Simulating Gas

Before broadcasting, you can simulate the transaction to estimate how much gas it will consume. This prevents out-of-gas failures.

Automatic Simulation with `"auto"`

The simplest approach — pass "auto" as the fee and the client simulates internally:

const result = await client.signAndBroadcast(senderAddress, [sendMsg], "auto");

Behind the scenes, the client:

Sends the transaction to the chain’s Simulate endpoint (without actually executing it)
Receives the estimated gasUsed
Multiplies by a safety buffer (default 1.4x) to get the gas limit
Calculates the fee from the gas limit and the configured gasPrice

You can override the multiplier by passing a number instead of "auto":

const result = await client.signAndBroadcast(senderAddress, [sendMsg], 1.6);

Manual Simulation

For more control, call simulate directly and build the fee yourself:

import { calculateFee, GasPrice } from "@cosmjs/stargate";

const gasEstimate = await client.simulate(senderAddress, [sendMsg], "optional memo");
const gasLimit = Math.ceil(gasEstimate * 1.5);
const fee = calculateFee(gasLimit, GasPrice.fromString("0.025uatom"));

const result = await client.signAndBroadcast(senderAddress, [sendMsg], fee, "optional memo");

Explicit Fee (No Simulation)

When you know the gas cost ahead of time, skip simulation entirely by passing a StdFee:

import { calculateFee } from "@cosmjs/stargate";

const fee = calculateFee(200_000, "0.025uatom");
const result = await client.signAndBroadcast(senderAddress, [sendMsg], fee);

Simulation is an estimate. If chain state changes between simulation and broadcast (e.g. many concurrent transactions), actual gas may differ. The safety multiplier accounts for this, but high-contention scenarios may need a larger buffer (1.5–2.0x).

CosmJS

Concepts

Guides

API Reference

Automatic Simulation with `"auto"`

Manual Simulation

Explicit Fee (No Simulation)

Next Steps

Gas & Fees

Signing & Broadcasting

​Automatic Simulation with "auto"

​Manual Simulation

​Explicit Fee (No Simulation)

​Next Steps

Gas & Fees

Signing & Broadcasting

Automatic Simulation with `"auto"`

Manual Simulation

Explicit Fee (No Simulation)

Next Steps