Convert acres to mi² acres ↔ square miles Free Online Secure
Type a value in acres and read it in square miles at once. The factor is exact by definition, the arithmetic happens in your browser, and nothing is sent anywhere.
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Common values — Convert acres to mi² (acres ↔ square miles)
| ac | mi² |
|---|---|
| 1 | 0.0015625 |
| 2 | 0.003125 |
| 3 | 0.0046875 |
| 5 | 0.0078125 |
| 10 | 0.015625 |
| 20 | 0.03125 |
| 50 | 0.078125 |
| 100 | 0.15625 |
Where these units come from
Square units need no definition of their own: a square metre is a metre by a metre, and everything follows from the length. The land measures are older and stranger. The acre began as the land a yoke of oxen could plough in a day, which is why it was long and thin rather than square — one chain by one furlong, 4840 square yards. The hectare and the are came out of the French Revolution, when land had to be measured in something decimal; neither belongs to the International System, but both were kept for land because a hectare is a comfortable size to picture and a square metre is not. Since the 1959 agreement on the yard, the acre is exactly 4046.8564224 square metres, so the conversions on this page are exact rather than approximate.
Where you meet them
Square metres price flats, size carpets and decide how much paint a wall needs; square feet do the same work in the United States and in much of the property market worldwide. Hectares belong to farmland, forests and planning permissions; acres to land deeds in the English-speaking world. Square kilometres and square miles are for countries, lakes and fires. Square centimetres and square millimetres live in workshops, on datasheets and anywhere a cross-section matters.
Questions people ask
Is an acre the same in the United States?
For every practical purpose, yes. The international acre used here is exactly 4046.8564224 m². The United States also kept a survey acre, based on a slightly different foot, which was larger by about four parts in a million — a difference of some 160 cm² in an acre, invisible except in large land surveys. That survey foot was formally retired at the end of 2022, so only one acre is now in use.
How many acres are in a square mile?
640 acres, and a square mile is exactly 2.589988110336 square kilometres. American land surveys divided territory into sections of one square mile, which is why 640 acres is such a recognisable figure. For larger regions the square kilometre is more convenient.
Is the answer exact?
Where both units are defined in terms of one another by whole numbers, yes — the page says so above the figure, and the only rounding is in how many digits are shown. Where the ratio has no finite decimal, the page shows twelve significant digits and says the relation is approximate rather than pretending otherwise.
Does anything I type leave my device?
No. The arithmetic runs in your browser; nothing is uploaded and there is no server to upload it to. Load the page once and it keeps working with the network switched off.
Six hundred and forty acres to the square mile
When land gets big, acres stop being a comfortable number. A ranch of 825,000 acres, a park of 748,000 acres, a wildfire of 963,000 acres: these figures are hard to picture, and American news and government reports often turn them into square miles for exactly that reason. To convert acres to mi² you divide the acres by 640, so 640 acres make exactly one square mile and 6,400 make ten. This page does the division as you type, and the text below shows what the results look like on real maps, from Manhattan to Texas.
You will find the rule with worked examples, short profiles of both units, a comparison, a long tour of situations where the two units meet and a list of common mistakes. The calculation happens in your browser and nothing you type leaves your device.
How to convert acres to mi²: divide by 640
The rule
One square mile equals exactly 640 acres. To convert acres to mi², divide the number of acres by 640. So 320 acres are 0.5 mi², 1,280 acres are 2 mi², 6,400 acres are 10 mi² and 64,000 acres are 100 mi². To go the other way, multiply square miles by 640.
Worked examples
A ranch of 5,000 acres is 7.81 mi². A county of 250,000 acres is 390.6 mi². A national park of 1,200,000 acres is 1,875 mi². When the number does not divide neatly, keep two decimals, or use fractions of a mile: 160 acres is a quarter of a square mile.
A mental shortcut
Divide by 600 to get a quick estimate and then subtract about six per cent. For 3,000 acres, 3,000 divided by 600 is 5, and subtracting six per cent leaves 4.7, close to the exact 4.69.
A quick check
The square mile is the larger unit, so the number of square miles must be much smaller than the acres. If a “convert acres to mi²” calculation produced a larger figure, you multiplied instead of dividing.
The acre
One six hundred and fortieth of a square mile
The acre is 4,046.86 square metres, a furlong by a chain. Its value is tied to the mile, because 640 acres fill a square mile exactly. This neat relationship is no accident: the English mile, furlong and chain were designed to fit together.
Where it is used
American and British land law, farm sales and property tax use acres. The unit is also common in Canada, Australia and Ireland.
The square mile
One mile on each side
A square mile is 2.59 square kilometres, 259 hectares or 27.88 million square feet. It is used in the United States and the United Kingdom for large areas: counties, cities, parks, lakes and states. Population density is quoted per square mile.
The land survey section
Across most of the United States, the Public Land Survey divided the country into sections of one square mile, each of 640 acres. That is why farm roads often run at one mile intervals in the Midwest.
Acre and square mile compared
The square mile is 640 times larger than the acre. Small plots belong to acres, whole counties to square miles, and the dividing line is somewhere around a few thousand acres, when a number such as 3,200 begins to feel unwieldy and 5 square miles is easier to read.
