Millennium:	2nd millennium
Centuries:	17th century 18th century 19th century
Decades:	1780s 1790s 1800s 1810s 1820s
Years:	1797 1798 1799 1800 1801 1802 1803

1800 by topic
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1800 in various calendars
Gregorian calendar	1800 MDCCC
French Republican calendar	8–9
Ab urbe condita	2553
Armenian calendar	1249 ԹՎ ՌՄԽԹ
Assyrian calendar	6550
Balinese saka calendar	1721–1722
Bengali calendar	1207
Berber calendar	2750
British Regnal year	40 Geo. 3 – 41 Geo. 3
Buddhist calendar	2344
Burmese calendar	1162
Byzantine calendar	7308–7309
Chinese calendar	己未年 (Earth Goat) 4497 or 4290 — to — 庚申年 (Metal Monkey) 4498 or 4291
Coptic calendar	1516–1517
Discordian calendar	2966
Ethiopian calendar	1792–1793
Hebrew calendar	5560–5561
Hindu calendars
- Vikram Samvat	1856–1857
- Shaka Samvat	1721–1722
- Kali Yuga	4900–4901
Holocene calendar	11800
Igbo calendar	800–801
Iranian calendar	1178–1179
Islamic calendar	1214–1215
Japanese calendar	Kansei 12 (寛政１２年)
Javanese calendar	1726–1727
Julian calendar	Gregorian minus 11 or 12 days
Korean calendar	4133
Minguo calendar	112 before ROC 民前112年
Nanakshahi calendar	332
Thai solar calendar	2342–2343
Tibetan calendar	阴土羊年 (female Earth-Goat) 1926 or 1545 or 773 — to — 阳金猴年 (male Iron-Monkey) 1927 or 1546 or 774

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1800 (MDCCC) was an exceptional common year starting on Wednesday of the Gregorian calendar and a leap year starting on Sunday of the Julian calendar, the 1800th year of the Common Era (CE) and Anno Domini (AD) designations, the 800th year of the 2nd millennium, the 100th and last year of the 18th century, and the 1st year of the 1800s decade. As of the start of 1800, the Gregorian calendar was 11 days ahead of the Julian calendar, which remained in localized use until 1923.

As of March 1 (O.S. February 18), when the Julian calendar acknowledged a leap day and the Gregorian calendar did not, the Julian calendar fell one day further behind, bringing the difference to 12 days until February 28 (O.S. February 16), 1900.

Events

World population approaches the 1 billion milestone which it will attain in 1802. The population distribution by region:
- Africa: 107,000,000
- Asia: 635,000,000
- Latin America: 24,000,000
- Northern America: 7,000,000
- Oceania: 2,000,000

January–March

January 1
- Quasi-War: Action of 1 January 1800 – A naval battle off the coast of Haiti, between four United States merchant vessels escorted by naval schooner USS Experiment, and a squadron of armed barges manned by Haitian pirates (known as picaroons), under the command of general André Rigaud, ends indecisively.
- The Dutch East India Company dissolves.
February 7 – A public plebiscite in France confirms Napoleon as First Consul, by a substantial majority.
February 11 – Infrared radiation is discovered by astronomer Sir William Herschel.
March 14 – Papal conclave, 1799–1800: cardinal Barnaba Chiaramonti succeeds Pius VI as Pius VII, the 251st pope. He is crowned on March 21, in Venice.
March 17 – The British Royal Navy ship of the line, HMS Queen Charlotte (1790), catches fire off the coast of Capraia, with the loss of 673 lives.^[1]
March 20 – Alessandro Volta describes his new invention, the voltaic pile, the first chemical battery, in a letter to the Royal Society of London.
March 26 – British Royal Navy officer Henry Waterhouse first charts the Antipodes Islands.

