How a realistic retirement calculator differs from the ones your bank offers

Open the retirement calculator at almost any major US bank. You'll see three or four fields: current age, current savings, monthly contribution, and target retirement age. You hit "calculate," and a single confident number appears — your "retirement nest egg."

That number is almost certainly wrong, and not in a small way. Here's what's missing.

The single-number problem

A bank calculator says: "If your portfolio earns 7% per year for 25 years, you'll have $1.4M at retirement." That's a mathematically clean answer. It's also a fantasy.

Real markets don't deliver 7% per year. They deliver -37% one year (2008), then +26% the next (2009), then a long quiet stretch, then a crash, then a recovery. The order of returns matters enormously — a portfolio that gets the bad years early in retirement is a fundamentally different outcome from one that gets them late, even if the average return is identical. This is called sequence-of-returns risk, and any calculator that ignores it is hiding the most important thing about retirement: variance.

Inflation isn't a footnote

Most bank calculators either ignore inflation entirely or apply a flat 2.5%. But inflation is volatile too — and it tends to spike in exactly the years that hurt retirees most (the 1970s for stocks; 2022 for bonds and cash). A retirement plan that quietly assumes a 2% inflation rate forever has hidden a roughly 30% downside risk to your real spending power.

Yearfold samples inflation jointly with returns from the same historical month, so the simulation captures the correlation: if 1973 returns are bad, 1973 inflation is also bad in the same path. Your plan gets stress-tested against scenarios that actually happened, not against a tidy average that never has.

Social Security is not a single number either

Most calculators either ignore Social Security or ask you to type in "your monthly benefit" — a number you typically don't know. The right calculation:

1. Take your earnings record (or a fallback Primary Insurance Amount estimate).
2. Apply the 2026 bend points and PIA formula.
3. Adjust for your claim age (62 to 70).
4. For couples, model spousal and survivor benefits.
5. Apply COLA each year of retirement.

That's not optional. For most middle-class households, Social Security is the largest single source of retirement income. Getting the claim age right is worth tens of thousands of dollars in expected lifetime value.

What "good enough" actually looks like

A calculator worth using should:

• Give you a probability, not a single number.
• Use historical returns with their actual variance, not a flat 7%.
• Sample inflation jointly with returns.
• Model Social Security from current rules, not a guess.
• Show you what the worst-case 10% looks like, because that's where retirement failures live.
• Suggest specific fixes ranked by probability gain per dollar of effort.

That's the bar. Yearfold's bar is to clear all of it for free, in your browser, with your data never leaving your device.

If you've been making decisions on a single-number bank calculator, run your numbers through ours — most people are surprised by what shifts when the variance becomes visible.