How we calculate

We build small, single-purpose calculators for money, health, and business. The goal is the same every time: give you a clear, honest answer fast, then get out of the way. No signup, no ads, no dark patterns. To keep that promise we hold ourselves to a few fixed rules about how results are produced and described.

How we pick a method

For any calculation there is usually more than one accepted formula. When we add a tool, we choose the method by the same order of preference every time:

1. A published standard or official guideline, where one exists — for example, the World Health Organization's BMI cut-offs, or the U.S. Department of Labor's overtime rules.
2. A widely cited, peer-reviewed equation, when there is no single official standard — for example, the Mifflin-St Jeor equation for resting metabolic rate, which is the formula most clinical dietitians reach for.
3. Plain arithmetic the math agrees on, for things like amortization or a debt snowball, where the result is not a matter of opinion once the inputs are fixed.

Where a respected alternative exists, we say so on the tool page and explain why we chose the default. We never silently average competing methods to manufacture a single tidy number.

What “estimate” actually means

Most of our results are estimates, and we use that word on purpose. An estimate is a careful, defensible projection from the numbers you typed — not a guarantee, and not a measurement of you. Two things make a result an estimate:

• The formula is a model of reality, not reality. A metabolic-rate equation describes the average person of your age, sex, height, and weight. Your real metabolism can sit above or below that average by several hundred calories.
• The future is assumed, not known. A savings or payoff projection holds your interest rate, contributions, and timing steady. Real life rarely is.

So when a projection says “$171,538 in 20 years,” the arithmetic is exact on the inputs — and still an estimate, because “6% every year for 20 years” is an assumption no market guarantees. We show the contributed-versus-growth split wherever we can so a number can't be mistaken for a promise.

How we cite our sources

When a tool relies on an external figure — a tax bracket, a recommended daily water intake, a fee schedule — we cite it the same way every time, with three fixed parts: the publisher who actually issued the figure (the agency, the journal, the standards body — never a blog that re-posted it), a direct link so you can confirm it in one click, and the retrieved date we read and recorded it. That convention is identical across every category: a finance tool pins the IRS the same way a health tool pins a clinical journal. One format, no exceptions. Here is exactly how a pinned source reads:

How often we review

A calculator is not “done” when it ships — it is maintained. We sort tools into review bands by how fast their inputs go stale, and the cadence is a published commitment:

Finance tools

Reviewed each December–January, when the IRS publishes the new tax year's figures. Tools pinned to a yearly figure carry a visible “Last reviewed” date.

Health tools

Reviewed when the CDC, WHO, or ACOG change their reference bands — the only events that move a guideline-based result.

Owner

The Osvian editorial process owns this cadence. The maintainers stand behind the math; we do not attach a named individual “reviewed by” byline (see below).

What we will never do

This is the rule that overrides the others. We do not invent numbers, sources, reviewers, accuracy claims, or outcomes to make a tool look more authoritative than it is. Concretely:

• No invented citations. If we can't pin a figure to a real, linkable publisher, we don't state it as fact — we either expose it as an editable assumption or leave it out.
• No fake precision. We don't show a body-fat percentage to two decimals when the method is only good to a few points. Where a result has a known margin, we say so.
• No fabricated authority. No invented reviewer names, no made-up certifications, no “doctor-approved” badge we can't back up, no star ratings or testimonials we didn't receive. We deliberately do not attach a “medically reviewed by” byline to any page, because we will not manufacture a credential we don't hold. If we ever add a qualified reviewer, their real name and credentials will appear here — never before.
• No silent advice. Estimates are framed as educational, not as financial, medical, or legal advice. The suite-wide disclaimer says so, and individual tools add their own honest caveats where the stakes are higher.

Per-tool methodology

Every calculator carries its own “How this is calculated” section with the exact formula, assumptions, and dated sources for that tool. The list below links straight to each one. Tools that already carry structured source citations show a “sources” count; time-sensitive tools show the date they were last reviewed.

More on how we work

Everything here is meant to be checkable. If a figure looks wrong or a source has moved, that's a bug we want to fix.

• About — who builds and stands behind these tools.
• Disclaimer — the limits of an estimate.
• Privacy — what happens to the numbers you type.