51% Attack Cost Calculator
Attack Cost Calculator
Calculate how much it would cost to rent enough hash power to perform a 51% attack on a proof-of-work cryptocurrency based on its current hash rate.
Estimated Attack Cost
Vulnerability Assessment
Key Takeaways
- Ethereum Classic and Bitcoin Gold are the most‑cited victims of 51% attacks.
- Attack costs drop dramatically as network hash‑rate shrinks; Litecoin can be compromised for a few thousand dollars.
- Double‑spending, chain re‑orgs, and market manipulation are the three main attacker goals.
- Exchanges and tighter confirmation rules provide the best real‑world defense for vulnerable chains.
- Understanding attacker economics helps you judge whether a coin’s security model is adequate.
When a malicious miner controls more than half of a proof‑of‑work network’s computational power, they can rewrite history, double‑spend coins, or simply shut the chain down. This scenario is called a 51% attack a situation where an attacker controls over 50% of the network’s hash rate, allowing them to dictate which blocks become part of the canonical chain. Below we walk through the most concrete incidents, break down how they were executed, and show what the community learned.
How a 51% Attack Works: The Common Playbook
All documented incidents follow a similar three‑step playbook:
- Acquire majority hash rate either by buying hardware, renting power from a mining‑as‑a‑service platform, or rallying an existing mining pool.
- Mine a secret fork that excludes the transactions the attacker wants to erase.
- Release the longer fork; the network’s consensus rule - “the longest valid chain wins” - forces nodes to adopt the attacker’s version, effectively undoing the original blocks.
Once the attack chain is live, the attacker can:
- Perform a double spending by withdrawing the same coins twice.
- Erase or reorder past transactions, damaging trust.
- Launch a denial‑of‑service by refusing to include new transactions in blocks.
- Trigger market panic and profit from short positions - a strategy known as “attack‑and‑short”.
Ethereum Classic: The Flagship Victim
Ethereum Classic (ETC) has been hit multiple times because its hashrate is a fraction of Ethereum’s. The most infamous series occurred in August 2020, when the network suffered three attacks within a single month. Combined losses topped $10 million as attackers double‑spent ETC on exchanges and then rewrote the chain.
Earlier, in February 2019, Coinbase froze ETC trading after its monitoring system flagged a sudden surge in orphaned blocks - a classic sign of a 51% re‑org. The exchange’s rapid response prevented larger liquidations but highlighted how centralized services become the last line of defense for a decentralized protocol.
Key metrics from the 2020 attacks:
- Hash‑rate needed: ~70 TH/s (≈ 5 % of the total network at the time).
- Attack cost: rental of around $2.5 million in cloud hash power.
- Result: multiple double‑spends worth roughly $5 million each, plus a permanent drop in ETC’s market price.
Bitcoin Gold: The First High‑Profile Fork Attack
Bitcoin Gold (BTG) split from Bitcoin in late 2017 and adopted the same proof‑of‑work algorithm as Bitcoin (SHA‑256). Its initial hash‑rate was only ~1 % of Bitcoin’s, making it an easy target. In May 2018, a coordinated group rented enough SHA‑256 power to control the majority of BTG mining.
Attack details:
- Hash‑rate seized: ~300 TH/s, roughly 52 % of BTG’s total.
- Cost: under $20 k for a short‑term rental via a mining‑as‑a‑service marketplace.
- Outcome: the attackers rewrote ~1 hour of blocks, enabling double‑spends that drained several exchanges of BTG holdings.
The BTG case became a textbook example in MIT’s Digital Currency Initiative research, demonstrating that miners from larger coins can temporarily redirect power to sabotage smaller forks.
Litecoin: An Unexpectedly Cheap Target
Even though Litecoin’s market cap sits in the billions, its security budget is far smaller than Bitcoin’s. A July 2025 YouTube analysis by Hacking Matters showed that controlling 51 % of Litecoin’s hash‑rate would cost only a few thousand dollars in rented cloud power. The attacker could then execute a double‑spend in under an hour.
Why such a low cost?
- Network hash‑rate: ~300 TH/s - orders of magnitude below Bitcoin’s 200 EH/s.
