Calculators that show their work

Calculate jackpot odds, prize-tier probabilities and expected value for any lottery — Powerball 5/69+1/26, Mega Millions, EuroMillions or Korean 6/45.

Compute the probability of disease given a positive test (PPV) from prevalence, sensitivity and false-positive rate — with full derivation.

Wald normal-approximation CI for a proportion — 90/95/99% two-sided intervals with standard error and adequacy check.

Test whether a sample proportion differs from a baseline — z statistic, one/two-sided p-value, and a plain-language decision.

Minimum sample size for estimating a proportion — margin of error, confidence level, optional finite population correction.

Convert decimal odds to implied probability, compute break-even win rate and expected value (EV) against your own probability estimate.

How likely is a shared birthday among 23 people? (50.7%) Should you switch doors in Monty Hall? (Yes — 2/3) Verified analytic values with explanations.

Compare monthly payments and total interest across the three repayment methods for any loan — principal, rate and term.

Future value of a lump sum plus monthly deposits with annual/quarterly/monthly compounding — growth breakdown by source.

For overseas Koreans and other non-residents selling Korean real estate — capital gains tax and withholding (when the buyer is a company). The primary-residence exemption does not apply to non-residents.

For overseas Koreans and other non-residents receiving a gift of Korean assets — the gift-tax deduction (spouse, children, other relatives) does not apply when the donee is a non-resident.

For overseas Koreans and other non-residents whose estate includes Korean assets — when the decedent was a non-resident, only the 200M KRW basic deduction applies (no spousal, lump-sum, or financial-asset deduction).