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1812 was a leap year starting on Wednesday of the Gregorian calendar and a leap year starting on Monday of the Julian calendar, the 1812th year of the Common Era (CE) and Anno Domini (AD) designations, the 812th year of the 2nd millennium, the 12th year of the 19th century, and the 3rd year of the 1810s decade. As of the start of 1812, the Gregorian calendar was 12 days ahead of the Julian calendar, which remained in localized use until 1923.
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- January 1 – The Allgemeines bürgerliches Gesetzbuch (the Austrian civil code) enters into force in the Austrian Empire.
- February 7
- February 12 – Napoleon authorizes the usage of Mesures usuelles, the basis of the metric system.
- February 13 – The first Chilean newspaper Aurora de Chile deals with political philosophy, and stands in favor of the new national government.
- February 27
- Argentine War of Independence: Manuel Belgrano raises the Flag of Argentina (which he designed) in the city of Rosario, for the first time.
- English poet Lord Byron gives his first address as a member of the British House of Lords, in defense of Luddite violence against industrialism, in his home county of Nottinghamshire.
- March 5 – Prussia and France sign the Treaty of Paris.
- March 15 – Luddites attack the wool-processing factory of Frank Vickerman in West Yorkshire.
- March 16–April 6 – Siege of Badajoz (Peninsular War): The Anglo-Portuguese Army, under the Earl of Wellington, besieges Badajoz, Spain and forces the surrender of the French garrison.
- March 19 – The Cádiz Cortes creates the first modern Spanish constitution.
- March 26
- In the United States, the Boston Gazette prints a political cartoon coining the term Gerrymander, after former Massachusetts Governor Elbridge Gerry's approval (on February 11) of legislation creating oddly shaped electoral districts, designed to help incumbents win re-election.
- The 1812 Caracas earthquake destroys Caracas, Venezuela.
- April 4 – U.S. President James Madison enacts a 90-day embargo on trade with the United Kingdom.
- April 8 – The capital of Finland is moved from Turku to Helsinki, when Russia's Tsar Alexander I signs an edict moving the Government Council of the Grand Duchy of Finland. 
- April 30 – Louisiana is admitted as the 18th U.S. state.
- May 11 – John Bellingham assassinates British Prime Minister Spencer Perceval, in the lobby of the British House of Commons.
- May 16 – Russian Field Marshal Mikhail Kutuzov signs the Treaty of Bucharest, ending the Russo-Turkish War (1806–12) and annexing Bessarabia to Imperial Russia.
- May 25 – Felling mine disaster: A mine explosion at the Felling colliery near Jarrow, England leaves 96 dead.
- June 1 – War of 1812: U.S. President James Madison asks the U.S. Congress to declare war on the United Kingdom.
- June 4 – Following Louisiana's admittance as a U.S. state, the territory by that name is renamed the Missouri Territory.
- June 16 – New York State charters the City Bank of New York (later Citibank).
- June 18 – The War of 1812 between the United States, Canada and the United Kingdom begins.
- June 24 – Napoleon's Grande Armée crosses the Neman River, and invades Russia.
- July 12 – Americans invade Canada at Windsor, Ontario.
- July 18 – Russia's Patriotic War – Battle of Klyastitsy: Kulnev defeats Oudinot, but sustains a mortal wound.
- July 22 – Peninsular War – Battle of Salamanca: British forces led by Lord Wellington defeat French troops near Salamanca, Spain.
- August 5 – War of 1812: Tecumseh's Indian force ambushes Thomas Van Horne's 200 Americans at Brownstone Creek, causing them to flee and retreat.
- August 12 – Peninsular War: The combined English and Portuguese army under the command of Wellington enters Madrid, following the Battle of Salamanca.
- August 15 – War of 1812 – Battle of Fort Dearborn: Potawatomi warriors overrun the United States fort in Illinois Territory.
- August 16 – War of 1812: American General William Hull surrenders Fort Detroit, without a fight, to the British Army.
