What is a unique visitor?
A unique visitor is one person counted once in a period however often they visit. How uniques are counted with and without cookies, and why totals never add up.
What is a unique visitor?
A unique visitor is one person counted once within a reporting period, no matter how many times they visit or how many pages they view. Analytics tools identify them with a cookie, a device identifier, or a hashed fingerprint of request signals. The count is always an estimate, because one person on three devices is three visitors.
The formula.
Unique visitors = count of distinct visitor identifiers in the period
- visitor identifier
- A cookie value, a stored id, or a hash of signals such as IP address, user agent and site domain combined with a rotating secret salt.
- period
- Uniques do not add across periods. Daily uniques summed over a week always exceed weekly uniques, because returning visitors are counted once per day but once per week.
Every method counts browsers, not humans. A person on a phone and a laptop is two unique visitors in every analytics tool ever built.
A worked example.
One site, one week, three ways of counting.
| Pageviews | 24,800 |
|---|---|
| Sessions | 9,100 |
| Sum of the seven daily unique counts | 7,350 |
| Weekly unique visitors, deduplicated | 6,020 |
The daily uniques over-count the week by 1,330
The difference is people who came back on more than one day. This is why you cannot add uniques, and why a monthly figure must be recomputed rather than summed from dailies.
How visitors are identified
There are three approaches in common use, and each one trades accuracy against privacy.
- A first-party cookie holding a random id. Persists across days and browser sessions, which makes long-window counting possible. It is also the thing that requires a consent banner in the EU and UK.
- A hash of request signals with a daily-rotating salt. No storage on the device at all, so no consent requirement, but the identifier deliberately stops working after 24 hours.
- A user id you provide from your own backend. The most accurate option and the only one that survives a device change, available only for signed-in visitors.
Mrkr uses the second by default and supports the first as an optional per-site cookie mode. Cookie mode gives you cross-day identity and, on that site, does require a consent banner.
What each approach can and cannot tell you
Cookieless, daily-rotating hash
- Accurate daily unique visitors
- No consent banner, because nothing is stored on the device
- Cannot recognise the same person tomorrow
- Weekly and monthly uniques are estimates, not exact counts
Cookie mode, or your own user id
- Unique visitors over any window, including 30 days
- Multi-session journeys and long attribution windows
- Cookie mode requires consent on that site
- Still counts browsers unless you supply a real user id
See it on real data
The live demo is a fully populated dashboard with real demo traffic. No signup and no card required.
Open the live demo
Why unique counts never quite agree
One person, several devices
Phone, laptop and work machine are three identifiers in every tool. There is no fix short of a login.
Privacy browsers and blockers
Safari and Firefox cap or clear first-party cookie lifetimes, which quietly inflates unique counts by turning one returning visitor into several new ones.
Bots that look human
Unfiltered crawler traffic lands directly in the unique count. On one live site running Mrkr, filtered bot hits ran at 4.1 times the human traffic over 30 days.
Different windows, different tools
One tool reporting 30-day uniques and another reporting summed dailies will never match, and neither is broken.
How Mrkr counts them
In its default cookieless mode, Mrkr derives a visitor identifier by hashing the IP address, user agent and site domain together with a secret salt that rotates every day. The same person on the same day produces the same hash; tomorrow they produce a different one, so nobody can be followed across days.
That is the deliberate trade. Daily uniques are accurate and require no banner. Cross-day identity is unavailable unless you switch that site into cookie mode, which restores it and brings a consent requirement with it.
Known bots and crawlers are classified and excluded before counting, so the unique visitor number reflects people rather than infrastructure.
Questions, answered.
Related terms.
- What is pageview?
A pageview is a single recorded view of a page on your site.
- What is session?
A session is one continuous visit to a site: a group of pageviews and events from the same visitor with no long gap between them.
- What is cookieless tracking?
Cookieless tracking is any method of recognising visits without storing an identifier in the browser.
- What is web analytics?
Web analytics is the collection, measurement and analysis of what happens on a website: who visits, where they came from, which pages they see, and what they do before they leave.
- What is cookie banner?
A cookie banner is the notice that asks a visitor's permission before storing or reading information on their device.
Where this shows up in Mrkr
- How Mrkr counts visitors without cookies
The daily-rotating salted hash, the legal reasoning, and the honest trade-off.
- See what website visitor tracking looks like in Mrkr
- Browse the full analytics glossary
Every metric, method and privacy term, defined in one place.
Your first visitor is already here.
Drop in the script and watch them land. It takes about a minute.