Optimistic and ZK RollupsZK
Rollups process transactions off the main chain and post them back in batches. Optimistic rollups assume batches are valid unless someone submits a fraud proof during a challenge period; ZK (zero-knowledge) rollups attach a cryptographic validity proof to every batch. A sequencer orders the transactions in both.
Rollups run transactions off the main chain and post the results back to it. Optimistic rollups assume batches are valid unless someone challenges them. ZK rollups attach a cryptographic proof. For traders the big differences are withdrawal time, fees and trust assumptions.
- Category
- Cryptocurrency and Blockchain
- Reading time
- 4 min
- Also called
- Layer 2 rollups, Optimistic rollup, Zero-knowledge rollup
- Updated
Rollups in plain words
Rollups, like state channels and commit-chains, move transaction processing off the main chain while users keep control of their tokens.1
Optimistic rollups assume batches are valid and don't post proofs on-chain.2
ZK rollups post a validity proof — cryptographic evidence that the new state really comes from executing the batch.3
How a rollup batch reaches Ethereum
Users transact on L2
Trades run on the rollup, cheaply and fast.
Batch the transactions
A sequencer bundles many transactions together.
Post to L1
The batch data and new state go to Ethereum.
Optimistic: wait for challenges
Optimistic rollups allow a challenge period in which anyone can dispute a result.2
ZK: verify the proof
ZK rollups process withdrawals once the contract has checked the validity proof.3
Punish bad batches
If a fraud challenge succeeds, the sequencer that included the bad transaction is penalised.2
Worked examples
Example 1 — Fee sharing in a batch
$0.05 eachA hypothetical L1 posting costs $60 and the batch holds 1,200 transactions.
| L1 cost per transaction | $60 ÷ 1,200 = $0.05 |
Offloading processing away from the root blockchain lets many users share one L1 cost.1
Example 2 — Fast withdrawal fee
9.99 ETH todayA hypothetical trader wants 10 ETH off an optimistic rollup now. A liquidity provider charges 0.1%.
| Fee | 10 × 0.1% = 0.01 ETH |
| Received now | 10 − 0.01 = 9.99 ETH |
A liquidity provider takes over the pending withdrawal and pays the user on layer 1 straight away, for a fee.2
Example 3 — Waiting out the challenge
About 7 daysA hypothetical trader withdraws natively from an optimistic rollup on day 0.
| Challenge period | 7 days |
| Funds usable on L1 | Day 7 |
Without such help, the user has to sit out the roughly one-week challenge period before funds reach Ethereum.2
Optimistic vs ZK rollups
| Aspect | Optimistic rollup | ZK rollup |
|---|---|---|
| How validity is shown | Assumed, open to challenge | Validity proof each batch |
| Native withdrawal time | About one week | As soon as the proof is verified |
| Main cost | Posting data to L1 | Generating proofs |
| Main trust point | At least one honest challenger | Correct proof system and setup |
Where rollups matter for traders
Cheaper trading
Lower Trading Fees and Commissions for on-chain swaps.
Moving between venues
Withdrawal times shape Settlement plans.
Arbitrage
Delays limit cross-chain Liquidity.
Risk checks
Sequencers and bridges add Counterparty Risk.
Advantages and disadvantages
Advantages
- Much lower fees than L1.
- Inherit Ethereum's security for settlement.
- ZK rollups allow fast exits.
- Optimistic rollups are simpler to build on.
Disadvantages
- Optimistic exits take about a week.
- Sequencers can be centralized.
- ZK proofs are costly and complex.
- Bridging adds smart-contract risk.
Going deeper
Read as far as you need. Each level builds on the one before it.
Many trades, one posting
Beginner
Moving the work off the main chain while users keep custody is what makes rollups cheaper.1
The optimistic bet
Intermediate
Optimistic designs accept state updates by default and only check them if someone objects.1
Proofs and exits
Advanced
ZK rollups have no exit delay, because withdrawals go through as soon as the proof is verified.3
Common mistakes
Planning on instant optimistic exits
Native withdrawals wait about a week.
Ignoring fast-exit fees
Liquidity providers charge for speed.
Treating all L2s as equal
Proof systems and sequencers differ.
Forgetting bridge risk
Bridges have been hacked before.
Check your understanding
Test yourself before moving on. Each answer comes with an explanation.
Key points to remember
- Rollups run trades off L1 and post results back.
- Optimistic rollups assume validity unless challenged.
- ZK rollups post a validity proof.
- Optimistic native exits take about a week.
- Fast exits cost a fee to a liquidity provider.
Frequently asked questions
Are rollups as safe as Ethereum?
Why do ZK rollups not need a week?
Can I avoid the 7-day wait?
Further learning
- Settlement
When funds are final.
- Trading Fees and Commissions
How costs add up.
- Liquidity
Moving funds between venues.
- Counterparty Risk
Sequencer and bridge trust.
Sources
This article is written in our own words. The sources below support the specific facts, rules and figures marked with a number in the text. Links open the original publisher’s page.
- NIST. NISTIR 8301: Blockchain Networks: Token Design and Management Overview (opens in a new tab) Checked .
- ethereum.org. Optimistic rollups (opens in a new tab) Checked .
- ethereum.org. Zero-knowledge rollups (opens in a new tab) Checked .
Educational content, not financial advice. Examples use hypothetical prices and simplified costs; actual fees, margin rules and regulations vary by broker, market and country. We are not affiliated with any publisher linked above. Last reviewed . Report an error or request a source credit.