Mining Pool

What Is a Mining Pool?

A mining pool is a coordinated group of Bitcoin miners who combine their computational power to increase the probability of finding valid blocks and earning rewards. Instead of each miner competing independently against the entire network, pool participants contribute their hashrate to a shared effort. When the pool finds a block, the reward is distributed among participants based on the work each contributed.

Solo mining became impractical for most participants as Bitcoin's network difficulty grew. A single modern ASIC miner might wait months or years to find a block on its own. Mining pools solve this by converting the all-or-nothing lottery of solo mining into a more predictable income stream. The tradeoff: pool operators charge fees (typically 0–4%) and control block template construction, which has implications for network decentralization.

How It Works

Mining pools operate through a client-server architecture where the pool operator runs a Bitcoin full node and coordinates work distribution among connected miners.

1. The pool operator selects transactions from the mempool and constructs a candidate block, including a coinbase transaction that encodes the pool's reward address
2. Work units (block headers with varying nonce space) are distributed to connected miners via the Stratum protocol
3. Each miner hashes candidate block headers, searching for a nonce that produces a hash below a target threshold
4. The pool sets a "share difficulty" much lower than network difficulty. When a miner finds a hash meeting this easier target, it submits that "share" as proof of work performed
5. If a submitted share also meets the full network difficulty, a valid block has been found and the pool broadcasts it to the Bitcoin network
6. The pool distributes the block reward (currently 3.125 BTC after the April 2024 halving) plus transaction fees to miners according to the pool's payout method

Share Difficulty and Hashrate Measurement

Shares serve two purposes: they prove a miner is actively hashing, and they allow the pool to estimate each miner's hashrate. The pool adjusts share difficulty per miner so that each submits approximately one share every few seconds. Too easy and the pool server is flooded with submissions. Too hard and the pool cannot accurately measure contribution.

A simplified Stratum V1 interaction between miner and pool looks like this:

// Miner subscribes to the pool
{"method": "mining.subscribe", "params": ["miner-agent/1.0"]}

// Pool assigns work with target difficulty
{"method": "mining.set_difficulty", "params": [4096]}
{"method": "mining.notify", "params": [
  "job_id", "prev_hash", "coinbase1", "coinbase2",
  ["merkle_branch"], "version", "nbits", "ntime", true
]}

// Miner submits a valid share
{"method": "mining.submit", "params": [
  "worker_name", "job_id", "extranonce2", "ntime", "nonce"
]}

Payout Methods

How a pool distributes rewards to its miners is one of the most important factors in pool selection. Each method distributes risk differently between the pool operator and miners.

FPPS (Full Pay Per Share)

The pool pays a fixed amount per valid share, covering both the block subsidy and an estimated transaction fee component. The pool absorbs all block-finding variance: miners receive consistent payouts regardless of whether the pool is lucky or unlucky in any given period. This is the most predictable payout model, but pools charge higher fees (typically 2–4%) to compensate for the risk they absorb.

PPLNS (Pay Per Last N Shares)

Rewards are distributed only when the pool finds a block, proportionally among miners who contributed shares in a recent window (the last N shares before the block was found). PPLNS offers lower fees (often 0–2%) but higher payout variance. If the pool goes through an unlucky stretch without finding a block, miners receive nothing during that period. Over long time horizons with consistent uptime, PPLNS can yield slightly higher returns than FPPS due to lower fees.

PPS+ (Pay Per Share Plus)

A hybrid model: the block subsidy portion is paid at a fixed per-share rate (like FPPS), while the transaction fee portion is distributed using a PPLNS-style calculation based on blocks the pool actually finds. This gives miners stable base income from the subsidy while allowing upside from high-fee blocks.

Comparison

The Mining Pool Landscape

Bitcoin mining is dominated by a small number of large pools. As of early 2026, the network hashrate hovers around 900–1,000 EH/s, with the following approximate distribution:

Exact percentages fluctuate by measurement period. Foundry USA has consistently held the largest share throughout 2025–2026. For a deeper look at how mining economics interact with Bitcoin's fee market, see the mining economics analysis and fee market dynamics research.

Centralization Concerns

The concentration of hashrate among a small number of pools is one of Bitcoin's most discussed security considerations. The top two pools (Foundry USA and AntPool) collectively control roughly 49–55% of total network hashrate, and the top six pools mine approximately 95% of all blocks.

