Example of how a block's base gas fee is calculated on Ethereum

eth_base_fee_calc.py

# Reference: https://github.com/ethereum/EIPs/blob/master/EIPS/eip-1559.md

BASE_FEE_MAX_CHANGE_DENOMINATOR = 8
ELASTICITY_MULTIPLIER = 2

# Calculate expected base fee for block 12965001

current_block = 12965000 # London Upgrade block
# Get the following values from Etherscan: https://etherscan.io/block/12965000
parent_base_fee_per_gas = 1000000000 # 1 Gwei
parent_gas_target = 15014561
parent_gas_used = 30025257

if parent_gas_used == parent_gas_target:
    	# Perfect, gas target was met, expected base fee stays the same
	expected_base_fee_per_gas = parent_base_fee_per_gas
elif parent_gas_used > parent_gas_target:
    	# Spent more gas, expected base fee increases
	gas_used_delta = parent_gas_used - parent_gas_target
	base_fee_per_gas_delta = max(parent_base_fee_per_gas * gas_used_delta // parent_gas_target // BASE_FEE_MAX_CHANGE_DENOMINATOR, 1)
	expected_base_fee_per_gas = parent_base_fee_per_gas + base_fee_per_gas_delta
else:
    	# Spent less gas, expected base fee decreases
	gas_used_delta = parent_gas_target - parent_gas_used
	base_fee_per_gas_delta = parent_base_fee_per_gas * gas_used_delta // parent_gas_target // BASE_FEE_MAX_CHANGE_DENOMINATOR
	expected_base_fee_per_gas = parent_base_fee_per_gas - base_fee_per_gas_delta

# In this example expected_base_fee_per_gas = 1124967822 (1.124967822 Gwei)
# Answer matches block exactly: https://etherscan.io/block/12965001
print("Expected base fee per gas for block " + str(current_block + 1) + ": " + str(expected_base_fee_per_gas))