# Exchange Rate

Each bToken is convertible into an ever increasing quantity of the underlying asset, as interest accrues in the market. The exchange rate between a bToken and the underlying asset is equal to:

```
exchangeRate = (getCash() + totalBorrows() - totalReserves()) / totalSupply()
```

**BErc20 / CEther**

```
function exchangeRateCurrent() returns (uint)
```

* `RETURN`: The current exchange rate as an unsigned integer, scaled by 1e18.

**Solidity**

```
BErc20 bToken = BToken(0x3FDA...);
uint exchangeRateMantissa = bToken.exchangeRateCurrent();
```

**Web3 1.0**

```javascript
const bToken = CEther.at(0x3FDB...);
const exchangeRate = (await bToken.methods.exchangeRateCurrent().call()) / 1e18;
```

Tip: note the use of call vs. send to invoke the function from off-chain without incurring gas costs.