Acres and Hectares: Land Area Units

Two units for the same job

Land is rarely measured in square meters or square feet, because the numbers become unwieldy. Instead, farmland, parks and building plots are usually quoted in larger units. The hectare is the standard choice in most of the world. The acre remains in everyday use in the United States and several other countries, and it is still seen in the United Kingdom, particularly for land.

If you read property listings, agricultural reports or planning documents from different countries, you will meet both. Knowing how they relate is mostly a matter of remembering two fixed numbers.

This article covers the exact definitions, the conversion in both directions, worked examples and a few cautions for anyone dealing with real land rather than abstract figures.

What a hectare is

A hectare is exactly 10,000 square meters. That is the area of a square measuring 100 meters on each side. It is not an SI unit in the strict sense, but the BIPM accepts it for use alongside the SI, and it is the usual unit for land across science, farming and forestry.

Because it is defined from the meter, the hectare fits neatly into the metric system. One square kilometer is 100 hectares, and one hectare is 10,000 m². Converting between them requires only moving the decimal point. Smaller metric areas follow the same pattern: a plot of 2,500 m² is simply 0.25 hectares.

What an acre is

The acre has a more historical shape. In its modern international form it is defined as exactly 43,560 square feet, which equals 4,840 square yards. Using the exact foot-to-meter relationship, this works out to 4,046.8564224 m², or 0.40468564224 hectares. A square with an area of one acre has sides of about 63.6 meters.

The acre also fits into a larger unit that is still used on maps: 640 acres make one square mile. Those relationships are exact, so a square mile comes to 258.9988 hectares when rounded. A parcel of 160 acres, a quarter of a square mile, is therefore about 64.75 hectares.

Converting between them

The two factors you need are:

• 1 acre = 0.40468564224 ha (exact)
• 1 hectare ≈ 2.4710538 acres (rounded reciprocal)

To convert acres to hectares, multiply by 0.4047. A 50-acre farm is 50 × 0.40468564 = 20.234 ha, which you would usually write as about 20.2 hectares. For a smaller plot, 3.5 acres is 3.5 × 0.40468564 = 1.4164 ha.

To convert hectares to acres, multiply by 2.4711. A 12.5-hectare field is 12.5 × 2.4710538 = 30.888 acres, so about 30.9 acres. A 250-hectare estate comes to roughly 617.8 acres.

A shortcut for quick estimates

For everyday judgement, remember that a hectare is about two and a half acres, and an acre is about four tenths of a hectare. Both are slightly rounded: the exact numbers are 2.47 and 0.405.

The shortcut is accurate enough to compare plots at a glance. A 40-acre parcel is about 16 hectares (the exact figure is 16.19). A 100-hectare farm is about 250 acres (the exact figure is 247.1). The error is about 1.2% in both cases, which is acceptable for general reading but not for a purchase, tax calculation or boundary description.

Do not confuse the two units when scanning numbers. An acre is smaller than a hectare, so the number of acres is always larger than the number of hectares for the same land.

The survey acre and why it rarely matters

In the United States there used to be two slightly different acres. The international acre above is based on the international foot. The older US survey acre is based on the US survey foot and is about 4,046.87261 m², larger by roughly 0.016 m². The difference is about four parts per million, so even for a very large tract it is tiny. On a 640-acre square mile, it adds up to only around 10 square meters, less than the floor of a small room.

The US National Geodetic Survey and NIST announced that the survey foot, survey mile and survey acre would be phased out at the end of 2022. For everyday conversion you can ignore the difference. For land surveying, legal descriptions or coordinates, always check which definition the source document uses.

Practical tips for land measurements

When working with land, keep these points in mind:

1. Check the unit before you compare. A number such as 12 means very different things in acres and hectares.
2. Carry full precision through the calculation and round at the end, because land values often multiply the area by a price.
3. Ask whether the area is given as a gross total or a net figure, since farmland, woodland and building plots are sometimes described in different ways.
4. Remember that a stated area may be measured on a flat plan rather than along the ground surface, which can differ on steep terrain.

A unit converter such as this one is convenient here. Select the area category, enter your value, pick acres and hectares, and read the result along with the equation. The values in all other area units appear below it, so you can also see the figure in square meters, square feet, square yards or square miles. For legal documents, deeds or financial decisions, treat the converted figure as a guide and rely on the original document.