PromptsEdge
FreeGitHubOther

Evm Token Decimals

Prevent silent decimal mismatch bugs across EVM chains. Covers runtime decimal lookup, chain-aware caching, bridged-token precision drift, and safe normalization for bots, dashboards, and DeFi tools. Use when handling token amounts across EVM chains, or when a balance, price, or transfer amount is off by orders of magnitude.

affaan-m on GitHub

Curated by PromptsEdge from a public repo · MIT license. All credit goes to the author.

What is this skill?

Silent decimal mismatches are one of the easiest ways to ship balances or USD values that are off by orders of magnitude without throwing an error.

When to use

  • Reading ERC-20 balances in Python, TypeScript, or Solidity
  • Calculating fiat values from on-chain balances
  • Comparing token amounts across multiple EVM chains
  • Handling bridged assets
  • Building portfolio trackers, bots, or aggregators

How It Works

Never assume stablecoins use the same decimals everywhere. Query decimals() at runtime, cache by (chain_id, token_address), and use decimal-safe math for value calculations.

Examples

Query decimals at runtime

python
from decimal import Decimal
from web3 import Web3

ERC20_ABI = [
    {"name": "decimals", "type": "function", "inputs": [],
     "outputs": [{"type": "uint8"}], "stateMutability": "view"},
    {"name": "balanceOf", "type": "function",
     "inputs": [{"name": "account", "type": "address"}],
     "outputs": [{"type": "uint256"}], "stateMutability": "view"},
]

def get_token_balance(w3: Web3, token_address: str, wallet: str) -> Decimal:
    contract = w3.eth.contract(
        address=Web3.to_checksum_address(token_address),
        abi=ERC20_ABI,
    )
    decimals = contract.functions.decimals().call()
    raw = contract.functions.balanceOf(Web3.to_checksum_address(wallet)).call()
    return Decimal(raw) / Decimal(10 ** decimals)

Do not hardcode 1_000_000 because a symbol usually has 6 decimals somewhere else.

Cache by chain and token

python
from functools import lru_cache

@lru_cache(maxsize=512)
def get_decimals(chain_id: int, token_address: str) -> int:
    w3 = get_web3_for_chain(chain_id)
    contract = w3.eth.contract(
        address=Web3.to_checksum_address(token_address),
        abi=ERC20_ABI,
    )
    return contract.functions.decimals().call()

Handle odd tokens defensively

python
try:
    decimals = contract.functions.decimals().call()
except Exception:
    logging.warning(
        "decimals() reverted on %s (chain %s), defaulting to 18",
        token_address,
        chain_id,
    )
    decimals = 18

Log the fallback and keep it visible. Old or non-standard tokens still exist.

Normalize to 18-decimal WAD in Solidity

solidity
interface IERC20Metadata {
    function decimals() external view returns (uint8);
}

function normalizeToWad(address token, uint256 amount) internal view returns (uint256) {
    uint8 d = IERC20Metadata(token).decimals();
    if (d == 18) return amount;
    if (d < 18) return amount * 10 ** (18 - d);
    return amount / 10 ** (d - 18);
}

TypeScript with ethers

typescript
import { Contract, formatUnits } from 'ethers';

const ERC20_ABI = [
  'function decimals() view returns (uint8)',
  'function balanceOf(address) view returns (uint256)',
];

async function getBalance(provider: any, tokenAddress: string, wallet: string): Promise<string> {
  const token = new Contract(tokenAddress, ERC20_ABI, provider);
  const [decimals, raw] = await Promise.all([
    token.decimals(),
    token.balanceOf(wallet),
  ]);
  return formatUnits(raw, decimals);
}

Quick on-chain check

bash
cast call <token_address> "decimals()(uint8)" --rpc-url <rpc>

Rules

  • Always query decimals() at runtime
  • Cache by chain plus token address, not symbol
  • Use Decimal, BigInt, or equivalent exact math, not float
  • Re-query decimals after bridging or wrapper changes
  • Normalize internal accounting consistently before comparison or pricing

Related skills

View all →