Fees and fee markets

TIP

The short version. Every transaction pays a fee in the chain's native coin. Fees pay the block producers and stop anyone from flooding the network. Block space is limited, so when many transactions compete, the higher fees go first: a fee market. Each family prices fees differently: gas on EVM chains, bytes on Bitcoin, resources on Tron, compute units on Solana. A fee that is too low leaves a payment stuck until it is replaced with a higher one.

Builds on: Transactions and Accounts, UTXOs and transaction order.

Why fees exist

Every node checks and stores every transaction, forever. If transactions were free, anyone could send millions of them and bring the network to a halt. A fee makes every transaction cost something, and it pays the block producers who do the work.

Fees are always paid in the native coin, even for a token payment. A wallet that holds 1,000 USDT and no ETH cannot send its USDT on Ethereum: it cannot pay the fee.

The fee market

A block has room for a limited number of transactions. When more are waiting in the mempool than fit, block producers pick the ones that pay the most. So the fee needed to be included soon goes up and down with demand: low on a quiet night, high during a market rush. A wallet estimates a fee from what recent blocks accepted, and usually offers a few speeds: slower and cheaper, or faster and dearer.

How each family prices a transaction

Family	A transaction's fee is…	The unit of price
EVM (EIP-1559 networks)	gas used × (base fee + priority tip), capped by a maximum fee per gas	wei per gas (often shown in gwei: 10^9 wei)
EVM (legacy networks, such as BNB Smart Chain)	gas used × gas price	wei per gas
Bitcoin	the transaction's size in virtual bytes × a fee rate	satoshis per virtual byte (sat/vB)
Tron	bandwidth (size) and energy (contract work), paid from staked resources or burned TRX	sun
Solana	a fixed fee per signature + compute units × a priority price	lamports; micro-lamports per compute unit
TON	gas and forwarding fees set by the network's configuration	nanograms
Avalanche X-Chain / P-Chain	a fixed fee / gas × a dynamic gas price	nAVAX

A few ideas recur:

• Gas (EVM) measures computation. A plain ETH transfer costs 21,000 gas; a token transfer costs more, because it runs contract code. The sender sets a gas limit; if execution needs more, the transaction fails and still pays.
• EIP-1559 (Ethereum and most EVM chains) splits the price in two. The base fee is set by the protocol from how full recent blocks were, and is burned. The priority fee, or tip, goes to the block producer. The sender also sets a max fee per gas, the most it will pay in total per gas, so a base-fee spike cannot drain it.
• Size matters on Bitcoin. The fee depends on bytes, not on the amount sent: every input makes a transaction bigger, so spending many small coins costs more than one large coin.

Unsticking a payment

A transaction whose fee is too low for current demand may sit in the mempool for hours, or be dropped. Lesson 7 showed why sending a second payment is wrong. The fix is to replace it: a new transaction in the same slot (the same nonce, or the same coins), with a fee high enough that block producers prefer it. Networks require a minimum bump, typically 10% more on EVM chains, so that replacements cannot be used to spam.

Chains with an expiry (Tron, Solana) or a seqno and deadline (TON) have no replacement: a transaction that does not land expires, and is then re-issued.

Under the hood: estimates are not prices

A fee estimate is a prediction made before the transaction runs. On Bitcoin, once the transaction is built, its fee is exact. On EVM chains the estimate is an upper bound: the sender commits to a maximum, and usually pays less. Some layer-2 chains (OP Stack chains such as Optimism and Base) add a second fee for posting data to Ethereum, which moves with Ethereum's own prices. And the estimate itself comes from a node: a buggy or malicious node can suggest an absurd fee, so careful systems cap what they are willing to sign.

Why a developer cares

• Fees are real money, and a hot wallet needs native coin for fees even when it only pays out tokens.
• Too low gets stuck, too high is wasted, and both happen. Your system needs a way to bump a stuck payment without paying twice.
• Never let a node decide your fee unchecked. Set your own ceiling, and refuse to sign above it.

In crypto-aio

bc.estimateFee(intent) returns the fee as a list of charges (one per asset and purpose, such as network, priority or l1-data) and a bound that says how much to trust it: exact, expected or upper. A transfer's fee is a speed, 'slow', 'normal' (the default) or 'fast', or a family-specific override:

import { feeTotal } from 'crypto-aio';
import { createFakeEnv } from 'crypto-aio/testing';

const env = await createFakeEnv();
const to = env.stranger();
for (const fee of ['slow', 'normal', 'fast'] as const) {
  const estimate = await env.run(env.bc.estimateFee({ to, amount: '0.001', fee }));
  console.log(fee, feeTotal(estimate, 'fakechain:local/native')?.format(), estimate.bound);
}
// Prints:
// slow 0.00000001 FAKE exact
// normal 0.00000002 FAKE exact
// fast 0.00000003 FAKE exact

Every family bounds the fees it will sign with a handle option no endpoint can change, such as maxFeePerGas on EVM chains and maxFeeRate on Bitcoin, and refuses a fee above it before signing. A stuck payment is replaced with bc.replace(operationId, { fee }) (or cancelled with bc.cancel) where the chain supports it. Send a transfer documents every family's fee options and bounds, and Fix a stuck transfer covers replacement.

Check yourself

1. Your service pays out USDT on Tron. What must the hot wallet hold besides USDT?
2. Why does a Bitcoin transaction that spends 50 small coins cost more than one that spends a single large coin, for the same amount?
3. An EVM payment is stuck with a low tip. What do you send?
4. A node suggests a fee rate 100 times higher than usual. What should your code do?

Answers

1. TRX (or staked resources) to pay for bandwidth and energy: fees are paid in the native coin.
2. Bitcoin fees depend on size, and every input adds bytes.
3. A replacement: the same nonce, with the tip and the max fee each raised by at least the network's minimum bump (10% on most EVM chains).
4. Refuse to sign above its own ceiling. The library clamps or refuses fees above the handle's bound.

Key terms

• Fee: what a transaction pays, in the native coin, to be included.
• Fee market: competition for block space that sets the fee needed to be included soon.
• Gas, gas limit: a unit of EVM computation; the most a transaction may use.
• Base fee, priority fee (tip), max fee per gas: the parts of an EIP-1559 fee.
• Fee rate (sat/vB): Bitcoin's price per virtual byte.
• Bandwidth, energy: Tron's resources for size and computation.
• Compute units: Solana's measure of work, priced by a priority fee.
• Replacement (fee bump): a new transaction in the same slot with a higher fee.

What's next

Estimating a fee, broadcasting, reading a block: every one of these is a question your code asks a server. Which server, and how? Nodes, RPC and providers.