Reporters often change the unit to help the reader picture the area. A fire of 320,000 acres sounds enormous; the same fire is 500 mi², about the size of the city of Los Angeles, which makes the scale easier to grasp.
Where the conversion appears
Cities and states
Manhattan covers about 14,600 acres, which is 22.8 mi². Washington, DC has 43,700 acres, 68.3 mi². Rhode Island has 777,000 acres, 1,214 mi². Texas is 171.9 million acres, 268,596 mi². Reading one unit and converting to the other shows how large each place really is.
National parks
Yosemite has about 748,000 acres, or 1,169 mi². The Grand Canyon National Park has 1.22 million acres, 1,904 mi². Yellowstone covers 2.2 million acres, 3,472 mi². Park services usually give both figures on their web pages, but news stories vary.
Wildfires and floods
The Dixie Fire of 2021 burned about 963,000 acres, 1,505 mi². A “convert acres to mi²” calculation puts the fire on the same scale as a county or a small state, which is what readers need to understand the damage.
The section system
The Public Land Survey System, begun in 1785, divided land west of the Appalachians into townships of 36 sections, each a mile square. Every section holds 640 acres. Farmers and surveyors speak of sections by number, and land descriptions give the section, township and range. A farm of two and a half sections is 1,600 acres, 2.5 mi², and the arithmetic of a “convert acres to mi²” calculation is the easiest way to move between the two scales.
Lakes
Lake Tahoe covers 191 mi², about 122,000 acres. Lake Okeechobee has 730 mi², 467,000 acres. Lake Michigan is 22,404 mi², 14.3 million acres. Surface area is one of the standard facts about a lake, and both units are used by park and water authorities.
A mile square or a square mile
A square mile is an area of 640 acres. A tract that is one mile square has sides of a mile and the same area. But "two miles square" means a square of 2 by 2 miles, 4 mi², while "two square miles" is only 2 mi². The difference is a factor of two, and writers often mix the two phrases.
Railroad land grants
In the nineteenth century, the US government gave railroad companies alternate sections of land along their tracks, a total of over 130 million acres, more than 200,000 mi². The checkerboard pattern of public and private land in the West still shows the result. The numbers are large enough to demand square miles.
Population density
Density is expressed as people per square mile in the United States. A density of 1 person per acre equals 640 per square mile, similar to a leafy suburb. A density of 10,000 per square mile is about 15.6 per acre, typical of dense urban neighbourhoods. A “convert acres to mi²” calculation on the area helps you compare with figures in people per hectare.
Zoning of one house per 40 acres
Many rural counties limit development to one house per 40 acres, which is a density of 16 houses per square mile. A parcel of 160 acres therefore has four building rights, and a section of 640 acres sixteen. These rules protect farmland and are described in acres because the original surveys used that unit.
Rounding
Because 640 is exact, no rounding is needed until the end. For a report, give one or two decimals when the result is under 100 square miles and whole numbers for larger areas. A figure such as 156.25 mi² can be written 156 mi² in a newspaper.
Spreadsheet use
Enter acres in column A and =A2/640 in column B. Add a column with =B2*2.58998811 for square kilometres. A total row helps compare the sum of parcels with the area of the county. If you need to go the other way, multiply by 640.
Teaching with grid paper
Draw a grid on paper where each square is a quarter mile on a side, which is 40 acres. Sixteen squares make a square mile, and a class can colour in farms of 160 or 320 acres. Students see why 640 is the right factor and not a random number.
Hunting and timber leases
Hunting clubs lease land by the acre, typically a few thousand acres for a camp. A lease of 3,200 acres is 5 mi². Timber companies lease or own tracts of hundreds of thousands of acres, and annual reports state the holdings in both units.
Mapping software
GIS programs display area in the unit you choose. A polygon of 12,800 acres appears as 20 mi² when you change the setting. Check the layer settings before you export a table, because two people can read the same shapefile and quote different numbers.
Quarter sections
A quarter section is 160 acres, 0.25 mi². It was the standard family farm under the Homestead Act. A quarter of a quarter, 40 acres, is the unit still used in many rural zoning rules. Eight such 40 acre parcels make half a square mile, 320 acres, and sixteen make a whole section.
The Louisiana Purchase
In 1803 the United States bought land of about 828,000 square miles, which is 530 million acres, for 15 million dollars, roughly 3 cents an acre. The comparison between the two units gives an idea of the scale of the deal: the Purchase doubled the size of the country.
Counties from small to huge
San Bernardino County in California covers 20,105 mi², 12.9 million acres, larger than nine US states. Arlington County in Virginia covers 26 mi², 16,600 acres. The gap between the biggest and smallest counties is a factor of 800.
Why the number 640 is so tidy
The English units of length fit neatly into each other: a chain is 66 feet, a furlong 10 chains, a mile 8 furlongs. A square mile, 8 furlongs by 8 furlongs, therefore holds 64 square furlongs, and each square furlong is 10 acres. That gives 640. When you convert acres to mi² you are using a relationship that medieval surveyors built into the system on purpose, so that every field, strip and mile would add up.
