Acreage calculator: measure a lot on a map or by dimensions

The calculator above measures land in acres three ways: outline a lot on aerial imagery, enter the dimensions of a rectangle, triangle, circle or trapezoid, or convert an area you already have. Every method uses one rule. Find the area in square feet, then divide by 43,560.

Pillar wrote this page and built the calculator. A figure from it is an estimate for screening a property. It is not a survey, and it does not locate a legal boundary.

How to calculate acreage

Measure the lot in feet, work out its area in square feet, and divide by 43,560. A rectangular tract 330 feet wide and 660 feet deep covers 217,800 square feet. Divide that by 43,560 and the answer is 5 acres.

If the measurements are in another unit, convert them to feet first or let the calculator do it. It accepts feet, yards, meters, chains and rods. Some older deeds give distances in chains and rods. NIST Handbook 44, Appendix C lists a chain as 66 feet, a rod as 16.5 feet and an acre as 10 square chains. A strip 10 chains long and 1 chain wide is 660 feet by 66 feet, which is 43,560 square feet, or 1 acre.

How many square feet are in an acre

One acre is 43,560 square feet. NIST lists that figure as exact. The same acre is 4,840 square yards, about 4,046.86 square meters and about 0.4047 hectares. A square mile holds 640 acres.

An acre has no fixed shape. As a square it is about 208.71 feet on each side. A lot 150 feet wide and 290.4 feet deep is also 1 acre, and so is a strip 66 feet wide and 660 feet long.

Older records can use a slightly different acre. The US survey foot was about two parts per million longer than the international foot, so an acre measured in survey feet was about four parts per million larger. NIST retired the US survey foot at the end of 2022, and the calculator uses the international foot of 0.3048 meters. On a 100 acre tract the two definitions differ by about 17 square feet.

How to measure acreage on a map

Open the map tab in the calculator and follow these steps.

1. Search for a street address, or pan and zoom until you can see the whole lot.
2. Click or tap each corner in order, working around the outline. On a phone you can also slide the map under the crosshair and press "Add point at crosshair."
3. Drag any point to adjust it. The acreage, square footage and perimeter update as you go.

The calculator measures the outline on the WGS84 ellipsoid, the model of the earth that GPS uses, with Karney's geodesic algorithms. Web maps use the Web Mercator projection, which stretches distances away from the equator. An area computed from the map's flat coordinates would come out about 49% too large at 35 degrees north and about 70% too large at 40 degrees north.

We checked the method against a 10 acre square laid out with the National Geodetic Survey's coordinate conversion tool. The result agreed to better than one part in a million.

The outline you draw stays in your browser. Map images load from USGS servers. If you search for an address, the text goes through Pillar's server to the Census Bureau geocoder, which returns a location for the map.

How accurate is acreage measured on a map?

Accuracy depends on the photo and on where you put the points. The math adds almost no error.

The imagery comes from The National Map, a USGS program whose map services are free and in the public domain. When you zoom in to lot scale, most of it is aerial photography from the National Agriculture Imagery Program. USGS says the program's standard ground resolution changed to 0.6 meters in 2018, which is about 2 feet per pixel. The photos are taken in the growing season, on a repeat cycle of up to three years.

A small error at the edge matters more on a small lot. On a square 1 acre lot, drawing every edge 2 feet outside the true line overstates the area by about 3.9%. On a square 10 acre tract the same 2 foot error is about 1.2%.

A photo shows fences, mowing lines, tree lines and pavement edges. A property line can sit somewhere else. Tree cover can hide a corner, and the photo can be a few years old. Use the result for screening, and get a survey when the acreage affects price, yield or a boundary.

Acreage formulas for each lot shape

Each formula gives square feet when the measurements are in feet. Divide the result by 43,560 for acres.

Rectangle or square

Multiply length by width. A lot 100 feet by 200 feet is 20,000 square feet, or 0.4591 acres.

Triangle

With a base and a height, multiply the two and divide by 2. The height is measured at a right angle to the base. A base of 400 feet and a height of 300 feet give 60,000 square feet, or 1.377 acres.

With three sides and no height, use Heron's formula. Call half the perimeter s. Multiply s by s minus each side in turn, then take the square root. Sides of 300, 400 and 500 feet give s = 600, so the product is 600 × 300 × 200 × 100 = 3,600,000,000. The square root is 60,000 square feet, or 1.377 acres.

Three lengths only make a triangle when the two shorter sides add up to more than the longest. The calculator rejects sides such as 100, 200 and 400 feet.

Circle

Multiply pi by the radius squared. A radius of 100 feet gives about 31,416 square feet, or 0.7212 acres.

