Gas and transaction fees in Blockchain / Solidity - Step-by-Step Execution

Concept Flow - Gas and transaction fees

User creates transaction

↓

Estimate gas needed

↓

Set gas price

↓

Calculate total fee = gas * gas price

↓

Send transaction with fee

↓

Miner validates and executes

↓

Transaction included in block

↓

User pays fee from balance

This flow shows how a blockchain transaction uses gas to measure work and fees to pay miners.

Execution Sample

Blockchain / Solidity

transaction = {
  'gas_limit': 21000,
  'gas_price': 50,
  'value': 1
}
fee = transaction['gas_limit'] * transaction['gas_price']
print(fee)

Calculate the transaction fee by multiplying gas limit by gas price.

Execution Table

Step	Variable	Value	Action	Output
1	transaction['gas_limit']	21000	Set gas limit for transaction
2	transaction['gas_price']	50	Set gas price per gas unit
3	fee	21000 * 50	Calculate total fee	1050000
4	print(fee)		Output fee to user	1050000
5	-	-	Transaction sent with fee
6	-	-	Miner executes transaction, fee paid
7	-	-	Transaction included in block

💡 Fee calculated and transaction processed with fee paid to miner

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
transaction['gas_limit']	undefined	21000	21000	21000	21000
transaction['gas_price']	undefined	undefined	50	50	50
fee	undefined	undefined	undefined	1050000	1050000

Key Moments - 3 Insights

Why do we multiply gas limit by gas price to get the fee?

Because gas limit is how much work the transaction can use, and gas price is how much you pay per unit of work. Multiplying gives total cost (see execution_table step 3).

What happens if the gas limit is too low?

The transaction may fail because it runs out of gas during execution, but you still pay the fee for used gas (not shown in table but important).

Why does the miner get the fee?

Miners include transactions in blocks and get fees as reward for processing (see execution_table step 6).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'fee' after step 3?

A 21000

B 1050000

C 50

D Undefined

💡 Hint

Check the 'Value' column for 'fee' at step 3 in execution_table.

At which step is the transaction fee actually paid to the miner?

A Step 3

B Step 4

C Step 6

D Step 7

💡 Hint

Look for the step mentioning miner execution in execution_table.

If the gas price doubles, how does the fee change in the execution_table?

A Fee doubles

B Fee halves

C Fee stays the same

D Fee becomes zero

💡 Hint

Fee = gas_limit * gas_price, so changing gas_price affects fee proportionally.

Concept Snapshot

Gas measures work units for a transaction.
Gas price is cost per unit of gas.
Fee = gas limit * gas price.
User pays fee to miner for processing.
If gas limit too low, transaction fails but fee is still paid.

Full Transcript

This visual execution shows how blockchain transactions use gas and fees. First, the user sets a gas limit and gas price. The fee is calculated by multiplying these two values. The transaction is sent with this fee. Miners execute the transaction and receive the fee as payment. Variables like gas_limit, gas_price, and fee change step by step. Key points include why we multiply gas limit by gas price, what happens if gas limit is too low, and why miners get the fee. The quiz tests understanding of fee calculation and payment steps.