April–June

April 2
- Ludwig van Beethoven's Symphony No. 1 premieres at the Burgtheater, in Vienna.
- The Treaty of Constantinople establishes the Septinsular Republic, the first autonomous Greek state since the Fall of the Byzantine Empire.
April 3 – The first voting, albeit indirect, begins in the 1800 United States presidential election as voters in the New York make their selections for the New York state legislature, which in turn will vote on October 31 for the state's 12 presidential electors. ^[2] A majority of state legislature members are from the Democratic-Republican Party, rather than the Federalist Party. At the time, only 4 of the 16 U.S. states (Kentucky, Maryland, North Carolina and Virginia) have a popular vote for the presidential electors and the President. The other 12, including New York, have a popular vote for the state legislature, but not for the president. At the time, neither of the major parties have picked their nominees for president and vice president. The result will not be certified until February, 1801. ^[3]
April 6 – War of the Second Coalition: Siege of Genoa – General André Masséna is surrounded by 40,000 Austrian troops under Field Marshal Michael von Melas and blockaded by a strong British squadron under Lord Keith.^[4]
April 24 – The U.S. Library of Congress is founded in Washington, D.C.
May 14 – Second Coalition: French forces under General Louis-Alexandre Berthier are halted by 400 Austro-Piedmont soldiers, at Fort Bard in the Aosta Valley.^[5]
May 15 – Napoleon and his French army (40,000 men)—not including the field artillery and baggage trains—(35,000 light artillery and infantry, 5,000 cavalry) begin crossing the Alps. He selects the shortest route through the Great St Bernard Pass, and invades after five days traversing the northern region of Italy.
June 2 – The first smallpox vaccination is made in North America, at Trinity, Newfoundland.
June 3 – U.S. President John Adams moves to Washington. Because the President's Mansion is still under construction, President Adams takes up residence at Tunnicliffe's City Hotel near the unfinished U.S. Capitol Building.^[6]
June 4 – War of the Second Coalition: Siege of Genoa – The French army is evacuated from Genoa. Marshal André Masséna is allowed to march out, with all the honours of war. A portion of his force joins General Louis-Gabriel Suchet, and the rest is conveyed in British ships to Antibes.^[7]
June 14
- War of the Second Coalition: Battle of Marengo – Napoleon defeats the Austrian troops near Marengo, Italy.
- French general Jean-Baptiste Kléber is assassinated in Cairo by Syrian Kurdish Muslim student Suleiman al-Halabi.
June 15 – Convention of Alessandria (Armistice of Marengo): Austria agrees to evacuate much of Italy.
June 19 – War of the Second Coalition: Battle of Höchstädt – General Jean Victor Marie Moreau leads French forces to victory, opening the Danube passageway to Vienna.
June 27 – War of the Second Coalition: Battle of Neuburg – General Claude Lecourbe leads French forces to victory, securing control of the Danube from Austria.

July–September

July 2 – The Union with Ireland Act 1800 is passed by the Parliament of Great Britain; the Irish Parliament passes similar legislation in the following month, uniting the two kingdoms^[8]^[9]^[10] and abolishing the Parliament of Ireland.
July 10 – Fort William College is established by Lord Wellesley, British Governor-General of India, in Calcutta, to promote Bengali, Hindi and other vernaculars of the Indian subcontinent.
August 1 – King George III gives royal assent to the second Act of Union to unite the Kingdom of Great Britain and Kingdom of Ireland (both ruled by him) into the United Kingdom of Great Britain and Ireland, effective on January 1, 1801.^[11]
August 30 – The plot by African-American blacksmith and slave Gabriel Prosser to seize Richmond, Virginia, and guide a slave uprising, is thwarted by a massive downpour on the evening that it is set to begin; two other slaves have revealed Prosser's plans to authorities, who have prepared to follow him to the rendezvous point and arrest the conspirators, so that "neither the geographical extent of the plot nor the number of insurgents in the conspiracy was revealed";^[12] eventually, 25 slaves, including Prosser, will be captured, tried and hanged.
September 4 – Siege of Malta (1798–1800): The French garrison in Valletta surrenders to British troops, who had been called at the invitation of the Maltese. The islands of Malta and Gozo become the Malta Protectorate.
September 30 – The Convention of 1800, or Treaty of Mortefontaine, is signed between France and the United States of America, ending the Quasi-War.^[13]