EIP-1559 Base Fee Calculations

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EIP-1559 Base Fee Calculations

Dynamic base fee mechanism adjusting based on block gas utilization.

Overview

EIP-1559 introduced a dynamic base fee that:

• Targets 50% full blocks (gasLimit / 2)
• Increases up to 12.5% per block when above target
• Decreases up to 12.5% per block when below target
• Enforces minimum of 7 wei

BaseFee

BaseFee(
  parentGasUsed: bigint,
  parentGasLimit: bigint,
  parentBaseFee: bigint
): BaseFee

Construct next block’s base fee using EIP-1559 formula.

Parameters:

• parentGasUsed - Gas used in parent block
• parentGasLimit - Gas limit of parent block
• parentBaseFee - Base fee of parent block (wei)

Returns: Next block’s base fee (wei)

Formula:

gasTarget = gasLimit / ELASTICITY_MULTIPLIER (= gasLimit / 2)

if gasUsed > gasTarget:
  increase = (baseFee * (gasUsed - gasTarget)) / gasTarget / 8
  newBaseFee = baseFee + max(increase, 1)

if gasUsed < gasTarget:
  decrease = (baseFee * (gasTarget - gasUsed)) / gasTarget / 8
  newBaseFee = max(baseFee - max(decrease, 1), MIN_BASE_FEE)

if gasUsed == gasTarget:
  newBaseFee = baseFee

Always: newBaseFee >= MIN_BASE_FEE (7 wei)

Examples:

import * as FeeMarket from './FeeMarket.js';

// At target (50% full): unchanged
const baseFee1 = FeeMarket.BaseFee(
  15_000_000n,   // 50% of 30M
  30_000_000n,
  1_000_000_000n // 1 gwei
);
// baseFee1 === 1_000_000_000n

// Full block: maximum increase (12.5%)
const baseFee2 = FeeMarket.BaseFee(
  30_000_000n,   // 100% full
  30_000_000n,
  1_000_000_000n
);
// baseFee2 === 1_125_000_000n (12.5% increase)

// Empty block: decrease
const baseFee3 = FeeMarket.BaseFee(
  0n,            // Empty
  30_000_000n,
  1_000_000_000n
);
// baseFee3 === 1_000_000_000n (stays same for empty blocks)

// Below target: decrease
const baseFee4 = FeeMarket.BaseFee(
  7_500_000n,    // 25% full (half of target)
  30_000_000n,
  1_000_000_000n
);
// baseFee4 < 1_000_000_000n (decreased)

Edge Cases:

// Minimum base fee enforced
const minFee = FeeMarket.BaseFee(0n, 30_000_000n, 10n);
// minFee >= 7n (MIN_BASE_FEE)

// Very high base fee
const highFee = FeeMarket.BaseFee(
  30_000_000n,
  30_000_000n,
  100_000_000_000n // 100 gwei
);
// highFee === 112_500_000_000n (still 12.5% increase)

getGasTarget

getGasTarget(state: State): bigint

Get target gas usage for a block (50% of gas limit).

Parameters:

• state - Block state with gasLimit

Returns: Target gas (gasLimit / 2)

Example:

const state: FeeMarket.State = {
  gasUsed: 20_000_000n,
  gasLimit: 30_000_000n,
  baseFee: 1_000_000_000n,
  excessBlobGas: 0n,
  blobGasUsed: 0n
};

const target = FeeMarket.getGasTarget(state);
// target === 15_000_000n (30M / 2)

State method:

const target = FeeMarket.State.getGasTarget.call(state);
// Same result

isAboveGasTarget

isAboveGasTarget(state: State): boolean

Check if block gas usage is above target (will increase base fee).

