Multiply square metres by 0.3025 to get pyeong. The exact links between pyeong, ares, hectares and acres, and the traps in quoted floor areas.
The most common mistake when converting area is reusing the multiplier for length. A metre is 100 cm, but a square metre is 100 times larger in both directions, so it equals 10,000 square centimetres. Likewise a square kilometre is 1,000,000 square metres. Double both the length and width of a room and its area quadruples. Whenever a unit has 'square' in front of it, remember that the length conversion factor must be squared too.
The pyeong is the area of a square six ja on each side. With one ja defined as 10/33 of a metre, each side is 6 × 10/33 = 20/11 m, and one pyeong is that squared, 400/121 m² or about 3.3058 m². Conversely, one square metre is exactly 121/400 pyeong, or 0.3025 pyeong. So to turn square metres into pyeong you multiply by 0.3025, and to go the other way you divide by 0.3025. The Japanese tsubo is the same size. In Korea, use of legal metric units has been mandatory since 2007, so official transactions and labels are in square metres, and the pyeong survives only by custom.
Working out a few figures that come up often in property listings and conversation gives you a feel for the sizes. Values are rounded.
The same Korean flat can be described with different numbers depending on the basis. Exclusive area is the space you alone use inside the front door. Supply area adds residential common parts such as stairs, corridors and lifts. Contract area further adds other common facilities such as car parks and management offices. The size people quote in pyeong is often based on supply area, which is why a flat with 84 m² of exclusive area is often described as a flat in the mid-30s in pyeong. Extended balcony space is not included in any of these. When comparing homes or prices per pyeong, compare figures on the same basis.
Larger areas such as land use ares and hectares. One are is 100 m², a square 10 m on each side, and one hectare is 10,000 m², a square 100 m on each side. In pyeong, an are is 30.25 and a hectare exactly 3,025. A square kilometre is 100 hectares. Hectares appear often in farmland and forestry statistics and park sizes, so keeping the picture of 'a hectare is a 100 m square' in mind makes sizes easier to judge.
American and British material often uses acres and square feet. Because a foot is exactly 0.3048 m, these units have exact values too.
Hearing 'a 100-pyeong plot', it is tempting to picture a square 10 pyeong by 10 pyeong, but the pyeong is an area unit and cannot be split that way. 100 pyeong is about 330.6 m², so as a square each side would be about 18.2 m. Even at the same area, a long or irregular shape changes how much space you can really use. When looking at land or a room, check the dimensions and shape as well as the area, and for anything that is not a rectangle, divide it into triangles and rectangles, calculate each, and add them up.
Most area errors come from forgetting to square or mixing bases. Where money is involved, as in a contract or sale, check against the square-metre figure in the official documents. Rounding the pyeong to 3.3 m² is off by about 0.6 m² at 100 pyeong and about 5.8 m² at 1,000 pyeong, so the larger the plot, the more the exact value matters.
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