Cookie Policy

What a Cookie Is, and What localStorage Is

Two different browser storage technologies get lumped together under the word "cookies", and the difference matters more than most cookie notices let on.

A cookie is a small piece of text a website asks your browser to keep. The defining feature is what happens next: your browser attaches that cookie to every subsequent HTTP request it makes to that domain. Load a page, load an image, load a script — the cookie rides along each time, and the server on the other end reads it. That is what makes cookies useful for logins and what makes them useful for tracking. The data travels.

localStorage is a separate store built into your browser. A page can write values into it with JavaScript and read them back later. Crucially, nothing in localStorage is attached to requests. It is never sent to a server unless a script explicitly reads a value and transmits it, and no script on this site does that. The data sits on your device.

A rough way to hold the distinction: a cookie is a note you hand back to the doorman every time you enter the building. localStorage is a note you leave in your own desk drawer.

There are related technologies worth naming, because ad systems use them: sessionStorage (like localStorage, but wiped when you close the tab), IndexedDB (a larger structured store), and pixels or web beacons (a tiny invisible image whose only job is to make your browser send a request somewhere, revealing your IP address and any cookies for that domain). This site uses none of these.

What One Calculator Stores Today

This site sets no cookies of its own. Not one. There is no analytics cookie, no tag manager, no session cookie, no A/B testing cookie, and — because nothing yet requires consent — not even a consent cookie. You can verify this yourself: open your browser's developer tools, go to the Application or Storage tab, and look at the cookie list for onecalculator.app. It will be empty.

What the site does use is localStorage, for exactly two purposes.

History

• What it is — a list of your past calculations on a given calculator. If you worked out an age from 14 March 1991, that entry is stored so you can look back at it without retyping.
• Why it exists — checking a result you calculated five minutes ago should not require entering the birthdate again. It exists for your convenience, and nothing else reads it.
• How much — capped at 20 entries per calculator. Entry 21 pushes out the oldest. The cap is deliberate: it keeps the store small and means an old calculation does not linger on a shared machine indefinitely.
• How long it lasts — until you delete it. localStorage has no expiry date, which is one more way it differs from cookies. It survives closing the tab, closing the browser, and restarting the machine.
• How to remove it — turn History off in Settings, clear it from Settings, or clear site data in your browser. It is optional; leave it off and nothing is written.

Settings

• What it is — your preferences: measurement units for the BMI Calculator, date format for the Age Calculator, and light or dark theme. The theme value is written by the next-themes library, which uses localStorage for exactly this purpose.
• Why it exists — so that choosing kilograms once means you are not shown pounds on your next visit, and so a dark theme does not flash white on every page load.
• How long it lasts — until you change it or clear it.
• How to remove it — clear it from Settings, or clear site data in your browser. The site falls back to its defaults.

Neither of these is transmitted anywhere. They are per-device, so your settings on a phone are unrelated to your settings on a laptop, and there is no account to sync them through — because there are no accounts.

Two things that are not cookies

Server logs. Firebase Hosting, which serves this site, records ordinary request logs: IP address, user agent, URL requested, timestamp. That is a log written by the hosting infrastructure, not a cookie, and it is not something you can decline in a cookie banner. It is covered in the Privacy Policy.

Fonts. The typefaces are compiled into the site at build time by next/font and served from this domain. No request goes to Google Fonts or any other font service when you load a page, so no font-service cookie is set and no third party learns you visited.

Advertising Cookies, Once Ads Are Running

One Calculator is free and is funded by advertising. We intend to display ads through Google AdSense. At the time of writing, advertising is not yet enabled and no advertising cookies are set on this site. This section describes what will happen when it is, so that the change is not a surprise.

Once ads are live, Google and other ad vendors may set and read cookies in your browser and use similar identifiers, including pixels and device identifiers. Broadly, that storage falls into three categories:

• Essential — storage needed for the ad to be delivered and displayed at all, including basic fraud prevention and frequency capping so you are not shown the same ad twelve times.
• Preferences — your own choices, such as a recorded consent decision or an opt-out setting, kept so you are not asked repeatedly.
• Advertising — identifiers used to measure ad performance and, where permitted, to select ads based on your prior browsing across sites.

These cookies are set by the ad vendors, not by us. They are read on their domains, under their privacy policies, and we cannot see them, delete them, or list their exact names in advance — the set of vendors serving a given ad slot changes. Google describes its side in How Google uses information from sites or apps that use our services.

One thing we will not do: nothing you type into a calculator is sent to an ad system. Your height, weight, BMI result, and date of birth stay in your browser. Ads on the BMI Calculator are not chosen from your weight.

How to Control and Delete Everything

On this site. Clear History and Settings from the Settings panel. Because both live in localStorage, that removes them completely — there is no server copy to also delete.

In your browser. Every major browser has a settings section covering privacy and site data. The wording varies — look for "Cookies and site data", "Privacy and security", or "Clear browsing data" — and inside it you can usually clear everything, clear data for one site only, block third-party cookies, or set data to clear automatically when you quit. Clearing site data for onecalculator.app removes your History and Settings along with anything else stored; the site itself keeps working, it just forgets your preferences. Private or incognito windows discard all of it when you close the window.

Opting out of personalised ads. These work at the ad-vendor level, independently of this site:

• Google Ads Settings, now presented as My Ad Center
• aboutads.info, from the Digital Advertising Alliance
• Your Online Choices, from the EDAA in Europe

Opting out does not remove ads. It makes them non-personalised — chosen from the subject of the page and coarse location rather than from a profile of your browsing. Note also that most of these opt-outs are themselves recorded in a cookie, so clearing all cookies clears the opt-out too.

Consent in the EEA, the UK and Switzerland

If you are in the European Economic Area, the United Kingdom, or Switzerland, you will see a consent message before any advertising cookies or similar identifiers are used. It is presented through a consent management platform certified by Google under the IAB Transparency and Consent Framework. Your choice is recorded, and you can change or withdraw it later by reopening the consent tool from the link in the site footer.

Declining means non-personalised or limited ads rather than no ads. Your History and Settings are not part of that consent request: they never leave your device, so there is no third party for you to consent to.

Changes to This Policy

The most likely change to this page is advertising going live, at which point the tense in the advertising section changes from future to present. We will update the text rather than leave it stale, and the effective date at the top tells you when it last changed.

Get in Touch

Questions about storage on this site, or anything in this policy: privacy@onecalculator.app. For the wider picture of what is and is not collected, read the Privacy Policy. For anything else, the contact page has the details.