How was the 2.5% rate for Zakat determined, and is it uniform across all Islamic countries?

The Messenger of Allah, peace and blessings be upon him, clarified to his Ummah all that it needs in its religion. The amounts and thresholds of Zakat are among the boundaries of Allah, which the Prophet, peace and blessings be upon him, explained most completely. Therefore, it is not permissible to increase or decrease them. The rate of a quarter of a tenth (2.5%) is for the Zakat on currency and trade goods, while agricultural produce, fruits, and livestock have other specific amounts. The proof for this is what Ali ibn Abi Talib narrated from the Prophet, peace and blessings be upon him, concerning the Zakat on silver and gold. This amount is fixed and does not change with the alteration of time or place.