This concentration creates several risks:

• Transaction censorship: with a small number of operators constructing most block templates, there is a practical risk that pools could be compelled by regulation or other pressure to exclude certain transactions
• Reorg vulnerability: in March 2026, Foundry USA mined seven consecutive blocks, triggering a rare two-block chain reorganization that orphaned valid blocks from AntPool and ViaBTC at block height 941,881. The protocol resolved it as designed, but the incident illustrated the stress that sustained hashrate concentration places on block propagation
• Double-spend potential: while a pool controlling 51% of hashrate could theoretically execute double-spend attacks, pool operators are economically incentivized against this since it would destroy trust and the value of their own mining infrastructure

It is important to distinguish between pool operator control and miner control. Individual miners can switch pools freely, meaning a pool's hashrate share can shift rapidly if miners lose confidence in the operator. This dynamic serves as a natural check on pool behavior.

Pool Hopping

Pool hopping is a strategy where miners switch between pools to maximize short-term profitability. It primarily exploits proportional (PROP) payout systems, where each share in a "lucky" (short) round earns more because the full block reward is split among fewer total shares.

Hoppers join pools at the start of rounds when expected value per share is highest and leave when rounds drag on. Research has shown this can reduce honest miners' payoffs by up to 43%. Pool hopping was the primary reason most pools abandoned proportional payouts in favor of PPLNS and PPS-style methods, both of which are resistant to this strategy.

PPLNS specifically mitigates hopping because it only rewards shares in a rolling window before a block is found. A miner who hops in briefly contributes few shares to the window and earns proportionally less.

Stratum V2 and Decentralization Efforts

The original Stratum protocol (V1) gives pool operators exclusive control over block template construction: they select which transactions to include and miners have no input. Stratum V2, a major protocol upgrade, addresses this with a Job Declaration sub-protocol that lets miners running a full node construct their own block templates and propose them to the pool.

Key improvements in Stratum V2:

• Miner-selected templates: miners propose which transactions to include, reducing censorship risk
• End-to-end encryption: binary protocol with authenticated encryption prevents ISPs or state actors from observing or redirecting mining traffic
• Bandwidth reduction: approximately 60–70% lower bandwidth usage compared to V1's JSON-based protocol

In May 2026, seven major mining entities (Foundry, AntPool, F2Pool, SpiderPool, MARA Foundation, Block Inc., and DMND) joined the Stratum V2 Working Group, representing roughly 75% of global hashrate. OCEAN Pool has taken an alternative approach with its DATUM protocol, which gives miners unilateral template control: miners construct the template and the pool cannot reject it.

Why It Matters

Mining pools are fundamental to Bitcoin's security model. They determine which transactions get included in blocks, how mining rewards are distributed, and ultimately how decentralized the network remains in practice. Every Bitcoin layer built on top of the base chain, from the Lightning Network to protocols like Spark, inherits its security from the mining process. The decisions pool operators make about transaction selection directly affect finality guarantees and settlement reliability for all layers above.

For anyone building on Bitcoin or holding Bitcoin, understanding mining pool dynamics provides insight into the network's actual security properties, not just its theoretical ones.

Risks and Considerations

Custodial Risk

Most mining pools hold miner rewards before distributing payouts, creating custodial risk. Pool operators can delay payouts, impose minimum thresholds, or in extreme cases fail to pay at all. Non-custodial alternatives like OCEAN's TIDES payout system address this by paying miners directly in the coinbase transaction, but at the cost of increased on-chain footprint.

Regulatory Pressure

As identifiable entities, pool operators are subject to regulatory pressure in their jurisdictions. This has led to concerns about KYC/AML requirements being imposed on mining or pools being compelled to censor specific transactions. The geographic concentration of major pools in the US and China amplifies this risk.

Fee Opacity

FPPS pools estimate transaction fees when calculating share payments. If the pool consistently mines blocks with higher-than-estimated fees, it keeps the surplus. Miners have limited visibility into whether the fee estimates they receive are fair, making it difficult to compare true returns across pools.

This glossary entry is for informational purposes only and does not constitute financial or investment advice. Always do your own research before using any protocol or technology.