What the projection actually assumes
Every retirement calculator hides a pile of assumptions behind one number. Here are the ones this tool makes, stated plainly, so you can judge whether the answer applies to you.
A retirement calculator that gives you a single number is telling you about its assumptions at least as much as it is telling you about your money. This one is no different. The difference is that these are written down.
Today’s dollars, inflated forward
The two figures you enter for spending and Social Security, retireSpend and
ssIncome, are today’s dollars. Internally they are inflated forward from
your current age to the year being projected, so a $70,000 spending target at
age 45 is not $70,000 at age 70; it is whatever $70,000 buys then.
This matters because the alternative convention, entering future dollars, is almost impossible to do consistently. Nobody has an intuition for what their grocery bill will be in 2051.
Social Security gets a full COLA. Pensions do not.
Social Security benefits are indexed to inflation, so the projection grows them at the full inflation rate every year. Most private pensions are not indexed at all, and public ones are usually partially indexed. So pension income is grown only by whatever COLA you give it, defaulting to none.
Over a thirty-year retirement that difference is enormous. A $30,000 pension with no COLA against 2.5% inflation is worth about $14,300 in today’s money by year thirty. Treating it as inflation-proof is the single most common way a plan looks fine on paper and does not hold.
Withdrawals are grossed up for tax
If the plan needs $60,000 of spending money out of a pre-tax 401(k), it does not withdraw $60,000. It withdraws enough that $60,000 survives the tax, at the effective rate you supply. Calculators that skip this step systematically overstate how long a portfolio lasts, because they quietly spend the government’s share too.
RMDs start at your applicable age, not at 70½ or 72
Required Minimum Distributions have moved twice in recent years, and a lot of material online is still out of date. Under SECURE 2.0 the applicable age depends on when you were born:
| Birth year | RMD applicable age |
|---|---|
| 1951–1959 | 73 |
| 1960 or later | 75 |
The projection derives this from your birth year rather than hardcoding a number, and applies it to pre-tax balances and governmental 457(b) balances, which are easy to forget.
Couples run on two clocks
If you are modelling a household, the projection is indexed to one person’s age, the primary’s, but the spouse has their own retirement age, their own RMD cohort and their own Social Security claim age. All of it gets converted onto the shared axis.
One consequence is worth stating outright, because the default surprises people: leaving the spouse’s retirement age blank means “retires in the same calendar year you do”, not “retires at the same age number”. For a couple six years apart those are wildly different plans, and the second one invents a six-year staggered window that nobody asked for.
These are estimates, not forecasts. The return rate, the inflation rate and the tax rate are inputs you choose. Change any of them and the answer changes. That sensitivity is the useful part: it tells you which assumptions your plan is actually resting on.
What it does not model
Being explicit about the gaps matters as much as the assumptions:
- Sequence-of-returns risk is not in the deterministic projection at all. It assumes a steady annual return. Use the Monte Carlo tab for that.
- Tax brackets are a single effective rate, not a bracket schedule. Good enough for planning; not a substitute for a tax preparer in the year you actually withdraw.
- Healthcare before Medicare is only whatever you put in your spending figure. The gap between retiring at 60 and turning 65 is a real and expensive thing this tool will not remind you about.