The recipe says 1 cup, the shoebox says 10 inches, the butcher says one geun, and the property listing says 25 pyeong. Convert them in one place. And this article adds something most converters do not: in Korea, using pyeong or geun in advertising is prohibited by statute.
1. What does it do. Converts length, mass, volume, temperature, speed, area and data in one place — the recipe's cup, the shoebox's inches, the butcher's geun, the listing's pyeong.
2. What gets missed. Using pyeong or geun in trade or advertising is prohibited in Korea — they are not legal units, and the statute says so directly. That is why property listings are required to show square metres.
3. What to watch. Some conversions are defined “exactly” and others are approximations — one inch is exactly 2.54cm, while others carry rounding. Which is which is marked below.
Type on either side and the other updates. 1 inch = exactly 25.4 mm and 1 yard = exactly 0.9144 m are values NIST states as “exactly.” Pyeong, geun and don are not legal units in Korea, and the factors used here are customary values we could not verify in legislation.
Why pyeong and geun cannot be advertised
Measures Act, Article 6 (Prohibition on Non-Legal Units) (1) No person shall manufacture or import a measuring instrument or product marked in a unit other than a legal unit (a “non-legal unit”) …
(2) No person shall use a non-legal unit in trade measurement or in advertising. …
(3) Notwithstanding paragraphs (1) and (2), a person may display a non-legal unit together with a legal unit where the display requirements prescribed by Ministerial Ordinance are met. <amended 1 Oct 2025>
(4) The Minister or a provincial governor shall police the use of non-legal units and may order a person who has used one to display the legal unit.
Paragraph (2) says plainly: not in trade measurement, not in advertising. That is why Korean property listings switched from “25 pyeong” to “59 sq m” and butchers price by 100 g.
But paragraph (3) permits pairing — display it “together with a legal unit.” Hence listings reading “59 sq m (about 25 pyeong).” The display requirements themselves are a Ministerial Ordinance matter we did not open.
Paragraph (1) carries five exceptions — goods for export, ships, aircraft and military supplies, research and development, instruments and goods for export, and inputs for export goods. That is where nautical miles and knots live (see our marine unit converter).
Whether pyeong and geun appear on the special-unit list is something we could not confirm — Decree Article 3 says only “special units and their meanings are as set out in table 2,” and annexed tables do not appear on the per-article page.
How legal units are layered
Measures Act, Article 4 (Legal Units) (1) Legal units are divided into base units, derived units and special units.
(2) Base units follow Article 10 of the Framework Act on National Standards. (3) Derived units follow Article 11 of that Act.
(4) Special units are units used for special measurement purposes, and their meanings shall be prescribed by Presidential Decree.
Framework Act on National Standards, Article 10 (Base Units) (1) The base units are:
1. the metre for length 2. the kilogram for mass 3. the second for time 4. the ampere for electric current 5. the kelvin for temperature 6. the mole for amount of substance 7. the candela for luminous intensity
| Layer | What it covers | Basis |
|---|---|---|
| 7 base units | metre, kilogram, second, ampere, kelvin, mole, candela | Standards Act art. 10 |
| Derived units | Units built by combining base units | Same Act art. 11 ( not opened) |
| Special units | Units for special measurement purposes | Measures Act Decree table 2 (unreadable) |
| Non-legal units | Everything not above | Measures Act art. 6 — banned in trade and ads |
Note that the base unit for temperature is the kelvin — not Celsius. That is why the temperature tab here includes K.
Exact value, or approximation?
Some conversion factors are fixed to the last decimal by international agreement; others are habit. Mixing them is how errors start.
