Solana: Blocks vs slots explained

• Solana reports two metrics: slots, which are scheduled opportunities to produce a block, and blocks, which only increment when a block is actually produced.
• This leads to a discrepancy: the slot count typically exceeds the block count by millions because many slots go unfilled.
• For transaction expiry, Solana uses block height (150 blocks), not slots, so empty slots do not affect how long your transaction remains valid.
• Be mindful that RPC calls and explorers often mix the terminology, so always verify whether you’re dealing with a slot or a block.

​

Main article

• What a slot is.
• What a block is.
• Why slot numbers and block numbers differ (sometimes by millions).
• How this affects transaction validity, especially when using the getLatestBlockhash RPC call.

​

What is a Solana slot?

A timeslot allocated for a potential block.

​

What is a Solana block?

• Solana Explorer: 96542000
• Solscan: 96542000
• SolanaFM: 96542000

% curl --request POST \
     --url CHAINSTACK_SOLANA_ARCHIVE_NODE \
     --header 'accept: application/json' \
     --header 'content-type: application/json' \
     --data '
{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "getBlock",
  "params": [
    96542000,
    {
      "encoding": "jsonParsed",
      "maxSupportedTransactionVersion": 1
    }
  ]
}
'
{"jsonrpc":"2.0","error":{"code":-32009,"message":"Slot 96542000 was skipped, or missing in long-term storage"},"id":1}

​

Example: Slot height vs. block height

• Slot height: 311118891
• Block height: 289445888

​

Where this matters: getLatestBlockhash method

• slot: The slot number at which the block hash was produced.
• blockhash: The latest block hash to use for a new transaction.
• lastValidBlockHeight: The block height until which the transaction is considered valid before expiring (150 blocks from the “current” block height).

curl --request POST \
     --url https://solana-mainnet.core.chainstack.com/mt_c524ad9781ae5dccd9ac6f5b0370e/ \
     --header 'accept: application/json' \
     --header 'content-type: application/json' \
     --data '
{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "getLatestBlockhash"
}
'

{
  "jsonrpc": "2.0",
  "result": {
    "context": {
      "apiVersion": "2.0.18",
      "slot": 311115149
    },
    "value": {
      "blockhash": "AAU3aKZUQbddu7Q6sP8sggmQme7B9H1FAZvjVvkz5MZq",
      "lastValidBlockHeight": 289442306
    }
  },
  "id": 1
}

• The "slot": 311115149 matches the slot height.
• The "lastValidBlockHeight": 289442306 is about 22 million behind the slot number.

​

How Solana decides transaction validity

• Blocks, not slots, are used in these calculations.
• If some slots are empty, they do not count toward that total of 150—because no block was produced there!

​

Quick illustration with getBlock

curl --request POST \
     --url https://<YOUR_SOLANA_NODE> \
     --header 'accept: application/json' \
     --header 'content-type: application/json' \
     --data '
{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "getBlock",
  "params": [
    311115149,
    {
      "encoding": "jsonParsed",
      "maxSupportedTransactionVersion": 1
    }
  ]
}
' | jq '.'

{
  "jsonrpc": "2.0",
  "result": {
    "blockHeight": 289442156,
    "blockTime": 1735703553,
    "blockhash": "AAU3aKZUQbddu7Q6sP8sggmQme7B9H1FAZvjVvkz5MZq",
    ...
  },
  "id": 1
}

• The slot passed in as a parameter is 311115149.
• The block height returned is 289442156.

​

Getting the latest slot vs. the latest block

• getSlot | Solana gives you the latest slot height.
• getBlockHeight | Solanagives you the latest block height.
• getBlockTime | Solana gives you the latest block time but you need to use the slot as a parameter in the call. If you are not using an archive node, make sure you provide the slot that is about within the last 24 hours.

curl --request POST \
     --url https://solana-mainnet.core.chainstack.com/mt_c524ad9781ae5dccd9ac6f5b0370e/ \
     --header 'accept: application/json' \
     --header 'content-type: application/json' \
     --data '
{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "getSlot"
}
'

curl --request POST \
     --url https://solana-mainnet.core.chainstack.com/mt_c524ad9781ae5dccd9ac6f5b0370e/ \
     --header 'accept: application/json' \
     --header 'content-type: application/json' \
     --data '
{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "getBlockHeight"
}
'

curl --request POST \
     --url https://solana-mainnet.core.chainstack.com/mt_c524ad9781ae5dccd9ac6f5b0370e/ \
     --header 'accept: application/json' \
     --header 'content-type: application/json' \
     --data '
{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "getBlockTime",
  "params": [
    166974442
  ]
}
'

​

Common pitfalls

1. Mistaking a slot for a block Many Solana explorers incorrectly label URLs or pages with the word “block” while they’re actually referencing a slot. For instance:
   • Solscan’s URL might say block/311115149, but that number is a slot.
   • The official Solana explorer also says block/311115149, but again, it’s a slot, not a block.
2. Transaction expiration If you assume 150 slots for expiration instead of 150 blocks, you’ll miscalculate how long your transaction can remain in the mempool. In reality, empty slots do not affect transaction expiry. Only blocks do.
3. RPC naming confusion The getBlock method uses a slot number as its parameter. But the response includes the block height (blockHeight). Don’t let the naming fool you—this is a “fetch block by slot” call.

​

Why slot and block numbers are always different

• Slots: Every slot increments by one whether or not a block is produced.
• Blocks: Only increment when a block is actually produced.

​

Conclusion

Ake

Director of Developer Experience @ Chainstack
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