public function convert($value, $fromUnit, $toUnit) {
		$fromUnit = strtoupper($fromUnit);
		$toUnit = strtoupper($toUnit);

		if (!isset($this->_units[$fromUnit]) || !isset($this->_units[$toUnit])) {
			throw new CalculatorException('Invalid Unit');
		}

		if ($fromUnit === static::UNIT_MILES) {
			$value *= $this->_units[$toUnit];
		} elseif ($toUnit === static::UNIT_MILES) {
			$value /= $this->_units[$fromUnit];
		} else {
			$value /= $this->_units[$fromUnit];
			$value *= $this->_units[$toUnit];
		}
		return $value;
	}