public function getConversions($unit)
    {
        if (!isset($this->units[$unit]['base'])) {
            throw new OutOfBoundsException('Unit is not set');
        }

        $units = array();

        foreach ($this->units as $key => $value) {
            if (isset($value['base']) && $value['base'] === $this->units[$unit]['base']) {
                array_push($units, $key);
            }
        }

        return $units;
    }