Blob costs: keep data gas separate from execution gas
A rollup batch can pay for execution gas and blob gas in the same Ethereum transaction. These are distinct resource quantities with distinct prices. Add their currency costs after multiplication, rather than adding raw gas quantities and applying one price. Otherwise a cost chart can attribute a change to the wrong resource market.
EIP-4844 defines blob gas independently from ordinary gas and describes its fee calculation. Check the network rules active at the observation height; later upgrades can change parameters. This worksheet avoids assuming the original proposal’s capacity constants remain current.
Build a two-resource ledger
For each batch-posting transaction, retain execution gas used, effective execution-gas price, blob gas used and applicable blob-gas price. Multiply each quantity by its own price, convert compatible denominations, then sum the resulting ETH amounts. A fee cap is not the realized unit price and a gas limit is not actual gas consumed.
Write posting cost as C = Ge × Pe + Gb × Pb, where e denotes execution and b denotes blobs. Preserve other costs separately: proof verification, additional posting transactions and operator overhead. This total should not be described as the full rollup operating cost unless those items are covered.
Link each transaction to the batches actually represented. One posting transaction may carry multiple blobs or cover many user transactions. A per-user allocation requires the batch’s defined population, not the base-layer transaction count. If the mapping is unavailable, publish cost per posting transaction and leave user allocation unresolved.
Hypothetical separate gas prices
Assume a fictional transaction uses 100,000 execution gas at 20 gwei and 262,144 blob gas at 2 gwei. Because one gwei is 10^-9 ETH, execution cost is 0.002 ETH and blob cost is 0.000524288 ETH. Their sum is 0.002524288 ETH.
| Resource | Quantity | Hypothetical unit price | Cost |
|---|---|---|---|
| Execution gas | 100,000 | 20 gwei | 0.002 ETH |
| Blob gas | 262,144 | 2 gwei | 0.000524288 ETH |
| Combined cost | Separate resources | No single gas price | 0.002524288 ETH |
Adding both quantities and multiplying by 20 gwei would produce 0.00724288 ETH, which is incorrect for this example. Resource labels cannot be erased merely because both names contain the word gas.
If the posting transaction covers 1,000 successful user actions under a stated mapping, equal allocation gives 0.000002524288 ETH per action. If only 500 actions are covered, that allocation doubles without any change in resource prices. Equal allocation is an accounting convention; more resource-intensive actions may justify a different rule.
Explain a cost movement
Compare quantities and prices independently. Lower blob cost may reflect a lower unit price, fewer blobs, better compression or a different batch population. Lower posting cost does not establish that execution became cheaper, and lower cost per action does not establish that user fees fell equally.
Create an unchanged-price comparison: price the later resource quantities at the earlier prices. Show the remaining repricing contribution under that chosen decomposition. Keep allocation changes, coverage changes and batch-fill changes visible rather than presenting them all as network efficiency gains.
If the provider reports an all-in fee, verify whether blob cost is already included before adding it again. Preserve original receipt fields and the calculation path. A dashboard exposing only ordinary gas fields may omit a material component instead of reporting a truly cheaper transaction.
The result should be a resource-cost bridge with a defined batch population. It cannot determine operator profit, token-holder revenue or future fee levels without additional evidence. Recheck when posting architecture, compression, batching policy or network accounting rules change.
Track batch occupancy as well as compression. Paying for a nearly empty data container can produce higher cost per action even at unchanged resource prices. Record the container count, encoded payload size and covered action count where available. Lower bytes per action and higher utilization are different operational explanations and should not be presented as a single unexplained fee improvement.
FAQ
Can execution gas and blob gas form one gas figure?
They are separate resources. Convert each into cost using its own price before adding currency amounts.
Does lower posting cost prove lower user fees?
No. User pricing, allocation, subsidies and other operating costs can differ from base-layer posting expenses.