs are not free either. Each one has a default, and each default is an assumption with a date. That is the next table.
The Dated 2026 Assumptions
Every default the calculator ships with, and where it comes from:
| Input | Dated 2026 assumption | Source, dated |
|---|---|---|
| Spending | Your own number, stated in 2026 dollars | Inflation 3.4 percent headline, 2.4 percent core, BLS, twelve months through August 2026 |
| Retirement age | 67, the full retirement age | FRA 67 for people born in 1960 or later, the 1983 Amendments to the Social Security Act; as recorded in the 2026 Trustees Report |
| End age | 97, a 30-year horizon from 67 | The 30-year test window of the 1994 study; the life table: 19.7 years past 65, to 84.7, and 43.8 percent past 85 — the 2024 U.S. life table, NCHS, August 2026 |
| Inflation | 3.4 percent | BLS, twelve months through August 2026; the withdrawal is indexed at this rate each year |
| Portfolio growth | 6 percent real and 3 percent real, both shown | 6 percent real, the long-run real record of the U.S. equity market since 1926; 3 percent real, a stated planning assumption for the lower end of the range. The current 2026 rates, for reference: the 10-year Treasury at 5.01 percent on September 16, 2026, the Federal Reserve at a 3.75 to 4.0 percent range |
| Withdrawal rate | 4 percent first-year, indexed for inflation, over thirty years | Bengen, Journal of Financial Planning, October 1994, against U.S. market data back to 1926; confirmed by Cooley, Hubbard and Walz, AAII Journal, February 1998, against data back to 1925 |
| Withdrawal edges | 3.5 percent and 3 percent | 3.5 percent, the worst thirty-year window in the 1926 to 2014 rolling data; 3 percent, a horizon longer than thirty years, per the dated studies. The full data is in The 4% Rule, Stress-Tested |
| Social Security | Your own benefit estimate at your claiming age | Dated public reference: $2,071 a month for all retired workers, the SSA’s January 2026 estimate after the 2.8 percent COLA announced October 24, 2025; the 2026 maximum at FRA is $4,152 a month |
Two things to read from that table. The first is the rule the site runs on: where two sources disagree on an input, both are shown, with dates. The growth row is the case in point — the 6 percent real since-1926 record and the 3 percent real planning floor are both in the calculator, not one of them. A second dated reference, for the record: the same program averaged $1,903 a month across all beneficiaries as of May 2025, per the 2026 revision of the Wikipedia entry on U.S. Social Security, which cites SSA data. The January 2026 retired-worker figure is the one the calculator uses, and the two are different populations, not a contradiction to resolve. The second thing is the dated column on the right: the 1994 test, the 1998 confirmation, the 2014 worst window, the 2024 life table, the August 2026 BLS print, the October 2025 COLA announcement. Every assumption in the calculator has a date, and the date is what makes it updatable. The next section is the formula.
The Formulas, Shown
Here is the arithmetic, step by step.
The retirement number. The plan pays a first-year withdrawal in the first year, and the portfolio must be large enough for that withdrawal to be the assumed rate of it:
first-year withdrawal = portfolio x withdrawal rate
Solve for the portfolio:
portfolio = first-year spending / withdrawal rate
At the dated 4 percent rate, that is first-year spending divided by 0.04, which is 25 times first-year spending. The 25x is not an invented number. It is the reciprocal of a tested rate, and the rate has a date: the 1994 test and the 1998 confirmation, both against data back to the 1920s. The edge rates invert to the same shape: 3.5 percent gives 28.6 times, and 3 percent gives 33.3 times.
The indexing. The withdrawal is not flat. The 1994 rule indexes it for inflation every year after the first, and the formula is:
year-n withdrawal = first-year withdrawal x (1 + inflation)^(n - 1)
At the dated 2026 inflation of 3.4 percent, a $52,000 first-year withdrawal is $70,257 by year ten. The portfolio target is set in first-year dollars; the indexing is what the plan carries through the horizon.
The horizon. The plan has to be alive at the end age. End age minus retirement age is the number of years the withdrawal runs: 97 minus 67 is 30, the test window of the 1994 study. The life table is the dated check on that window: 19.7 years past 65, to 84.7, and 43.8 percent of Americans live past 85, by the 2024 table. A 30-year plan from 67 runs past the average life, and that is exactly why the 3 percent edge is in the calculator at all.
