Why does the Bureau of Land Management describe a township as nominally 36 square miles rather than exactly 36 square miles?

Because nominal signals that the figure describes the ideal, intended size the survey system is designed around, not a guaranteed measurement for every township on the ground. The grid establishes a consistent legal framework, but actual survey conditions mean a specific township's true acreage can come out slightly above or below that ideal figure.

Why are survey corrections concentrated in the northern and western edges of a township rather than spread evenly across all 36 sections?

Because historical surveys were run progressing north and west from a fixed starting point, so accumulated measurement error naturally built up on the last lines surveyed, the township's northern and western boundaries. The convention places that error in the north tier and west column, with the northwest corner section absorbing the most.

Does a region covering exactly one square mile have to be shaped like a square?

No. A square mile describes area, not boundary shape. An irregular park, lake or district can cover exactly 640 acres and therefore have an area of one square mile. Only a square-shaped region of one square mile must measure one mile along each side.

Why doesn't the 640-acre survey grid appear in the original thirteen states or in Texas the way it does across the Midwest?

Because those areas had already established their own land-description systems before the federal rectangular survey system was created, and land already surveyed under an older system generally was not re-platted onto the new grid. The clean, rectangular checkerboard pattern visible from above is mainly a feature of the Midwest and parts of the West.

Square Miles to Acres: How Survey Grids Shaped America

Convert square miles to acres and discover why the number 640 shaped the layout of farms, roads and property lines across much of the United States. One square mile always equals exactly 640 acres — a relationship with a specific historical origin, explored below.

Square Mile to Acre Conversion Formula

One square mile contains exactly 640 acres. This relationship follows from one square mile containing 27,878,400 square feet and one acre containing 43,560 square feet.

The conversion formula is:

1 mi² = 640 ac

Or simply:

Acres = Square miles × 640

This conversion uses international-foot-based area values: 1 mi² = 2,589,988.110336 m² and 1 ac = 4,046.8564224 m².

These conversion values and the exact relationship 1 mi² = 640 ac are documented in the NIST revised unit conversion factors and the NIST Guide to the SI conversion tables .

Common Square mile to Acre Conversions

1 mi² = 640 ac
10 mi² = 6,400 ac
100 mi² = 64,000 ac
1,000 mi² = 640,000 ac
10,000 mi² = 6,400,000 ac

Square Miles to Acres: Reading the Grid Across the American Landscape

Square miles are useful for describing forests, parks, ranches and districts, but individual parcels are often recorded in acres. Converting between the units allows a large mapped area to be compared with farms, development sites and legal property descriptions.

The conversion is exact:

1 square mile = 640 acres

To convert any area from square miles to acres, multiply by 640:

Acres = square miles × 640

How One Square Mile Became a Survey Section

Beginning in the early history of the United States, federal surveyors used a rectangular system to identify and divide much of the public domain. A principal meridian provided a north–south reference, while a base line provided an east–west reference. Surveyed land could then be described by township, range and section.

The ideal survey township measured six miles from north to south and six miles from east to west:

6 miles × 6 miles = 36 square miles

Converting that nominal area into acres gives:

36 mi² × 640 acres per mi² = 23,040 acres

Each township was normally divided into 36 numbered sections. An ideal section measured one mile on each side and contained 640 acres. The sections were numbered in a serpentine (boustrophedon) pattern beginning in the northeastern corner of the township: numbering runs west across the top row to Section 6, drops down a row and runs east back to Section 12, and continues zigzagging back and forth until it ends at Section 36 in the southeastern corner.

The Bureau of Land Management's guide to legal land descriptions explains how townships, ranges, sections and smaller subdivisions are used to identify land.

From a Section to an Individual Property

A section could be divided repeatedly into halves and quarters. Under the ideal grid, every division also produced a familiar acreage:

Half-section: 0.5 square mile or 320 acres
Quarter-section: 0.25 square mile or 160 acres
Half of a quarter-section: 0.125 square mile or 80 acres
Quarter-quarter section: 0.0625 square mile or 40 acres

These subdivisions, known as aliquot parts, allowed a parcel to be identified by its position inside a larger section. A description such as the northeast quarter of a section identifies a particular part of the grid rather than relying only on a street address.

