Land measurement guide
How many acres are in a section of land?
A section is 640 acres, and it is exactly one square mile. That single fact is the backbone of how most of the United States west of the original colonies is surveyed, described, and deeded. This guide shows where the 640 comes from, how sections stack into townships, and how to read the legal descriptions built from them.
A section is 640 acres and one square mile
A section measures one mile, 5,280 feet, on each side. One mile squared is 27,878,400 square feet, and dividing by the 43,560 square feet in an acre gives exactly 640 acres. So the two facts are the same fact: a square mile and a 640-acre section are one and the same block of ground.
This comes from the Public Land Survey System, or PLSS, the rectangular grid the federal government has used to survey public land since the Land Ordinance of 1785. It is why so much of the Midwest, the Plains, and the West is a checkerboard of square fields and mile-apart roads. The original thirteen colonies, Texas, and a few other areas use older metes-and-bounds descriptions instead, so the section grid does not cover the whole country.
How a township holds 36 sections
Sections are grouped into townships. A survey township is 6 miles by 6 miles, so it holds 36 sections of one square mile each. The sections are numbered in a fixed back-and-forth pattern that surprises people the first time they see it: number 1 sits in the northeast corner, the count runs west to 6, drops one row south and runs east to 12, and keeps snaking down until it ends at 36 in the southeast corner.
A full township therefore contains 36 times 640 acres, which is 23,040 acres. One caution on the word township: the survey township here is a measurement unit. A civil township is a unit of local government, and its borders do not always line up with the survey grid.
Quarter-sections and the aliquot parts
A whole section is rarely bought or sold as one 640-acre piece. It gets split into halves and quarters, and those into quarters again, in a nested system called aliquot parts. Each split names a compass position, so the pieces describe themselves:
- Split a 640-acre section in four and each piece is a quarter-section of 160 acres: the NW 1/4, NE 1/4, SW 1/4, and SE 1/4.
- Split a quarter again and each piece is 40 acres, the classic American "forty." The top-right 40 above is the NE 1/4 of the NE 1/4.
- Split once more and you reach a 10-acre quarter-quarter-quarter, about as fine as aliquot descriptions usually go.
How to read a legal description
Put those pieces together and you can read a PLSS legal description, which is written from the smallest part to the largest, then decoded from the largest to the smallest. Take NW 1/4 of Section 12, T2N R3E:
- T2N R3E is the township: the second tier north (Township 2 North) and the third column east (Range 3 East) of that survey's starting point, its principal meridian and baseline.
- Section 12 is one of the 36 square-mile sections inside that township. From the grid above, section 12 sits on the east edge, second row down.
- NW 1/4 is the northwest 160-acre quarter of that section.
Read together, that description points to one specific 160-acre block of ground, no map needed, which is exactly why the system has lasted since 1785. Longer descriptions just nest more parts: "SE 1/4 of the NW 1/4 of Section 12" is a 40-acre piece, the southeast forty of that northwest quarter.
The grid tells you where a parcel is and how big the survey block is. It does not tell you what the ground is like, how the modern deeded boundary falls inside the section, or what the soil will grow. For those, keep going: what a plat map shows covers how ownership is recorded on top of the survey, how to calculate acreage handles parcels that do not follow the grid, how big an acre is anchors the base unit, and how much an acre of land costs takes it to value.
Sections and townships: frequently asked questions
- How many acres are in a section of land?
- A section is 640 acres. It is also one square mile, measuring one mile (5,280 feet) on each side. Because it is defined by the survey grid, a full section is always 640 acres, even though field boundaries inside it rarely follow the section lines exactly.
- How many acres are in a township?
- A survey township is 6 miles by 6 miles and holds 36 sections, so it contains 36 times 640, which is 23,040 acres. Do not confuse a survey township with a civil township, which is a unit of local government whose borders may not match the survey grid.
- How big is a quarter-section?
- A quarter-section is 160 acres, half a mile on each side. It is named by its position in the section, so the northwest quarter is the NW 1/4. A quarter of a quarter, such as the NE 1/4 of the NE 1/4, is 40 acres, the classic "forty" of American land.
- What does a legal description like "NW 1/4 of Section 12, T2N R3E" mean?
- Read it from the largest piece to the smallest. T2N R3E names the township, two tiers north and three ranges east of the survey origin. Section 12 is one of the 36 square-mile sections in that township. NW 1/4 is the 160-acre northwest quarter of that section. Together they point to one specific 160-acre block on the ground.
- Is every section exactly 640 acres?
- Almost, but not always. Most sections are a full 640 acres, but the survey system corrects for the curve of the earth along certain lines, so sections on the north and west edges of a township can be slightly larger or smaller. These are called fractional sections, and their odd acreages are recorded in the original survey.
From a section number to the actual ground.
Outline a parcel and get its computed acreage, soil, water, and revenue options from federal data. Ohio and Indiana have pre-loaded parcel boundaries; modeled data covers all 50 states.