There are exactly 640 acres per square mile. A square mile measures 5,280 feet on each side, which gives 27,878,400 square feet, and one acre is 43,560 square feet, so 27,878,400 / 43,560 = 640. In metric terms, a square mile is about 2.59 square kilometers (259 hectares), and an acre is about 4,047 square meters.
Convert Acres to Square Feet, Hectares and More
Enter any number of acres below to see the same area in square feet, square yards, square meters and hectares. The default is 640 acres, which is one square mile. Because the converter has no square-mile unit, divide your acres by 640 to get square miles, or multiply square miles by 640 to get acres.
The square mile and the acre are the two land units most Americans meet first. For instance, you see square miles on maps, in census figures and in news about wildfires or city limits. Acres, on the other hand, show up on property listings, farm records and deeds. So knowing how the two connect lets you translate between a county map and a real estate ad in seconds.
Also, the link is a fixed ratio, not an estimate. In addition, both units come from the same old English survey measures, and the US government’s legal tables still list them side by side. For example, the National Institute of Standards and Technology (NIST) defines 1 acre as exactly 43,560 square feet and lists the square mile at about 259 hectares in its Handbook 44 tables of units. As a result, 640 is exact by definition.
Below you will find a chart, the arithmetic behind the number, the land-survey sections that use it, and the phrases that trip people up. If you also need the length side of the math, our guide to how many yards are in a mile covers it.
Acres per Square Mile Chart
Use this table for quick lookups, because it covers the most common sizes. In addition, the metric columns use the exact international foot (0.3048 m) and are rounded to two decimals.
| Square miles | Acres | Hectares | Square kilometers |
|---|---|---|---|
| 1/4 sq mi | 160 | 64.75 | 0.65 |
| 1/2 sq mi | 320 | 129.50 | 1.29 |
| 1 sq mi | 640 | 259.00 | 2.59 |
| 2 sq mi | 1,280 | 518.00 | 5.18 |
| 3 sq mi | 1,920 | 777.00 | 7.77 |
| 5 sq mi | 3,200 | 1,294.99 | 12.95 |
| 10 sq mi | 6,400 | 2,589.99 | 25.90 |
| 36 sq mi (one township) | 23,040 | 9,323.96 | 93.24 |
| 100 sq mi | 64,000 | 25,899.88 | 259.00 |
Going the other way, 1 acre is 1/640 of a square mile, or 0.0015625 sq mi. Likewise, 40 acres is 1/16 sq mi, 160 acres is 1/4 sq mi, and 1,000 acres is 1.5625 sq mi.
Printable tip: print this page in landscape mode and the chart fits on one sheet.
Why There Are Exactly 640 Acres per Square Mile
At first, the number looks random until you see where the acre came from. Historically, an acre was a strip one furlong long and one chain wide. A furlong is 660 feet and a chain is 66 feet, so the acre is 660 x 66 = 43,560 square feet. Meanwhile, a mile is 8 furlongs, or 80 chains.
Therefore, a square mile is 8 furlongs long and 80 chains wide. As a result, you get 8 x 80 = 640 strips, and each strip is one acre. You can check it with feet, too:
- First, 5,280 ft x 5,280 ft = 27,878,400 square feet in a square mile.
- Then, 27,878,400 / 43,560 = 640 acres.
- Similarly, in square yards, 3,097,600 / 4,840 = 640 acres as well.
An acre does not have to be any particular shape, however. A square acre is about 208.7 feet on each side, but a long, narrow lot can hold the same area. For more on the chain and its cousins, see our explainer on the rod as a unit of measurement.
Recommended Tools for Measuring Acres
You rarely pace out a full square mile. However, measuring a lot or field is common. For example, a measuring wheel is the fastest way to walk a boundary. Next, a long open-reel tape is better for short, exact sides. Also, a laser distance meter helps when you cannot walk the line, for instance across a pond or a fence.
As an Amazon Associate, Measuring Expert earns from qualifying purchases.
Sections, Quarter Sections and Townships
Across much of the western and midwestern US, land was surveyed under the Public Land Survey System. Under that grid, a township is roughly 6 miles square and is then split into 36 sections. Each section is meant to be about 1 square mile. Therefore, the 640 figure is built into millions of deeds.
| Survey unit | Nominal size | Acres |
|---|---|---|
| Township | 6 mi x 6 mi (36 sections) | 23,040 |
| Section | 1 mi x 1 mi | 640 |
| Half section | 1 mi x 1/2 mi | 320 |
| Quarter section | 1/2 mi x 1/2 mi | 160 |
| Quarter-quarter section | 1/4 mi x 1/4 mi | 40 |
This is also where the phrase “the back forty” comes from: a 40-acre quarter-quarter section at the far end of a farm. Similarly, the 160-acre homestead claim was one quarter section.
