Happy Birthday to US.

The British government presented a massive custom-made coin to the American people Monday to mark 250 years since the signing of the Declaration of Independence as King Charles III and Queen Camilla kicked off a four-day state visit.

The bespoke piece is loaded with symbolism — from its 13 sides representing the 13 colonies who declared independence from Great Britain in 1776 to the 55 leaves accompanying a portrait of the monarch on one face and representing the 45 American presidents and 10 British monarchs over the previous two-and-a-half centuries.

The reverse face features an American eagle, while the names of the 55 heads of state are engraved around the edges.

The Post understands the coin was presented on behalf of the British people by UK Ambassador Sir Christian Turner to Monica Crowley, US Chief of Protocol, who accepted on behalf of the American people hours before Charles and Camilla touched down at Joint Base Andrews outside Washington.

President Trump has also seen the coin and may display it at Tuesday’s White House engagements with the royal couple, at which personal gifts will also be exchanged.

When the first couple paid a state visit to the UK in September, the Trumps gave the King a replica of a President Eisenhower sword, while Queen Camilla received a vintage 18-karat gold, diamond, and ruby flower brooch from Tiffany & Company.

For their part, the royals gave President Trump a “hand-bound leather volume specially crafted by the Royal Bindery in Windsor Castle, to celebrate the 250th Anniversary of the Declaration of Independence,” and the Union Flag that flew above Buckingham Palace on the day of his inauguration.

Melania Trump received a personalized Anya Hindmarch handbag, and a silver and enamel bowl crafted by Northern Irish artist Cara Murphy.

Notably, the queen wore a Union Flag/Stars and Stripes brooch set with rubies, emeralds and diamonds, given to Queen Elizabeth II by New York City Mayor Robert Wagner during her first state visit to the US in October 1957.