How to Convert Square Miles to Acres
Converting square miles to acres is a fundamental calculation in American land management, real estate, and geography. Both units belong to the imperial system, but they serve different scales — square miles describe large geographic areas like counties, cities, and states, while acres describe individual properties, farms, and development sites. One square mile equals exactly 640 acres, a clean conversion factor rooted in the US Public Land Survey System established in the late 18th century. This relationship is deeply embedded in American land law, property records, and agricultural planning. When a county reports its total area in square miles, developers and farmers need to think in acres to understand how much usable land exists. The 640-acre section became the basis for dividing the American West into manageable parcels, with quarter sections (160 acres) serving as the standard homestead allotment. Today, this conversion remains essential for land use planning, environmental conservation, natural resource management, and real estate development. Understanding that one square mile contains 640 acres helps contextualize the scale of large land transactions, conservation easements, and municipal planning decisions that shape communities across the United States.
Conversion Formula
One mile equals 5,280 feet, so one square mile equals 5,280 × 5,280 = 27,878,400 square feet. One acre equals 43,560 square feet. Dividing these: 27,878,400 ÷ 43,560 = 640 acres per square mile. This is an exact conversion. The factor 640 is the foundation of the US rectangular survey system, where one "section" of land equals one square mile or 640 acres.
Acres = Square Miles × 640
4 square miles = 2560 acres
Step-by-Step Example
To convert 4 square miles to acres:
1. Start with the value: 4 mi²
2. Multiply by the conversion factor: 4 × 640
3. Calculate: 4 × 640 = 2,560 acres
So 4 square miles equals exactly 2,560 acres. This is equivalent to 4 full sections of land in the US Public Land Survey System, or the size of a mid-sized ranch in the western United States.
Understanding Square Miles and Acres
What is a Square Mile?
The square mile derives from the mile, which has roots in the Roman "mille passus" (thousand paces), equivalent to about 5,000 Roman feet. The English statute mile was standardized at 5,280 feet by an act of Parliament in 1593. The square mile, as the area of a square with sides one mile long, became the standard large-area measurement in English-speaking countries. It remains widely used in US geography, real estate, and government statistics. The square mile is also the basis for the "section" in the US Public Land Survey System, where one section equals exactly one square mile (640 acres).
What is a Acre?
The acre has a long history dating back to medieval England, where it was originally defined as the amount of land a yoke of oxen could plow in one day. The word comes from the Old English "æcer" meaning open field. Over centuries, the acre was standardized to exactly 43,560 square feet or 4,840 square yards. In the Middle Ages, an acre was described as a strip of land one furlong (660 feet) long and one chain (66 feet) wide. The acre remains widely used in the United States for real estate and agriculture, and is still commonly understood in the United Kingdom, Canada, and Australia.
Practical Applications
Land use planners convert county and city areas from square miles into acres for zoning and development calculations. Agricultural agencies assess farmland availability by converting geographic regions into acreage. Conservation organizations calculate preserve sizes for habitat management. Real estate developers evaluate large land acquisitions. Government land management agencies like the Bureau of Land Management use both units extensively in managing federal lands.
Tips and Common Mistakes
The conversion factor of 640 is exact and easy to remember. A common memory aid: 640 = 2 × 2 × 2 × 2 × 2 × 2 × 2 × 5 = 128 × 5. A frequent mistake is confusing square miles with miles — one square mile is a unit of area, not length. Also be careful not to mix up the direction: multiplying square miles by 640 gives acres (a larger number), while dividing acres by 640 gives square miles (a smaller number). For reference, a township in the US survey system is 36 square miles or 23,040 acres.
Frequently Asked Questions
One square mile contains exactly 640 acres. This is a precise, historically defined relationship that forms the basis of the US Public Land Survey System. A square mile is also known as a "section" in land surveying terminology.