Bitcoin Block Difficulty Success Probabilities

• Bitcoin block difficulty-based success probability is defined by the inverse relationship between the network difficulty and individual hash success, which forms the basis for closed-form revenue and risk assessments.
• The model employs Bernoulli and Binomial distributions to quantify expected block rewards, while risk metrics such as zero-revenue probability, VaR, and upside potential inform mining strategy and protocol security.
• Extensions like the Bobtail protocol demonstrate how altering the mining order statistics can reduce variance and mitigate centralization risks, offering valuable insights into alternative proof-of-work schemes.

Bitcoin mining operates as a stochastic process characterized by block difficulty-based success probabilities for individual hashes. Each hash attempt is a Bernoulli trial with a rigorously defined probability of success determined by the current network difficulty parameter. This establishes direct, closed-form relationships between mining parameters, expected revenue, risk metrics, miner advantage, and attack models. The following sections detail the mathematical formulation, probabilistic structure, model generalizations, and implications, citing key references from the literature.

1. Definition of Block Difficulty-Based Success Probability

Let $D$ denote the network difficulty and $T_1$ the "difficulty-1 target," a fixed threshold on SHA-256 outputs. The active block target $\tau$ satisfies $\tau = T_1/D$, and a hash is successful if it produces an integer $h \in [0, 2^{256}-1]$ with $h < \tau$. The probability that a single hash attempt is successful is

$$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$

where the normalization $T_1/2^{256} = 1/2^{32}$ is conventional in Bitcoin (Cai et al., 23 Dec 2025, Bissias et al., 2017). Thus, block difficulty $D$ inversely determines the per-hash success probability—higher $D$ reduces $T_1$0, thereby increasing the expected number of trials per block.

2. Statistical Structure of Mining and Revenue

Mining consists of $T_1$1 independent Bernoulli($T_1$2) trials over a time window $T_1$3 for hash rate $T_1$4. The number of successfully mined blocks, $T_1$5, follows a Binomial($T_1$6, $T_1$7) distribution. Expected BTC-denominated revenue is

$T_1$8

with $T_1$9 the current block reward (Cai et al., 23 Dec 2025). Dollar-denominated revenue includes the conversion via the BTC-USD price, $\tau$0,

$\tau$1

This enables closed-form quantification of expected revenue per hash rate unit, facilitating fleet sizing, profitability estimates, and ex ante risk assessment for distinct operating conditions.

3. Risk Metrics: Downside, VaR, and Upside Analytics

The lottery-like nature of mining is quantified by analyzing the full distribution of $\tau$2. Key risk metrics include:

• Probability of Zero Revenue:

$\tau$3

utilizing the Poisson approximation for rare-event regimes.

• Value-at-Risk (VaR) and Lower-Tail Analysis:

For loss tolerance $\tau$4 and confidence $\tau$5,

$\tau$6

where $\tau$7 is the standard normal CDF and a normal approximation to $\tau$8 is employed.

• Upside Potential:

Probability of $\tau$9, $\tau = T_1/D$0,

$\tau = T_1/D$1

These formulations support ex ante, scenario-based risk quantification rather than ex post proxies (Cai et al., 23 Dec 2025).

4. Exact Block-Winning Probability and the Matthew Effect

The naive theory posits that a miner's share of block wins is proportional to its computational power $\tau = T_1/D$2. However, an exact analysis for $\tau = T_1/D$3 miners, each with share $\tau = T_1/D$4 and $\tau = T_1/D$5 valid solutions per contest, reveals a bias benefiting larger miners.

