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calculators

Calculators that show their work

Each one runs real arithmetic on your inputs, names the source behind every default, and explains what moves the number. Every input is a parameter in the address, so an answer is a link you can share, and the same link with .json on the end is the same answer as a document any agent or script can read.

11 calculatorsUpdated · 2026-08-21By Tejas Shah, Co-founder

Home and mortgage

Debt and loans

Retirement and investing

How these calculators work

Each calculator asks for the few numbers you know cold and fills the rest with published defaults: a survey rate, a Census statistic, a limit from a lender's or the IRS's own guide. Every default names its source and as-of date beside the field, and every one can be replaced with your own figure. The answer is one number first, then the costs behind it, then the arithmetic and the warnings, so you can stop reading at whichever depth you need.

Nothing on these pages is typed by hand. The worked examples in the prose, the tables, and the figures in the calculator all come from the same function that answers the JSON twin, so the page and the document never disagree. When a default changes, the source line changes with it.

They are instruments, not advice. A calculator shows you the ceiling a rule allows or the tax a set of facts produces; it does not know your credit score, your plan's rules, or your state's quirks unless you tell it. Each page says what it leaves out.

For agents and scripts

Every calculator lives at https://carlo.finance/tools/<slug> and answers as JSON at https://carlo.finance/tools/<slug>.json with the same query parameters. No browser, no API key, no rate limit beyond ordinary courtesy. The JSON carries the parsed inputs, the result, every assumption with its source, any warning about a value that was clamped or ignored, and a canonical URL that reproduces the answer.

For example, https://carlo.finance/tools/house-affordability.json?income=150000&down=50000 is the full answer to "How much house can I afford?" for those inputs. Leave a parameter out and the published default applies; pass a value outside the allowed range and the twin clamps it and says so.

The parameter list for every calculator is in llms.txt, and each page's "For agents" section documents its own fields, units, ranges, and defaults.

Want the number for your actual finances?

Carlo is a personal finance agent. It knows your accounts, debts, and goals, so it can run any of these with your real numbers already filled in, and tell you which one you should be asking. Text it the question.

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