The saving side, if you are still saving. If the start date is in the future, the target has to be reached, and the formula is the standard compound-growth one:
portfolio at start = current savings x (1 + g)^years + contribution x ((1 + g)^years - 1) / g
g is the real-growth assumption from the dated set, 6 percent or 3 percent, and the contribution is the dated 2026 IRS limit: $24,500 a year in a 401(k) or 403(b), plus $8,000 of catch-up at 50, plus $11,250 of super catch-up for ages 60 to 63. The age-by-age version of this formula, with the gap to the number at each age, is in How Much Do You Need to Retire?. This section keeps it to one line, because the number is the same either way.
The Worked Output
One set of inputs, and every number in the output traceable to one of them.
- Spending: $52,000 a year, in 2026 dollars. A life, not a sum.
- Retirement age: 67, the 2026 FRA.
- End age: 97, the 30-year window.
- Inflation: 3.4 percent, BLS through August 2026, indexing the withdrawal.
- Withdrawal rate: 4 percent, the 1994 test and the 1998 confirmation, with the edges at 3.5 percent and 3 percent.
- Social Security: $2,071 a month, the 2026 SSA estimate for all retired workers.
The calculation, step by step:
- What the portfolio has to cover: $52,000 minus $24,852 ($2,071 x 12) is $27,148 a year. The benefit pays 48 cents of every spending dollar; the portfolio pays the other 52.
- At 4 percent: $27,148 / 0.04 = $678,700. That is the number, dated: the 2026 average benefit, the 1994 and 1998 rate, the 2026 inflation.
- Run it at the edge rates and read the gap: $27,148 / 0.035 = $775,657; $27,148 / 0.03 = $904,933. The 3 percent edge carries the years past 97 that the life table holds.
- Without Social Security, the same spending at the same rates: $52,000 / 0.04 = $1.3 million; $1,485,714 at 3.5 percent; $1,733,333 at 3 percent. The benefit is the difference between a $678,700 number and a $1.3 million number, and it is the largest input after the rate.
- If you are still saving: $100,000 saved today, plus $24,500 a year — the 2026 base 401(k) limit, the figure for a pre-50 contributor — for ten years, compounds to $502,014 at 6 percent real and to $415,257 at 3 percent real. Same years, same contribution, $86,757 of gap, and it came entirely from the assumption, not from the discipline.
Two things the worked output does not do. It does not round the rate up, and it does not drop the edges. The 4 percent number is the center of the range, not the answer. And a withdrawal rate is two numbers: one that survives the sequence and one that does not. The 30-year average is what the formula uses; the order of the first decade is what the plan actually meets. That is the 4% Rule, Stress-Tested, with the dated start years and the counted outcomes.
What a Monte Carlo Calculator Does — and What This One Does
The question is asked often enough to be on the search engines: what is a Monte Carlo retirement calculator.
A Monte Carlo retirement projection does not output one number. It draws thousands of simulated sequences of returns from a distribution fitted to market data, runs the withdrawal plan over each sequence, and reports the share of the sequences in which the portfolio survives the horizon: a probability of success. It is the most common method in commercial retirement projections, and the figure it gives is a share of simulated futures, not a dollar amount.
This site’s calculator answers the deterministic question first: the number, and every dated assumption behind it. The two are the same contract at different resolutions. A Monte Carlo’s probability is only as good as the distribution it draws from, which is the same dated-assumption rule this calculator makes visible, applied to a return model instead of a stated rate. The edge rates in the table above — 4 percent, 3.5 percent, 3 percent, each with its dated test — are the sequence-of-returns risk expressed as multiples rather than probabilities: the number that survives the worst window, and the number for the horizon that outlives the test. Whether a given number survives its first decade, order by order, is the stress-test article.
How to Update the Assumptions When the 2027 Data Arrives
The calculator is a dated object, and the update process is the reason for that.
Each input re-dates on its own schedule:
- Inflation: the BLS prints every month; the site re-dates the indexing rate monthly. The 2026 figure here is the twelve months through August 2026.
- The rate snapshot: the 10-year Treasury and the Federal Reserve range are the site’s dated rate set, re-dated with the set. The 2026 value is September 16, 2026.
- Contribution limits: the IRS, every January. The 2026 set is $24,500 / $8,000 / $11,250.
- The life table: NCHS, every year. The 2024 table was published in August 2026; the next table lands on the same cycle.
- Social Security: the COLA is set by the CPI-W from the third quarter of one year to the third quarter of the next, and announced in October. The 2026 figure was 2.8 percent, announced October 24, 2025; the average and maximum benefits re-publish each January.
- The rate edges: the rolling window extends each year new data is added — 1926 to 2014 becomes 1926 to 2015 — and the worst window is re-measured.
The process is one line. Take the new data, replace the dated input, re-run every formula on this page. The number moves, the date moves, and the plan is the same plan with a newer number on it.
That is the whole contract of the calculator: the number, dated; change one input and re-run it.