Magento – Get currency rate by currency code

You cannot convert one price currency price to base currency.

because magento did not calculate base currency rate from other current rate.

Magento define a price in BASE currency which will convert using rate for other currency

Magento is calculate other price from base Currency

Please got see Mage_Directory_Model_Currency class on function

 public function convert($price, $toCurrency=null)
    {
        if (is_null($toCurrency)) {
            return $price;
        }
        elseif ($rate = $this->getRate($toCurrency)) {
            return $price*$rate;
        }

        throw new Exception(Mage::helper('directory')->__('Undefined rate from "%s-%s".', $this->getCode(), $toCurrency->getCode()));
    }

Here show that you can not get base currency from other currency

That means that you code format is

Mage::helper('directory')->currencyConvert($amt, 'baseCurrecy', 'your currency');

revise it not possible using this function

Mage::helper('directory')->currencyConvert($amt, 'your currency', 'baseCurrecy');

Magento uses webservicex.net to pull the currency conversion rates. You can check that by visiting

System->Configuration -> Currency setup -> webservicex.

then, take a look at

Mage_Directory_Model_Currency_Import_Webservicex
and

Mage_Directory_Model_Observer

Hope this helps.