What is Bitcoin uptime?

We measure the share of time the Bitcoin network has been “up” — that is, producing blocks and usable for transactions  — since the genesis block. The first block was mined on Jan 3, 2009 at 02:54:25 GMT, so that timestamp is the start of the clock. Uptime is simply (total time minus known downtime) divided by total time.

Why do some years show less than 100%?

There have been two widely counted network-level incidents. In 2010, a value overflow bug (CVE-2010-5139) led to a chain split and about 8.5 hours of effective downtime until the chain was fixed and reorganized. In 2013, a consensus bug (CVE-2013-3220) required a coordinated upgrade (BIP50); that event is counted as about 6 hours and 20 minutes of downtime. Both are linked above with more detail.

How is this calculated?

Everything on this page runs in your browser. The counter uses the genesis timestamp and the published downtime figures above; no server is involved. The numbers update in real time as time passes.