Acres per Square Mile vs Similar-Sounding Areas
In general, most mistakes come from wording, not math. In particular, “half a square mile” and “a half mile square” sound alike but describe very different areas.
| Phrase | What it means | Acres |
|---|---|---|
| 1 square mile | Any shape with an area of 1 sq mi | 640 |
| A mile square | A square 1 mile on each side | 640 |
| Half a square mile | Half the area of a square mile | 320 |
| A half mile square | A square 1/2 mile (2,640 ft) on each side | 160 |
| A quarter mile square | A square 1/4 mile (1,320 ft) on each side | 40 |
| Square kilometer | Metric area, about 0.386 sq mi | about 247.1 |
Another common slip is treating the acre as a length. After all, an acre is an area, so it has no single “length in miles.” Instead, you can only give the side of a particular shape. For example, a square 10-acre plot is 660 feet on each side, and its perimeter is exactly 0.5 mile.
How to Find the Acres in Any Plot
- Measure the sides. First, use a measuring wheel or tape and record each side in feet.
- Find the area. For a rectangle, multiply length by width to get square feet.
- Convert to acres. Divide the square feet by 43,560, or enter the result in the converter above.
- Convert to square miles if needed. Divide the acres by 640, since there are 640 acres per square mile.
- Check irregular shapes piece by piece. Split the plot into rectangles and triangles, then add the areas.
- Compare with the deed. If your figure differs a lot from the recorded acreage, ask a licensed surveyor.
Next, try a quick example. A field 1,320 feet by 660 feet is 871,200 square feet. Then 871,200 / 43,560 = 20 acres, and 20 / 640 = 0.03125 square mile. Our overview of area units and how they compare shows the same steps for metric land.
Picturing 640 Acres per Square Mile
Because large areas are hard to picture, comparisons help. Keep in mind that these are rough guides:
- American football fields. A field with end zones is 360 by 160 feet, or 57,600 square feet (about 1.32 acres). As a result, about 484 such fields fit in a square mile.
- The playing field alone. Without end zones, the field is 300 by 160 feet, about 1.10 acres. In other words, an acre is a bit smaller than the playing area.
- Soccer pitches. A common pitch size of 105 by 68 meters is about 1.76 acres, so roughly 360 fit in a square mile.
Do and Don’t When Converting Land Area
Do
- Multiply square miles by 640 to get acres.
- Also square the side length before converting.
- Keep feet and miles separate until the last step.
- Finally, use the deed or plat for legal acreage.
Don’t
- Confuse “half a square mile” with “a half mile square.”
- Treat an acre as a length, because it is an area.
- Assume every survey section is exactly 640 acres.
- Mix hectares and acres in one sum; instead, convert first.
Honest Limits of 640 Acres per Square Mile
The ratio of 640 acres per square mile is exact, but real land is messier. First, survey sections were laid out on a curved earth with chains and compasses, so many sections are slightly larger or smaller than a perfect square mile. Second, roads, rivers and easements can reduce the usable acreage on a parcel.
There is also a small history note. Before 2023, the acre was tied to the US survey foot, which was a hair longer than today’s international foot. According to NIST, the US survey foot was retired on January 1, 2023. Overall, the change shifts a square mile by about 10 square meters, roughly 0.0004 percent. So it never matters for everyday use, although old survey records can still reflect the earlier definition.
When Accuracy Really Matters
For maps, school work and rough planning, 640 acres per square mile is all you need. However, a few situations call for more care:
- Buying or selling land. Because price per acre multiplies any error, confirm the area on the survey.
- Property taxes and zoning. Assessors and planning offices also rely on recorded legal descriptions, not on your own estimate.
- Fencing and farm inputs. Seed, fertilizer and fence orders depend on true field size and perimeter.
Frequently Asked Questions
How many acres per square mile are there?
There are exactly 640 acres in one square mile, because a square mile is 27,878,400 square feet, and an acre is 43,560 square feet.
Is 640 acres per square mile exact or rounded?
It is exact because both units are defined in feet, and 27,878,400 divided by 43,560 is precisely 640.
How many acres are in half a square mile?
Half a square mile is 320 acres. By contrast, a square that is half a mile on each side is only 160 acres.
How many acres are in a quarter section?
A quarter section is nominally 160 acres, which is one quarter of a 640-acre section.
How many square miles is 1,000 acres?
1,000 acres is 1.5625 square miles. In short, divide the acres by 640 to get square miles.
How many acres are in a square kilometer?
A square kilometer is about 247.1 acres, or about 0.386 square mile.
How many hectares are in a square mile?
A square mile is about 259 hectares, because one acre is about 0.4047 hectare.
How many acres per square mile fit in a township?
A township is about 36 square miles, so it holds about 23,040 acres. Real townships can vary slightly because of survey corrections.
How long is one side of a square acre?
A square acre is about 208.7 feet, or about 63.6 meters, on each side.
How many football fields are in a square mile?
About 484 American football fields with end zones fit in a square mile, since each field covers about 1.32 acres.
The Bottom Line
One square mile always equals 640 acres, because a mile is 8 furlongs and 80 chains and an acre is one furlong by one chain. In metric, that is about 259 hectares or 2.59 square kilometers.
Finally, watch the wording: half a square mile is 320 acres, while a half mile square is 160. For legal or financial decisions, rely on the recorded survey rather than nominal section sizes.
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