Parameters:

• state - Block state

Returns: true if gasUsed > gasTarget

Example:

const state: FeeMarket.State = {
  gasUsed: 20_000_000n,   // Above target
  gasLimit: 30_000_000n,  // Target is 15M
  baseFee: 1_000_000_000n,
  excessBlobGas: 0n,
  blobGasUsed: 0n
};

const isAbove = FeeMarket.isAboveGasTarget(state);
// isAbove === true (20M > 15M target)

// Base fee will increase in next block
const nextState = FeeMarket.nextState(state);
// nextState.baseFee > state.baseFee

State method:

const isAbove = FeeMarket.State.isAboveGasTarget.call(state);
// Same result

Constants

See constants.mdx#eip1559:

FeeMarket.Eip1559.MIN_BASE_FEE                  // 7n wei
FeeMarket.Eip1559.BASE_FEE_CHANGE_DENOMINATOR   // 8n (12.5% = 1/8)
FeeMarket.Eip1559.ELASTICITY_MULTIPLIER         // 2n (target = limit/2)

Base Fee Dynamics

Increase Rate

Block above target by X% → base fee increases by up to X/2 * 12.5%

// 100% full (100% above target = 50% above target of 50%)
const fee100 = FeeMarket.BaseFee(30_000_000n, 30_000_000n, 1_000_000_000n);
// Increases by 12.5% (maximum)

// 75% full (50% above target)
const fee75 = FeeMarket.BaseFee(22_500_000n, 30_000_000n, 1_000_000_000n);
// Increases by ~6.25%

// 62.5% full (25% above target)
const fee625 = FeeMarket.BaseFee(18_750_000n, 30_000_000n, 1_000_000_000n);
// Increases by ~3.125%

Decrease Rate

Block below target by X% → base fee decreases by up to X/2 * 12.5%

// 0% full (maximum below target)
const fee0 = FeeMarket.BaseFee(0n, 30_000_000n, 1_000_000_000n);
// Stays same for empty blocks

// 25% full (50% below target)
const fee25 = FeeMarket.BaseFee(7_500_000n, 30_000_000n, 1_000_000_000n);
// Decreases by ~6.25%

// 37.5% full (25% below target)
const fee375 = FeeMarket.BaseFee(11_250_000n, 30_000_000n, 1_000_000_000n);
// Decreases by ~3.125%

Equilibrium

Constant 50% utilization maintains stable base fee:

let baseFee = 1_000_000_000n;
const gasLimit = 30_000_000n;
const gasUsed = 15_000_000n; // Exactly at target

for (let i = 0; i < 100; i++) {
  baseFee = FeeMarket.BaseFee(gasUsed, gasLimit, baseFee);
}
// baseFee === 1_000_000_000n (unchanged)

Projecting Base Fees

See BaseFee projections:

// Project 10 blocks with high utilization
const state: FeeMarket.State = {
  gasUsed: 0n,
  gasLimit: 30_000_000n,
  baseFee: 1_000_000_000n, // 1 gwei
  excessBlobGas: 0n,
  blobGasUsed: 0n
};

const fees = FeeMarket.projectBaseFees(
  state,
  10,          // blocks
  25_000_000n, // 83% full (above target)
  0n           // no blobs
);

// fees[0] > 1 gwei (increasing)
// fees[9] >> 1 gwei (compounded increase)

Usage Patterns

Fee Estimation

// Get current state
const state = getCurrentBlockState();

// Calculate next block's base fee
const nextState = FeeMarket.nextState(state);
console.log(`Next base fee: ${FeeMarket.weiToGwei(nextState.baseFee)} gwei`);

// Check if fees are rising
if (FeeMarket.State.isAboveGasTarget.call(state)) {
  console.log('⬆ Fees rising - blocks above target');
} else {
  console.log('⬇ Fees falling - blocks below target');
}

Gas Target Monitoring

const state: FeeMarket.State = { /* ... */ };

const target = FeeMarket.getGasTarget(state);
const utilization = Number(state.gasUsed * 100n / state.gasLimit);
const targetPct = Number(state.gasUsed * 100n / target);

console.log(`Gas: ${utilization.toFixed(1)}% of limit`);
console.log(`Target: ${targetPct.toFixed(1)}% of target (50%)`);

if (state.gasUsed > target) {
  const abovePct = Number((state.gasUsed - target) * 100n / target);
  console.log(`${abovePct.toFixed(1)}% above target → base fee rising`);
}

Implementation

Location: /src/primitives/FeeMarket/BrandedFeeMarket/BaseFee.js

See branded-feemarket.mdx for implementation details.

References

• EIP-1559 Specification
• constants.mdx
• calculations.mdx