How We Build and Check Our Calculators

A calculator is only useful if its answer is right. This page explains where the formulas come from, how they are tested, what "last reviewed" means on a calculator page and where the numbers stop being reliable.

1. Formulas come from published sources

Every formula on Calc Vault is a standard, published one — the Kelly criterion from Kelly's 1956 paper, the Sharpe ratio from Sharpe's own definition, option payoffs and futures P&L from standard derivatives textbooks, pip values and margin from how brokers and exchanges define them, odds conversions from the standard bookmaking definitions. No calculator uses a proprietary or "secret" model. Each calculator page ends with a Sources list naming the references its formula follows.

2. The math is separated from the page and covered by tests

The calculation behind each tool lives in its own small module, separate from the page layout, so it can be tested on its own. Those modules are covered by an automated suite of more than 1,000 unit tests (run with Vitest) that check:

• textbook cases with known answers (for example, even-money odds at 55% give a 10% Kelly stake);
• edge cases — zero or negative inputs, extreme odds, empty legs — that must not produce nonsense;
• internal consistency, such as implied probabilities that sum to 100% once the vig is removed.

The full suite is run before every release, and a change that breaks it is not shipped.

3. What "last reviewed" means

Every calculator page shows a last reviewed date. It is the date the page's published worked examples were last re-run against the calculator's actual math and found to match. It is not a cosmetic "updated" stamp: the date changes only when someone actually re-checks the examples, and a page whose examples no longer match the math is corrected before the date moves.

4. Inputs, rounding and live data

• Results are rounded for display, so a worked example done by hand can differ from the tool in the last digit.
• Some forex and crypto tools pre-fill live reference prices from public data feeds. If a feed is unavailable the tool falls back to a recent reference value — always check prices against your own broker or exchange before acting.
• Broker-specific details (contract sizes, swap rates, margin tiers, exchange fees) vary. Where they matter, the calculator lets you enter your own values.

5. Limits

A calculator is exact about arithmetic and silent about judgement. It cannot tell you whether your win probability estimate is realistic, whether a market will move, or how a tax authority will treat your gains. Tax calculators give simplified estimates and are not tax advice. See the terms of use for the full disclaimer.