> ## Documentation Index
> Fetch the complete documentation index at: https://cosmos-docs-cosmjs-docs.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Simulating Gas

> Estimate gas consumption with auto fees, manual simulation, or explicit fee objects

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:

```typescript theme={"system"}
const result = await client.signAndBroadcast(senderAddress, [sendMsg], "auto");
```

Behind the scenes, the client:

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

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

```typescript theme={"system"}
const result = await client.signAndBroadcast(senderAddress, [sendMsg], 1.6);
```

## Manual Simulation

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

```typescript theme={"system"}
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`:

```typescript theme={"system"}
import { calculateFee } from "@cosmjs/stargate";

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

<Info>
  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).
</Info>

## Next Steps

<CardGroup cols={2}>
  <Card title="Gas & Fees" icon="gas-pump" href="/cosmjs/v0.38.x/concepts/fees-gas/gas-and-fees">
    Understand gas pricing, fee calculation, and dynamic fee markets in depth.
  </Card>

  <Card title="Signing & Broadcasting" icon="paper-plane" href="/cosmjs/v0.38.x/guides/transactions/signing-broadcasting">
    Sign and send your transaction to the chain.
  </Card>
</CardGroup>