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.
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.
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.
Compared
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
Live demo
See it on real data. The live demo is a fully populated dashboard on demo traffic, with no signup and no card.
Open the live demoWhy 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.
FAQ
What is the difference between a unique visitor and a session?
A unique visitor is a person counted once in the period. A session is one visit with a defined start and end. One unique visitor can produce many sessions, and one session always belongs to exactly one visitor.
Why do daily unique visitors not add up to the monthly figure?
Because returning visitors are counted once in each day they appear but only once across the month. Summing dailies always over-counts. A monthly unique figure has to be computed over the whole month, not aggregated from daily numbers.
Can you count unique visitors without cookies?
Yes, within a day. A salted hash of request signals identifies a returning visitor for 24 hours without storing anything on their device, which is why it needs no consent. It cannot recognise them tomorrow, by design, so multi-day unique counts from cookieless data are estimates.
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