What is the value of a coin dated 1821 that says georgius iiii d g britanniar rex f d?

The coin you described is likely a British gold or silver coin featuring King George IV. The value of such a coin can vary significantly based on its condition, rarity, and whether it is a gold sovereign or another denomination. Generally, if it's in good condition, it could be worth anywhere from a few hundred to several thousand dollars. To get an accurate appraisal, it’s best to consult a coin expert or a reputable coin dealer.