Gas and transaction fees in Blockchain / Solidity

Introduction

Gas and transaction fees help pay for the work computers do to process and confirm your actions on a blockchain.

When you want to send cryptocurrency to a friend.

When you want to run a smart contract on the blockchain.

When you want to create or update digital assets like NFTs.

When you want to vote or participate in blockchain governance.

When you want to move tokens between different blockchain accounts.

Syntax

Blockchain / Solidity

transaction {
  gasLimit: number
  gasPrice: number
  data: bytes
  to: address
  value: number
}

gasLimit is the maximum amount of gas you allow for the transaction.

gasPrice is how much you pay per unit of gas, usually in a small currency unit like gwei.

Examples

This example shows a simple transfer of 1 ether with a standard gas limit and gas price.

Blockchain / Solidity

transaction {
  gasLimit: 21000
  gasPrice: 50 gwei
  to: 0xabc123...
  value: 1 ether
}

This example is for calling a smart contract function, which needs more gas and possibly no ether sent.

Blockchain / Solidity

transaction {
  gasLimit: 100000
  gasPrice: 100 gwei
  to: 0xdef456...
  data: smart contract call data
  value: 0
}

Sample Program

This simple JavaScript program calculates the total transaction fee in wei by multiplying the gas limit by the gas price in wei.

Blockchain / Solidity

const gasLimit = 21000;
const gasPriceGwei = 50;
const gasPriceWei = gasPriceGwei * 1e9;
const totalFeeWei = gasLimit * gasPriceWei;

console.log(`Gas limit: ${gasLimit}`);
console.log(`Gas price (gwei): ${gasPriceGwei}`);
console.log(`Total transaction fee (wei): ${totalFeeWei}`);

Important Notes

Gas fees can change depending on network demand.

If you set gasLimit too low, your transaction might fail but you still pay fees.

Higher gasPrice can make your transaction confirm faster.

Summary

Gas measures the work needed to process a transaction.

Transaction fees = gas used x gas price.

Fees pay miners or validators to include your transaction.

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