Block Size
Average block size per day in megabytes
Short ranges plot one point per block, about one every ten minutes. Longer ranges plot one point per day, computed from every block in that day. How it is computed depends on the chart: some average, some total the day up, some are a ratio of the day's sums, and each chart says which. Either way a brief spike inside a day is smoothed away.
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About this metric
Definition
How much data each block carries, in serialized bytes. The consensus limit is 4,000,000 weight units rather than a byte count, so Weight Utilization is the measure of how full a block is and this is the raw size beside it. A larger block is not better or worse; it means more, or larger, transactions were included.
How it is measured
The serialised size of the block as my node stores it, witness data included. Consensus limits weight rather than bytes, to 4 million weight units, and witness bytes count a quarter as much toward that. This is why blocks pass the old one-megabyte figure. Weight utilisation has its own chart.
Measured from my own Bitcoin node. Methodology