How the US Income Percentile Calculator works
This tool estimates where an annual income ranks among US earners by fitting a log-normal curve to the income distribution — a standard approximation economists use because income is strongly right-skewed (a long tail of high earners) but becomes roughly bell-shaped once you take its logarithm.
The formula
For an income I, an assumed median income M, and a log-standard-deviation σ (sigma, which controls how spread out incomes are):
z = (ln(I) − ln(M)) / σ
Percentile = Φ(z) × 100
where Φ is the cumulative distribution function of the standard normal distribution — the same function used for z-scores and bell-curve probabilities in statistics. The calculator uses different median and sigma values for household income (all earners in a home combined) and individual income (one wage earner), since the two have different centers and spreads.
Worked example
Using the household-income parameters (median around $75,000, σ ≈ 0.62), an income of $100,000 gives z = ln(100000/75000) / 0.62 ≈ 0.463, which maps to roughly the 68th percentile — higher than about two-thirds of household incomes, or the top 32%. Doubling that income to $200,000 pushes z to about 1.58, close to the 94th percentile, illustrating how income percentiles compress as income rises: each additional dollar buys less percentile movement near the top.
Why household and individual income differ
Household income sums every earner living in one home, so it is typically higher and more spread out than any single person's pay. Individual income reflects one wage or salary. Selecting the matching type keeps the percentile comparison meaningful — comparing a two-earner household's combined income against individual-income benchmarks (or vice versa) produces a misleading rank.
Limits of the log-normal approximation
A log-normal curve is a convenient, widely used approximation, not a lookup of exact government percentile tables. It fits the middle of the distribution reasonably well, but real income data has a fatter top tail (closer to a Pareto distribution) above roughly the 95th percentile, so estimates for very high incomes are directional rather than precise. Treat the result as a planning estimate, and consult primary sources such as the US Census Bureau for exact official statistics.