Guides

The maths behind the calculators

A calculator gives you a number. These explain where the number comes from, using figures worked through by the same engine that runs the tools, with the source for anything that is not arithmetic.

Loans

Why Early Loan Payments Are Mostly InterestOn a $300,000 mortgage at 6.5%, $1,625.00 of the first $1,896.20 payment is interest, and principal only overtakes it in year 19. Here is the one rule behind it.
Why Your Final Loan Payment Is DifferentA payment of $1,896.20407 is charged as $1,896.20. That fraction of a cent compounds for 30 years, and the last payment collects it: $1,900.91.
APR vs Interest Rate: What Each One Tells YouA 6.5% mortgage with $6,000 of fees has a 6.695% APR, but only if you keep it 30 years. Refinance after five and the real cost is 6.99%.

Tax

Marginal vs Effective Tax RateA single filer on $100,000 in 2026 is in the 22% bracket and pays 13.17% of their income in federal tax. Both are right. Here is which to use when.

Interest

How Day-Count Conventions Work: 30/360, ACT/360, ACT/365, ACT/ACTThe same $1,000,000 at 6% for one year earns $60,000.00 under 30/360 and $60,833.33 under ACT/360. How each convention counts, with worked figures.