State sizes in acres
Delaware has 1.3 million acres, 2,489 mi². Alaska has 365 million acres, 570,641 mi². California, at 104 million acres, 163,696 mi², is the third largest state. Writers often prefer square miles because the numbers stay short, while land agencies prefer acres because their records are kept in that unit.
Farm size classes
The US Department of Agriculture groups farms by size: under 50 acres are small hobby farms, 500 to 999 acres are medium, and 2,000 acres or more are large. A farm of 2,000 acres is 3.1 square miles. Farmers talk about acres in daily life, and economists look at square miles when they study regions, so the “convert acres to mi²” calculation moves a figure between the two languages.
Airports and military bases
Edwards Air Force Base spans about 308,000 acres, 481 mi², larger than the city of Los Angeles. Denver International Airport covers 33,531 acres, 52.4 mi², among the largest in the world. Facilities of this size are always described in both units in official documents.
The whole planet
The entire land area of the Earth is about 57.5 million square miles, or 36.8 billion acres. Dividing by the world population gives around 4.6 acres, or 0.0072 square miles, per person, including deserts and ice. These figures are rough, but they help students see how a conversion links human scales to planetary ones.
Reading old maps
On old county maps, each section is labelled with the owner and acreage. Sections near rivers are often smaller, because of irregular boundaries. A researcher comparing maps from different decades can convert each parcel to square miles to check that the totals still add up to 36 per township.
Townships
A township is six miles on a side, 36 square miles, or 23,040 acres. In the Midwest, townships are also units of local government. A county of 24 townships has 864 mi², 552,960 acres. The same grid appears in satellite photographs as a pattern of squares about a mile across.
Manhattan and other islands
Manhattan has about 14,600 acres, 22.8 mi². The Isle of Wight has 147 mi², 94,000 acres. Jersey has 45 mi², 28,800 acres. Islands are a good test of whether a conversion makes sense, because they have well defined edges and are easy to compare with each other.
Washington and Rhode Island
Washington, DC, is 68.3 mi², 43,700 acres. Rhode Island, the smallest US state, is 1,214 mi², 777,000 acres, or about 18 Washingtons. Both are used by journalists as yardsticks for larger events.
The size of Texas
Texas covers 268,596 square miles. In acres that is 171.9 million, nearly 400 times the size of Rhode Island. The state's ranches are famous for their size: the King Ranch has about 825,000 acres, 1,289 mi², larger than Rhode Island.
National parks compared
The largest US national park, Wrangell St Elias in Alaska, has 13.2 million acres, 20,600 mi². The smallest, Thaddeus Kosciuszko Memorial, has 0.02 acre. Between them lie the great western parks, and a table that lists both acres and square miles helps visitors choose a trip by scale.
Big ranches
Ted Turner owns about 2 million acres, 3,125 mi², across several states. The Waggoner Ranch in Texas has roughly 535,000 acres, 836 mi². Real estate brokers quote acres, newspapers often convert to square miles to compare ranches with cities.
Wildfires
Large fires are measured in acres in the United States. A fire of 100,000 acres is 156 mi², roughly the size of the city of Chicago. In 2020, California's fires burned more than 4 million acres, 6,250 mi². Fire agencies, insurers and news outlets use both units, and a “convert acres to mi²” calculation is routine.
More values for convert acres to mi²
A longer list of conversions
If the value you need is not in the first list, these further conversions may be close enough to read off directly:
- 0.1 ac = 0.00015625 mi²
- 0.5 ac = 0.00078125 mi²
- 3 ac = 0.0046875 mi²
- 4 ac = 0.00625 mi²
- 7 ac = 0.0109375 mi²
- 15 ac = 0.0234375 mi²
- 20 ac = 0.03125 mi²
- 75 ac = 0.1171875 mi²
- 200 ac = 0.3125 mi²
- 500 ac = 0.78125 mi²
- 2,000 ac = 3.125 mi²
- 10,000 ac = 15.625 mi²
Mistakes when you convert acres to mi²
Multiplying instead of dividing
Acres are the smaller unit, so the number shrinks. If 3,200 acres became 2,048,000, you multiplied.
Confusing square miles with miles
A square mile is an area, not a length. Ten square miles are not a ten mile line.
Mile square versus square mile
Two miles square is 4 mi², not 2. Read such phrases carefully.
Quick answers
How many acres in a square mile?
640.
How do I convert acres to mi²?
Divide by 640.
How many square miles is 1,000 acres?
1.5625 mi².
What is half a square mile in acres?
320 acres.
Does my number leave my device?
No. The calculation runs in your browser.
Related pages and a last word
If you work with land and area, these conversions are close at hand: Convert acres to ft², Convert acres to m², Convert km² to mi², Convert mi² to km², Convert km to mi, Convert mi to km, Convert °C to °F, Convert °F to °C, Convert dm to in. The reverse of this page is Convert mi² to acres.
In short, to convert acres to mi² divide by 640, remember that a section is one square mile, and keep two decimals unless you need more. Your number stays on your device: nothing is uploaded and no account is needed.