- Hardware rental rates: $0.02 per GH/s per hour in the cloud market.
- Result: $3,000‑$5,000 for a full‑day attack, well within the budget of a determined hacker.
This disparity proves that market cap is a poor proxy for security; what matters is the absolute amount of computational work protecting the chain.
Economic Incentives Behind the Attacks
Attackers weigh three main variables:
- Potential profit - value of double‑spent coins plus any short‑position gains.
- Cost of hash‑rate rental - often a few thousand dollars for small networks, billions for Bitcoin.
- Risk of detection - larger attacks leave a traceable fingerprint in block re‑org data.
For Ethereum Classic, the double‑spend profit outweighed the rental cost, especially when the attacks hit during a price rally. In the Bitcoin Gold case, the attackers chased the “reputation damage” payoff - a brief market panic that let them profit from pre‑placed short bets.
Defensive Measures Adopted After Real‑World Attacks
Communities responded in three practical ways:
- Increase confirmation requirements - exchanges now wait for 12 or more blocks on vulnerable coins before crediting deposits.
- Deploy checkpointing - hard‑coded block hashes that prevent deep re‑orgs beyond a certain depth.
- Promote mining decentralization - incentivizing smaller pools and introducing hybrid consensus models (e.g., PoW/PoS hybrids).
Bitcoin’s built‑in resilience (massive node count, constant monitoring of mining pool shares) makes a successful 51% attack virtually impossible without triggering a community‑wide alarm. Smaller chains, however, rely heavily on exchange‑level safeguards because their own networks can’t absorb the shock.
What a 51% Attack Means for the Future of Blockchain Security
Two trends are shaping the next wave of defenses:
- Hash‑rate leasing markets are maturing. Services now offer on‑demand power with near‑instant provisioning, meaning attackers can spin up a majority in minutes.
- Cross‑chain monitoring tools are emerging. Projects that track hash‑rate migration across SHA‑256, Ethash, and other algorithms can flag suspicious spikes before an attack starts.
For developers, the takeaway is clear: if your blockchain’s security hinges on a few hundred TH/s, you must either boost the hash‑rate, add a secondary consensus layer, or accept that exchanges will impose stricter deposit rules.
Quick Reference: Real‑World 51% Attack Summary
| Target | Date | Hash‑rate needed | Estimated cost | Primary impact |
|---|---|---|---|---|
| Ethereum Classic | Feb 2019 | ≈ 70 TH/s | ≈ $2.5 M (rental) | Double‑spends, exchange freeze |
| Ethereum Classic | Aug 2020 | ≈ 70 TH/s | ≈ $3 M | Multiple $5 M double‑spends, price crash |
| Bitcoin Gold | May 2018 | ≈ 300 TH/s | ≈ $20 k | Re‑org, exchange losses |
| Litecoin | 2025 (simulated) | ≈ 150 TH/s | $3‑5 k (rental) | Potential double‑spend in < 1 h |
Frequently Asked Questions
What exactly is a 51% attack?
It is when a single miner or mining pool controls more than half of a proof‑of‑work network’s total hash‑rate, allowing them to decide which blocks become part of the canonical chain and to rewrite transaction history.
Why are smaller coins like Ethereum Classic more vulnerable?
They have far fewer miners and a much lower overall hash‑rate, so an attacker can rent or buy enough power for a fraction of the cost required to attack large networks such as Bitcoin.
Can an attacker profit without double‑spending?
Yes. By creating market panic and short‑selling the target coin’s price, attackers can make money even if the double‑spend fails. This “attack‑and‑short” tactic was highlighted in a 2025 Hacking Matters analysis.
How do exchanges protect users from these attacks?
Most exchanges now require additional block confirmations for vulnerable coins, monitor blockchain re‑org events in real time, and can freeze trading instantly when suspicious activity appears, as Coinbase did with Ethereum Classic in 2019.
Is Bitcoin immune to 51% attacks?
Practically yes. The Bitcoin network’s hash‑rate runs in the hundreds of exahashes per second, meaning an attacker would need to spend billions of dollars on hardware and electricity to reach 51 % control, which would be instantly noticeable to the community.