The exact block-winning probability for miner $\tau = T_1/D$6 is

$\tau = T_1/D$7

where $\tau = T_1/D$8 and $\tau = T_1/D$9 (Zeng et al., 2019). When $h \in [0, 2^{256}-1]$0 (number of solutions) is small, the block-winning probability $h \in [0, 2^{256}-1]$1 deviates above $h \in [0, 2^{256}-1]$2 for the largest miner, and below $h \in [0, 2^{256}-1]$3 for smaller miners. This is the "Matthew effect": mining proceeds super-linearly favor larger participants. For sufficiently high $h \in [0, 2^{256}-1]$4 (small $h \in [0, 2^{256}-1]$5), coalitions can achieve a greater than $h \in [0, 2^{256}-1]$6 chance of block wins with less than $h \in [0, 2^{256}-1]$7 of aggregate hash rate, exposing structural risks in PoW (Zeng et al., 2019).

$h \in [0, 2^{256}-1]$8	$h \in [0, 2^{256}-1]$9	$h < \tau$0	$h < \tau$1	$h < \tau$2
0.10	0.05556	0.02778	0.01111	0.00556
0.30	0.21429	0.15000	0.06600	0.03300
0.50	0.50000	0.37500	0.23125	0.14688

When $h < \tau$3, the exact probability reverts to the share-based approximation.

5. Block-Arrival Process, Difficulty Retargeting, and Generalizations

Bitcoin’s block-generation dynamics are formally modeled as a Poisson process in the classical (single-target, $h < \tau$4) case with exponential inter-block times parameterized by $h < \tau$5 (Bissias et al., 2017). The time between blocks $h < \tau$6 satisfies

$h < \tau$7

Difficulty retargeting recalibrates $h < \tau$8 every $h < \tau$9 blocks to stabilize the mean block interval $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$0. This induces a dependence between consecutive periods: during each $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$1-block epoch, block arrival times are i.i.d. exponential with mean $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$2, but $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$3 is functionally dependent on the sum of previous intervals. The marginal law for inter-arrival times, mixing over the random $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$4, is the Lomax distribution: $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$5 with mean $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$6 and variance $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$7 (Fullmer et al., 2018). Thus, $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$8 targeting overshoots the mean block interval by $$p(D) = \frac{\tau}{2^{256}} = \frac{T_1}{D \cdot 2^{256}} = \frac{1}{D \cdot 2^{32}}$$9, inflating both expectation and variance slightly.

6. Alternative Proof-of-Work Schemes and Block Variance Mitigation

The "Bobtail" protocol generalizes Bitcoin's mining by triggering block production when a function of the $T_1/2^{256} = 1/2^{32}$0 smallest observed order statistics falls below a $T_1/2^{256} = 1/2^{32}$1-dependent target, not just the single minimum (Bissias et al., 2017). For Bobtail ($T_1/2^{256} = 1/2^{32}$2), inter-block times follow a phase-type distribution rather than exponential, with substantially reduced variance for higher $T_1/2^{256} = 1/2^{32}$3. Security outcomes, such as double-spend or selfish-mining success probabilities, become less favorable to attackers as $T_1/2^{256} = 1/2^{32}$4 increases, even for the same average block interval. The per-hash success probability remains $T_1/2^{256} = 1/2^{32}$5, but the statistical structure of block finding and attack modeling fundamentally changes.

7. Implications and Applications

The block difficulty-based success probability framework provides a unified ex ante foundation for quantifying expected mining revenue, risk (VaR, CV, zero-revenue), and upside, forming the basis for decision-theoretic sizing, pool behavior, and comparative assessment of protocol modifications (Cai et al., 23 Dec 2025, Zeng et al., 2019, Bissias et al., 2017). It reveals the inadequacy of ex post hash-price proxies for risk-sensitive planning and motivates analyses of economic incentives, strategic pooling, and protocol-level defenses against centralization and exploitation phenomena.

Calibration against large public Bitcoin mining operations confirms the predictive accuracy of the Bernoulli-trial model and quantifies the economic impact of variance reduction, pool fee structure, and hardware improvements (Cai et al., 23 Dec 2025). The outlined probability structure and its generalizations remain essential for robust economic, security, and protocol design analyses in proof-of-work blockchains.