Diagram dividing a one-mile-square survey section containing 640 acres into smaller land parcels — An ideal one-square-mile section contains 640 acres and can be divided into halves, quarters and smaller aliquot parts.

Common Square-Mile Areas Expressed in Acres

Nominal survey areas converted from square miles to acres
Square miles	Acres	Survey-grid comparison
0.0625 mi²	40 acres	Quarter-quarter section
0.125 mi²	80 acres	Half of a quarter-section
0.25 mi²	160 acres	Quarter-section
0.5 mi²	320 acres	Half-section
1 mi²	640 acres	Full section
6 mi²	3,840 acres	Six ideal sections
10 mi²	6,400 acres	Large park or ranch area
36 mi²	23,040 acres	Ideal survey township

Why the Grid Is Visible From Above

Across many parts of the Midwest and western United States, aerial photographs show long straight roads and rectangular fields. Some of these features follow section lines or smaller divisions inherited from the public-land survey grid.

The pattern influenced more than property records. Survey boundaries could shape the placement of roads, fences, farms and drainage systems. As neighbouring parcels were divided or combined, later landscapes often retained traces of the original rectangular structure.

The pattern is not universal. The PLSS was not used in the same way across the original thirteen states, Texas and certain other areas governed by older or separate land-description systems.

Why Real Sections Can Differ From 640 Acres

A flat sheet of squares cannot be extended indefinitely across a curved planet without adjustment. Survey lines running toward true north gradually approach one another because meridians converge. Measurement limitations, waterways, earlier boundaries and other physical conditions also affected the layout.

Surveyors used standard parallels, guide meridians, correction lines and fractional lots to manage these differences. Under the general survey plan, an excess or deficiency was commonly placed along the northern and western sides of a township rather than distributed evenly among every section — specifically the top row of sections and the westernmost column, with the northwest corner section typically bearing the largest adjustment of all, since it sits where both correction edges meet.

The Bureau of Land Management describes townships and sections as nominally six miles square and one mile square in its Specifications for Descriptions of Land . That wording is important: the grid establishes a legal survey framework, but it does not guarantee that every recorded section contains precisely 640 acres.

An Exact Conversion and a Surveyed Acreage Answer Different Questions

If an area is stated as exactly 2.5 square miles, its mathematical conversion is straightforward:

2.5 mi² × 640 = 1,600 acres

A historic property described as two and a half survey sections may not contain exactly 1,600 surveyed acres, however, because one or more sections could be fractional. For legal or purchasing decisions, the official plat, deed and surveyed acreage should be consulted.

The converter answers the unit question: how many acres represent a stated number of square miles. A land survey answers a different question: how much area lies inside the property's legally established boundaries.

Square Mile and Acre Questions and Answers

Because the survey grid is laid over a curved planet using lines that are supposed to run true north-south, and true meridians gradually converge toward each other rather than staying perfectly parallel. That unavoidable geometric fact, combined with measurement limitations and physical obstacles like waterways, means a flat grid of identical 640-acre squares can't actually be extended indefinitely without some sections absorbing the resulting excess or shortage.

Because "nominal" signals that the figure describes the ideal, intended size the survey system is designed around, not a guaranteed measurement for every individual township on the ground. The grid establishes a consistent legal framework for describing land, but actual survey conditions mean a specific township's true acreage can come out slightly above or below that ideal figure.

Because historical surveys were run from a fixed starting point progressing north and west, so any accumulated measurement error naturally built up and landed on the last lines surveyed, which were the township's northern and western boundaries. Rather than spreading that error thinly across every section, the convention places it specifically in the north tier and west column, with the northwest corner section absorbing the most since it sits where both of those edges meet.

This back-and-forth numbering, known as a boustrophedon pattern, keeps consecutively numbered sections physically adjacent to each other even as the numbering crosses from one row to the next. If every row simply restarted counting from the same side, Section 6 and Section 7 would sit at opposite ends of the township rather than sharing a border, which would make the numbering far less intuitive to navigate on an actual map.

Because those areas had already established their own land-description systems before the federal rectangular survey system was created, and land already surveyed and claimed under an older system generally wasn't re-platted onto the new grid. That's why the clean, rectangular checkerboard pattern visible from above is a feature mainly of the Midwest and parts of the West, rather than something found uniformly across the entire country.

Square mile to Acre Converter (convert mi² to ac)