- August 19 – War of 1812: The USS Constitution defeats the British frigate Guerriere, off the coast of Nova Scotia. The British shot is said to have bounced off the Constitution's sides, earning her the nickname "Old Ironsides".
- September 7 – Napoleonic Wars – French invasion of Russia – Battle of Borodino: The bloodiest battle of the Napoleonic Wars ends in a tactical victory for Napoleon. There are at least 70,000 casualties, with a minimum of 6,562 dead from the French Grande Armée alone.
- September 14 – French invasion of Russia: Napoleon's troops enter Moscow, which is deliberately set on fire by Muscovites, on orders of Fyodor Rostopchin.
- October 9 – War of 1812: American naval forces under Lieutenant Jesse Duncan Elliott capture two British warships, HMS Detroit and HMS Caledonia.
- October 12 – The capital of the Commonwealth of Pennsylvania, United States is permanently moved from Lancaster to Harrisburg. I. Daniel Rupp, History of Lancaster County: To which is Prefixed a Brief Sketch of the Early History of Pennsylvania 
- October 13 – War of 1812 – Battle of Queenston Heights: As part of the Niagara campaign in Ontario, Canada, United States forces under General Stephen Van Rensselaer are repulsed from invading Canada by British and native troops, led by Sir Isaac Brock (who dies during the battle).
- October 18–20 – Second Battle of Polotsk – Russians attack and defeat a Franco-Bavarian force in Belarus.
- October 19 – Napoleon begins his retreat from Moscow.
- October 23 – Malet coup of 1812: General Claude François de Malet attempts unsuccessfully to overthrow the Napoleonic régime in Paris.
- October 24 – Napoleonic Wars – Battle of Maloyaroslavets: An inconclusive encounter between the French vanguard and a Russian force leads Napoleon to decide to retreat along the same line as his advance, with disastrous results.
- November 3 – Napoleonic Wars – Battle of Vyazma: The rearguard of Napoleon's retreating army is defeated.
- November 5 – U.S. presidential election, 1812: James Madison defeats DeWitt Clinton.
- November 10 – United Kingdom general election, 1812: The Tory Party is victorious, under Robert Jenkinson, 2nd Earl of Liverpool.
- November 15–18 – Napoleonic Wars – Battle of Krasnoi: Napoleon's retreating army is again defeated in a series of skirmishes.
- December 8 – The M6.9–7.5 San Juan Capistrano earthquake affects Alta California with a maximum Mercalli intensity of VII (Very strong) to IX (Violent), killing 40 parishioners at Mission San Juan Capistrano.
- December 14 - The French invasion of Russia comes to an end as the remnants of the Grande Armée are expelled from Russia.
- December 20 – The first volume of Grimms' Fairy Tales is published in Germany.
- December 29 – War of 1812: The USS Constitution defeats the British frigate HMS Java, off the coast of Brazil.
- December 30 – The Convention of Tauroggen is signed.
- The Bishop James Madison Society is founded at the College of William & Mary, Williamsburg, Virginia.
- The Woodford Reserve Bourbon whiskey distillery, established in 1780 along Glenn's Creek in Woodford County, Kentucky, passes to the control of Oscar Pepper.
- The Ranikot Fort is reconstructed in Sindh.
- Approximate date – Battle of Shela: The people of Lamu Island are victorious, against those from other parts of the Kenya coast.
