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Bitcoin Data GuideBitcoin Reference

Network Stories · Congestion

The 2017 Congestion

In late 2017 the queue of unconfirmed transactions grew faster than blocks could clear it, and the fee needed to be included in the next block rose to record levels. The episode made block space the year's central argument.
Written by
The Bitcoin Data Guide editorial team
Reviewed by
Protocol review desk
Dates
Published 2026-09-21 · Last reviewed 2026-09-21

Fact

The mempool held a sustained backlog and median fees rose sharply during the period.

Forecast

Which single cause drove the demand is contested, and this page does not pick one.

What the record shows

A block has a fixed capacity, and when more transactions are broadcast than the next block can hold, the remainder waits in the mempool. Through late 2017 the mempool did not clear: the backlog persisted across many blocks, and the fee a sender had to offer to be included in the next block rose accordingly.

The fee market is an auction for the limited space in a block. Senders who value confirmation sooner bid higher, and miners generally include the highest-paying transactions first. When the queue is short, a low fee is enough; when it is long, the clearing price rises. The episode is a clear illustration of that mechanism under sustained load.

The period also overlapped with the activation of Segregated Witness, which changed how block capacity is measured. The block-weight rule replaced the simple byte limit and gave witness data a discount, which raised the effective capacity of a block without changing the block-size parameter.

Timeline

  1. 2017The mempool holds a sustained backlog and median fees rise to record levels.
  2. August 2017Segregated Witness activates, introducing the block-weight rule and a discount for witness data.
  3. December 2017The backlog and fee levels reach their peak for the episode before easing into 2018.

What the record does not settle

The queue and the fees are measurable. Why demand rose is not settled by the block record alone: several explanations were offered at the time, and the chain does not label a transaction with its motive. This page describes the mechanism and stops there.

The mempool reference explains how the queue works, and the SegWit explainer covers the capacity change that followed.

Sources

Bitcoin Core release notes — 0.15
The release that shipped the block-weight accounting and fee estimation changes in the period.
BIP 141 — Segregated Witness
The proposal that separated witness data and introduced the block-weight rule.