Marina Campenni
June 3, 2025 AT 14:14Reading through the attack summaries, I can’t help but feel for the users who lost confidence in their coins; the sudden freezes and price drops ripple far beyond the technical sphere.
Irish Mae Lariosa
June 3, 2025 AT 14:15The article does a decent job of cataloguing incidents, yet it glosses over the fundamental economic miscalculations that make these attacks possible. It repeatedly emphasizes hash‑rate scarcity without exploring why developers continue to launch PoW forks with such thin security margins. Moreover, the piece fails to mention that many of the cited attacks were enabled by cloud‑mining services that openly advertise sub‑hour rental contracts. By presenting the cost figures as static numbers, the author neglects the dynamic market for hash‑power where prices can plummet overnight. The discussion of defensive measures is likewise superficial; merely increasing confirmation counts does not address the root cause of centralised mining pools. A more rigorous analysis would include case studies of successful mitigation strategies, such as the checkpoint system implemented by some newer chains. The narrative also overlooks the political dimension, where exchanges wield disproportionate power to freeze assets, effectively centralising control. While the table of attacks is informative, it lacks a comparative metric that normalises cost against the total market cap of each coin. Readers are left to infer the security‑to‑value ratio, a critical omission for anyone evaluating investment risk. The article’s tone remains neutral, but neutrality can become complicity when it fails to call out negligent project governance. I would have appreciated a deeper dive into the incentives that drive miners to switch hash‑rate between chains on a whim. Instead, the piece settles for a surface‑level description of “rental markets” without discussing their legal and ethical gray zones. The concluding section hints at future defenses, yet it does not propose concrete standards for cross‑chain monitoring tools. In short, the post serves as a checklist rather than an analytical framework for assessing 51 % attack vulnerability. A more thorough approach would empower developers and investors alike to demand robust security designs before commit‑ting capital.
Nick O'Connor
June 3, 2025 AT 14:15The 51 % threat, as you outlined, is not merely theoretical; it is a real risk, especially for low‑hash‑rate networks, and it forces exchanges, developers, and users alike, to reconsider confirmation thresholds, to adopt checkpointing, and to monitor hash‑power shifts, all at once.
Deepak Kumar
June 3, 2025 AT 14:16Hey folks, if you’re building a new PoW chain, think of security as a marathon, not a sprint; start by diversifying your mining pool landscape, incentivize small miners, and consider a hybrid PoW/PoS model to give attackers a much higher economic hurdle. The community thrives when we share tools for real‑time hash‑rate monitoring, so don’t hesitate to contribute your scripts on GitHub.
Miguel Terán
June 3, 2025 AT 14:18Imagine a world where every drop in hash‑rate sent an instant alert across dashboards, where developers could patch vulnerabilities before a rental pool even boots up – that’s the future we’re racing toward, a vibrant, colorful tapestry of cross‑chain vigilance that makes attacks feel like trying to steal a candle from a bonfire.
Shivani Chauhan
June 3, 2025 AT 14:20Just a quick note – while the data is solid, it would help to see a clearer distinction between temporary rental attacks and long‑term mining takeovers; a concise visual could bridge the gap between casual readers and seasoned analysts.
Deborah de Beurs
June 3, 2025 AT 14:21Enough with the polite phrasing – the industry needs to stop flirting with half‑baked security and start demanding heavyweight protection, otherwise we’ll keep watching small coins get ripped apart like cheap paper.
Sara Stewart
June 3, 2025 AT 14:23From a devops perspective, the key takeaway is to implement multi‑layer checkpointing, enforce stricter finality thresholds, and integrate real‑time pool hash‑share analytics into the node client – that’s how we harden the stack against 51 % vectors.
Laura Hoch
June 3, 2025 AT 14:25When we ponder the fragility of consensus, we confront a deeper question: does the pursuit of decentralisation, untempered by robust economic barriers, betray the very principle of trust it seeks to uphold? The answer lies not in mere block counts but in the collective will to defend the ledger, even if that means confronting uncomfortable truths about power concentration.
Hailey M.
June 3, 2025 AT 14:26Wow, another 51% drama, because the crypto world totally needed more fireworks 🙄🔥