- January 6 – Melchora Aquino, Filipino revolutionary hero (d. 1919)
- January 13 – Victor de Laprade, French poet, critic (d. 1883)
- February 3 – William Fraser Tolmie, Scottish-Canadian scientist, politician (d. 1886)
- February 7 – Charles Dickens, English writer (d. 1870)
- February 11 – Alexander Hamilton Stephens, Vice President of the Confederate States of America (d. 1883)
- February 15 – Charles Lewis Tiffany, American jeweler, co-founder of Tiffany & Co. (d. 1902)
- February 16 – Henry Wilson, 18th Vice President of the United States (b. 1875)
- February 18 – Nils Johan Berlin, Swedish chemist, professor (d. 1891)
- February 29 – Sir James Milne Wilson, Premier of Tasmania (d. 1880)
- March 1
- March 6 – Aaron Lufkin Dennison, American watch manufacturer (d. 1895)
- March 22 – Stephen Pearl Andrews, American anarchist, abolitionist (d. 1886)
- April 14 – George Grey, 11th Premier of New Zealand (d. 1898)
- April 20 – Pauline Åhman, Swedish harpist (d. 1904)
- April 22 – Solomon Caesar Malan, Swiss-born orientalist (d. 1894)
- April 23 – Frederick Whitaker, English-New Zealand lawyer, politician, 5th Prime Minister of New Zealand (d. 1891)
- April 27
- April 29 – Emilie Högquist, Swedish dramatic star (d. 1846)
- May 6 – Madame Restell, American abortionist (d. 1878)
- May 7 – Robert Browning, English poet (d. 1889)
- May 12 – Edward Lear, English artist, poet (d. 1888)
- June 9 – Johann Gottfried Galle, German astronomer (d. 1910)
- June 13 – Gustavus H. Scott, American admiral (d. 1882)
- June 14 – Fernando Wood, New York City mayor (d. 1881)
- July 2 – Nathaniel de Rothschild, French wine grower (d. 1870)
- August 8 – John Rodgers, American admiral (d. 1882)
- August 25
- August 27 – Bertalan Szemere, 3rd Prime Minister of Hungary (d. 1869)
- September 2 – William Fox, 2nd Premier of New Zealand (d. 1893)
- September 18 – Herschel Vespasian Johnson, American politician (d. 1880)
- October 12 – Ascanio Sobrero, Italian chemist (d. 1888)
- October 20 – Austin Flint I, American cardiologist (d. 1886)
- November 14 – Aleardo Aleardi, Italian poet (d. 1878)
- December 6 – Ana María Martínez de Nisser, Colombian heroine, writer (d. 1872)
- December 8 – Henry Varnum Poor, American financial analyst (d. 1905)
- December 14 – Charles Canning, 1st Earl Canning, British Viceroy of India (d. 1862)
- December 17 – Vilhelm Petersen, Danish painter (d. 1880)
- December 24 – Karl Eduard Zachariae von Lingenthal, German jurist (d. 1894)
- January 23 – Robert Craufurd, British general (mortally wounded in battle) (b. 1764)
- March 7 – Isaac Swainson, English botanist (b. 1746)
- March 11 – Philip James de Loutherbourg, English artist (b. 1740)
- April 20 – George Clinton, 4th Vice President of the United States (b. 1739)
- April 25 – Edmond Malone, Irish scholar (b. 1741)
- May 11 – Spencer Perceval, Prime Minister of the United Kingdom (assassinated) (b. 1762)
- May 12 – Martha Ballard, American diarist, midwife (b. 1734 or 1735)
- May 18 – John Bellingham, British assassin of Spencer Perceval (b. 1769)
- September 19 – Mayer Amschel Rothschild, German banker (b. 1744)
- September 21 – Emanuel Schikaneder German dramatist, actor, and singer (b. 1751)
- September 24 – Juana Galán, Spanish heroine (b. 1787)
- October 13 – Isaac Brock, British general (killed in action) (b. 1769)
- December 20 – Sacagawea, Shoshone guide (b. ca. 1788)
- December 24 – George Beck, American artist, poet (b. 1749)
- Helsinki 200: 8 April 1812 Emperor Alexander I promotes Helsinki to the capital of the Grand Duchy.
- (Gilbert Hillis, 1844; reprinted by Heritage Books, 1990) p434
- "Life in the Desert, or, Recollections of Travel in Asia and Africa". World Digital Library. 1860. Retrieved 2013-09-23.