eth_getUncleCountByBlockNumber

eth_getUncleCountByBlockNumber returns the number of uncle blocks referenced by the block at a given height or block tag ("latest", "earliest", "safe", "finalized", etc.), as a hex-encoded integer. On Arbitrum One — the ETH-fee Optimistic Rollup built on Nitro/ArbOS — that count is always "0x0", because the chain's single sequencer orders blocks with no proof-of-work fork race and therefore never produces ommers. It is the height-addressed counterpart of eth_getUncleCountByBlockHash, retained for Ethereum JSON-RPC compatibility. The endpoint is https://arbitrum.therpc.io/YOUR_API_KEY (chain ID 42161, 0xa4b1).

Use cases

• Scan a sequential range of block heights for uncle presence to compute an uncle rate; on Arbitrum One every height answers "0x0", so the computed rate is a constant zero.
• Run a quick count check before fetching uncle details with eth_getUncleByBlockNumberAndIndex; since Arbitrum One always reports zero, the detail fetch can be skipped.

Parameters

Response

Example request

1curl https://arbitrum.therpc.io/YOUR_API_KEY \
2  -X POST \
3  -H "Content-Type: application/json" \
4  -d '{
5    "jsonrpc": "2.0",
6    "method": "eth_getUncleCountByBlockNumber",
7    "params": ["latest"],
8    "id": 1
9  }'

Try it live in the Arbitrum One playground.

Errors & troubleshooting

Common pitfalls

• Arbitrum One always returns "0x0" regardless of the block height or tag, the same as post-Merge proof-of-stake chains; do not build logic that expects a non-zero uncle count here.
• The result is a hex string such as "0x0", not a number — parse it (for example with parseInt(value, 16)) before any numeric comparison.

Supported networks

• Mainnet — Chain ID: 42161
• Sepolia — Chain ID: 421614

See also

• eth_getUncleByBlockNumberAndIndex
• eth_getUncleCountByBlockHash
• eth_getBlockByNumber

1curl https://arbitrum.therpc.io/YOUR_API_KEY \
2  -X POST \
3  -H 'Content-Type: application/json' \
4  -d '{"jsonrpc":"2.0","id":1,"method":"eth_getUncleCountByBlockNumber","params":[""]}'