Methodology & Assumptions

Exactly how each calculator works, where every default comes from, and what we deliberately do not model.

One engine, no hand-typed numbers

Every figure on this site — calculator outputs, guide examples, home-page samples — is produced by the same calculation engine at build time. When the engine changes, every example rebuilds. We never type statistics into articles by hand.

Default return: 7% nominal, 5% after inflation

Long-run US equity market returns have averaged roughly 10% nominal before inflation; a blended 60/40-style portfolio is closer to 7–8%. We default to 7% nominal growth for accumulation projections and show a 5% / 7% / 9% band so you can stress-test. This is an assumption, not a prediction — change it.

Fees are deducted from returns

A fund's expense ratio is charged against your balance every year. We model it as a simple deduction from the annual return (net = return − fee), which is how the drag actually compounds. The average 401(k) fund fee sits around 0.3–0.4%; we default to 0.35%.

What we do not model

Sequence-of-returns risk inside a single projection (our scenarios are constant-rate bands, not Monte Carlo draws), taxes on contributions beyond the simple federal bracket you enter, state taxes, and future law changes. Deterministic math keeps every number auditable; for Monte Carlo simulation use a dedicated tool.

Sources

IRS annual contribution limits (IR-2025-111 for 2026), SSA bend points and claiming rules (ssa.gov), and FERS rules from OPM. Each calculator page lists its specific references.