Methodology

Choosing a formula

Most quantities have more than one accepted formula, and the choice changes the answer. Basal metabolic rate can be estimated with Mifflin-St Jeor, Harris-Benedict, or Katch-McArdle; body fat has half a dozen competing estimators. Where alternatives exist, we pick the one with the strongest published validation for a general adult population, name it explicitly on the page, and mention the main alternative.

Where a formula is fixed by definition rather than by research — compound interest, loan amortisation, percentage change — we implement the standard form and show it in the method section so you can check our arithmetic against your own.

Stating assumptions

Every calculator embeds assumptions, and hiding them is how a tool becomes misleading while remaining technically correct. A mortgage calculator that returns principal and interest but not tax and insurance is accurate about what it computes and silent about what you will actually pay each month.

Each calculator page therefore carries an assumptions note covering what the formula excludes, what it holds constant, and the range over which it is reasonable. When a tool cannot answer the question you probably came with, we would rather say so than return a confident number.

Validation before publishing

Each tool is checked three ways before it goes live. The implementation is compared against the published formula term by term. A worked example is calculated by hand and published on the page, so any drift between the documented example and the live tool is visible to you as well as to us. And edge cases are exercised deliberately: zero interest rates, zero divisors, negative inputs, and values large enough to lose floating point precision.

The zero-rate case deserves specific mention because it breaks the standard amortisation formula through division by zero. Our loan and mortgage tools branch to a simple division in that case rather than returning an error or a nonsensical figure.

Rounding

Rounding is applied only to the final displayed value, never to intermediate steps. Rounding mid-calculation compounds error, and in multi-period financial projections that error accumulates in a visible direction over hundreds of periods.

Currency results are shown to two decimal places, percentages generally to one, and projections over long horizons are deliberately rounded to a coarser figure — a thirty-year retirement projection displayed to the nearest cent would imply a precision the underlying assumptions cannot support.

Everything runs in your browser

All computation happens locally in JavaScript. Nothing you enter leaves your device, which means results are instant, work offline once the page has loaded, and generate no record on our side. It also means we cannot silently change a result after you have seen it — the code producing your number is the code your browser downloaded.

The trade-off is that we cannot offer live external data such as current exchange rates or market prices. Where a rate is needed, you supply it and we show exactly how it was applied.

Corrections and updates

When an error is reported we reproduce it first, then fix the underlying calculation, then re-check every other tool sharing that logic — a rounding bug in one place is rarely confined to one place. Content pages carry a revision date so you can see how current the material is.

Reports that affect the accuracy of a result take priority over new features. If you believe a tool is wrong, email hello@useutilityatlas.com with the tool name, the inputs you used, the result you got, and the result you expected. That last detail resolves most reports immediately.

Limits of this approach

These are general-purpose tools built on population-level formulas. They cannot know your medical history, tax jurisdiction, or the specific terms of a contract in front of you. A calculator is a way to prepare for a decision — to arrive at a conversation with a professional already understanding the shape of the problem — not a substitute for one. Our terms of use set this out formally.