Gas and transaction fees in Blockchain / Solidity - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is 'gas' in blockchain transactions?

Gas is a unit that measures the amount of computational effort required to execute operations on a blockchain like Ethereum. It helps pay for transaction processing.

beginner

Why do blockchain transactions require fees?

Fees pay miners or validators to process and confirm transactions. They prevent spam and ensure network security by rewarding those who maintain the blockchain.

intermediate

What happens if you set a low gas price for a transaction?

The transaction may take longer to be processed or might not be processed at all because miners prioritize transactions with higher fees.

intermediate

How is the total transaction fee calculated?

Total fee = Gas used × Gas price. Gas used is how much work the transaction needs, and gas price is how much you pay per unit of gas.

intermediate

What is the difference between 'gas limit' and 'gas price'?

Gas limit is the maximum gas you allow for a transaction to use. Gas price is how much you pay per unit of gas. Together they control the max fee you pay.

What does 'gas' measure in a blockchain transaction?

A Computational work needed

B Amount of cryptocurrency sent

C Time taken to confirm transaction

D Number of users on the network

Gas measures the computational work required to execute a transaction or contract.

Why might a transaction with a low gas price take longer to confirm?

A Low gas price means more work needed

B It uses less gas overall

C Miners prefer transactions with higher fees

D It is automatically rejected

Miners prioritize transactions that pay higher fees to maximize their rewards.

How do you calculate the total fee paid for a transaction?

A Gas price divided by gas used

B Gas limit plus gas price

C Gas limit minus gas used

D Gas used multiplied by gas price

Total fee = Gas used × Gas price.

What is the purpose of setting a gas limit?

A To limit the maximum gas spent on a transaction

B To set the price per gas unit

C To speed up the transaction

D To increase the transaction size

Gas limit sets the maximum gas the transaction can consume to avoid overspending.

Which of these is NOT true about transaction fees?

A They reward miners or validators

B They guarantee instant transaction confirmation

C They depend on gas price and gas used

D They prevent network spam

Fees do not guarantee instant confirmation; higher fees increase priority but confirmation depends on network conditions.

Explain how gas and gas price work together to determine transaction fees.

Think about how much work is done and how much you pay for each unit of work.

Describe why transaction fees are important for blockchain networks.

Consider what motivates people to keep the network running and safe.

Practice

1. What does gas represent in a blockchain transaction?

easy

A. The amount of computational work required to process the transaction

B. The total amount of cryptocurrency sent in the transaction

C. The number of transactions in a block

D. The time it takes to confirm a transaction

Solution

1. Step 1: Understand the role of gas in blockchain

   Gas is a unit that measures how much computational effort is needed to execute operations like transactions or smart contracts.

2. Step 2: Differentiate gas from other transaction elements

   Gas is not the amount of cryptocurrency sent, nor the number of transactions or confirmation time.

3. Final Answer:

   The amount of computational work required to process the transaction -> Option A

4. Quick Check:

   Gas = computational work [OK]

Hint: Gas measures work, not money or time [OK]

Common Mistakes:

• Confusing gas with transaction value
• Thinking gas is transaction speed
• Mixing gas with block size

2. Which of the following is the correct formula to calculate transaction fees?

easy

A. Transaction fees = gas price ÷ gas used

B. Transaction fees = gas price - gas used

C. Transaction fees = gas used + gas price

D. Transaction fees = gas used x gas price

Solution

1. Step 1: Recall the transaction fee formula

   Transaction fees are calculated by multiplying the gas used by the gas price.

2. Step 2: Verify the formula correctness

   Dividing, adding, or subtracting gas used and gas price does not represent the fee calculation.

3. Final Answer:

   Transaction fees = gas used x gas price -> Option D

4. Quick Check:

   Fees = gas used x gas price [OK]

Hint: Multiply gas used by gas price for fees [OK]

Common Mistakes:

• Using division instead of multiplication
• Adding gas used and gas price
• Subtracting gas values

3. Given a transaction that used 21000 gas units and a gas price of 50 gwei, what is the total transaction fee in gwei?

medium

A. 21000 gwei

B. 1,050,000 gwei

C. 420 gwei

D. 50 gwei

Solution

1. Step 1: Identify given values

   Gas used = 21000 units, Gas price = 50 gwei per unit.

2. Step 2: Calculate total fee

   Total fee = gas used x gas price = 21000 x 50 = 1,050,000 gwei.

3. Final Answer:

   1,050,000 gwei -> Option B

4. Quick Check:

   21000 x 50 = 1,050,000 [OK]

Hint: Multiply gas used by gas price directly [OK]

Common Mistakes:

• Multiplying incorrectly
• Confusing gas units with gwei
• Using gas price as total fee

4. Identify the error in this transaction fee calculation code snippet:

gas_used = 30000
gas_price = 20
transaction_fee = gas_used + gas_price
print(transaction_fee)

medium

A. Using addition instead of multiplication for fee calculation

B. Incorrect variable names

C. Missing print statement

D. Gas price should be divided by gas used

Solution

1. Step 1: Review the code logic

   The code adds gas_used and gas_price instead of multiplying them.

2. Step 2: Understand correct fee calculation

   Transaction fee should be gas_used x gas_price, not addition.

3. Final Answer:

   Using addition instead of multiplication for fee calculation -> Option A

4. Quick Check:

   Fee = gas_used x gas_price, not + [OK]

Hint: Fees multiply gas used by gas price, not add [OK]

Common Mistakes:

• Adding gas values instead of multiplying
• Confusing variable names
• Ignoring correct formula

5. A user wants to speed up their transaction by increasing the gas price from 40 gwei to 60 gwei. If the gas used remains 21000, how much more will the user pay in transaction fees compared to before?

hard

A. 600,000 gwei more

B. 1,260,000 gwei more

C. 420,000 gwei more

D. 840,000 gwei more

Solution

1. Step 1: Calculate original fee

   Original fee = 21000 x 40 = 840,000 gwei.

2. Step 2: Calculate new fee

   New fee = 21000 x 60 = 1,260,000 gwei.

3. Step 3: Find difference

   Difference = 1,260,000 - 840,000 = 420,000 gwei more.

4. Final Answer:

   420,000 gwei more -> Option C

5. Quick Check:

   Increase = (60-40)x21000 = 420,000 [OK]

Hint: Subtract old fee from new fee to find extra cost [OK]

Common Mistakes:

• Calculating total fee instead of difference
• Mixing gas price and gas used
• Incorrect multiplication