Trapezoid

Add the two parallel sides, divide by 2, and multiply by the distance between them. Parallel sides of 200 and 300 feet set 150 feet apart give 37,500 square feet, or 0.8609 acres.

How to calculate the acreage of an irregular lot

A lot with angled or uneven sides needs one of two approaches.

The first is to draw it. On the map, put a point at every bend in the boundary. The calculator takes up to 100 points and measures the whole outline at once.

The second is to split the lot into triangles. Measure a diagonal between two opposite corners, which divides a four-sided lot into two triangles with three known sides each. Take a lot with sides of 210, 180, 160 and 230 feet in order and a 250 foot diagonal. The first triangle has sides of 210, 180 and 250 feet and covers 18,573 square feet. The second has sides of 250, 160 and 230 feet and covers 17,960 square feet. Together they are 36,533 square feet, or 0.8387 acres.

You need the diagonal. The same four sides can be pushed into a flatter shape with less area, so four side lengths alone do not give an answer.

If an outline crosses over itself, the calculator reports the crossing and gives no area. Move a point or undo until the outline is a simple loop. It also declines an outline that runs more than about 69 miles north to south or east to west, which is far larger than any single property.

Gross acres vs. net acres

Gross and net acreage are defined locally. The American Canyon, California municipal code is one example. It defines a gross acre as the area of a lot or site "including easements of right-of-way to be dedicated, but excluding existing public rights-of-way" and a net acre as the area "remaining after dedication of all required rights-of-way."

Here is an illustration with made-up numbers. A 10 acre tract must dedicate 1.2 acres for streets, which leaves 8.8 net acres. At 4 homes per acre, a density counted on gross acres allows 40 homes and one counted on net acres allows 35. Narrow strips add up as well. A right-of-way 30 feet wide along 660 feet of frontage is 19,800 square feet, or 0.4545 acres.

Each jurisdiction writes its own definition, so read the one in the code that applies to the site before you use either number. The calculator measures whatever you outline. To estimate net acres, draw only the area left after the dedication, or measure the strip on its own and subtract it.

Why measured acreage differs from deeded or GIS acres

A property can carry three acreage figures. The deed or survey states one. The county assessor or GIS parcel layer shows another. The outline you draw on a photo gives a third. Each comes from a different source, so they can disagree.

Under the map result, the calculator has an optional row for comparing with the record. Enter the deeded acreage and the assessor or GIS acreage, and it shows how far the measured figure sits from each in acres and percent. A measured 9.82 acres against a deeded 10 acres is 0.18 acres smaller, a difference of 1.8%.

A gap does not say which number is wrong. Our guide to deeded acres vs. GIS acres covers where each recorded figure comes from and which records to check before you change a number. A licensed surveyor is the person who can settle where a boundary is.

Common acreage questions

How many acres is a 100 x 100 foot lot?

A 100 by 100 foot lot is 10,000 square feet, which is 0.2296 acres. That is a little under a quarter acre.

How many acres is a 200 x 200 foot lot?

A 200 by 200 foot lot is 40,000 square feet, or 0.9183 acres.

How many acres is 660 x 660 feet?

A 660 by 660 foot square is 435,600 square feet, which is 10 acres. Its perimeter is 2,640 feet, or half a mile.

How big is an acre in feet?

An acre is 43,560 square feet. A square acre is about 208.71 feet on each side. Other 1 acre shapes include 150 by 290.4 feet and 100 by 435.6 feet.

How many square feet are in a half acre or a quarter acre?

A half acre is 21,780 square feet, about 147.6 feet on each side as a square. A quarter acre is 10,890 square feet, about 104.4 feet on each side.

How do I convert square feet to acres?

Divide the square feet by 43,560. For example, 20,000 square feet is 0.4591 acres. To go the other way, multiply acres by 43,560, so 2 acres is 87,120 square feet.

How many acres are in a hectare?

One hectare is about 2.471 acres, and one acre is about 0.4047 hectares. A hectare is 10,000 square meters.

How many acres are in a section?

About 640. NIST Handbook 44, Appendix C lists a section of land as approximately 1 mile square, and a square mile is 640 acres. A quarter section is about 160 acres and a quarter-quarter section about 40. A township is about 6 miles square, or 36 sections. NIST marks these figures as approximate, so the acreage of an actual section comes from its survey.

Where Pillar fits

Pillar's property search finds a parcel by address, APN or owner name. The record includes acreage along with ownership, zoning, land use and last sale, and you can save a parcel to your acquisition pipeline. That recorded acreage is one of the figures the compare row is for. The calculator on this page is free and needs no account.