NIST (US National Institute of Standards and Technology) — “The value for the inch, derived from the value of the Yard effective July 1, 1959, is exactly equivalent to 25.4 mm”. The same page gives 1 yd = 0.9144 m.
| Value | Status | Source |
|---|---|---|
| 1 inch = 25.4 mm | Exact | NIST |
| 1 yard = 0.9144 m | Exact → 1 ft = 0.3048 m and 1 mi = 1,609.344 m are derived from it (our own calculation) | |
| 1 lb = 0.45359237 kg | Widely used but not verified against a source this time | — |
| 1 pyeong ≈ 3.3058 sq m, 1 geun = 600 g, 1 don = 3.75 g | Customary — not confirmed in legislation | — |
The customary values are in the converter — because people really do use them. But they are not values the law fixes. Contracts, advertising and official documents must use sq m, g and kg.
A few values worth having
| Situation | Conversion |
| Body temperature 37°C | 98.6°F (our own calculation: 37 × 9/5 + 32) |
| Oven 180°C | 356°F (our own calculation) |
| Motorway 100 km/h | about 62.1 mph (our own calculation) |
| One Korean cup | 200 mL — a US recipe cup is about 240 mL |
| Exclusive area 85 sq m | about 25.7 pyeong — the threshold in the officetel broker fee test |
| A 1 TB drive | shows as about 931 GiB on a computer (1012 ÷ 230, our own calculation) |
That is the whole story behind “my 1 TB drive says 931 GB.” Manufacturers sell 1 TB = 1012 bytes (decimal); the operating system counts in 230-byte units (binary). 1012 ÷ 230 = about 931.32 (our own calculation) — no capacity vanished; the counting unit differs. The data tab carries both families.
Units tied to a bill, like electricity, need more than conversion — see our electricity bill calculator.
Questions that keep splitting opinion
Can property listings use pyeong?
Measures Act Article 6(2) says non-legal units may not be used in trade measurement or advertising. But paragraph (3) permits display alongside a legal unit, so “59 sq m (about 25 pyeong)” reads as permissible. The display requirements are a Ministerial Ordinance matter we did not open.
Is a geun 600 g or 375 g?
Both are in use — 600 g for meat, 375 g for fruit and vegetables. These are customary values we could not confirm in legislation, so the converter offers both. In a transaction, check the g or kg figure.
Is 1 GB 1,024 MB or 1,000 MB?
It depends on context. Internationally, kB, MB and GB are decimal (1,000) while KiB, MiB and GiB are binary (1,024). Confusion arises because many operating systems label binary quantities “GB.” This converter keeps the two families apart.
Why does temperature convert differently?
Because the zero points differ. Length and mass need only multiplication (zero is zero in any unit), but temperature adds an offset: °F = °C × 9/5 + 32, and K = °C + 273.15.
Which units belong in a contract?
Legal units. The property register records area in sq m (see our property register guide), and so does the exclusive area you check before signing a lease (jeonse contract checklist).
Units are not a matter of taste. Some values are fixed to the decimal by agreement, some are only habit, and some units may not lawfully appear in an advertisement at all.
Sources
- Ministry of Government Legislation, National Law Information Center — statutory text — Measures Act, Article 6 (Prohibition on Non-Legal Units). Source of “no person shall use a non-legal unit in trade measurement or in advertising,” paragraph (3) on paired display, the five exceptions in paragraph (1), and paragraph (4) on policing and display orders.
- Ministry of Government Legislation, National Law Information Center — statutory text — Framework Act on National Standards, Article 10 (Base Units). Source of the seven base units.
- Ministry of Government Legislation, National Law Information Center — statutory text — Measures Act, Article 4 (Legal Units). The three-way classification and the delegation of special units to Presidential Decree. The delegated Decree Article 3 says only that they are “as set out in table 2.”
- NIST — US National Institute of Standards and Technology — SI Units – Length. Source of “exactly equivalent to 25.4 mm” and 1 yd = 0.9144 m. The foot and mile values are derived by us.
- Our own calculations. 37°C = 98.6°F, 180°C = 356°F, 100 km/h ≈ 62.1 mph and 1 TB ≈ 931 GiB (1012 ÷ 230) were computed here.
Where to check further
- What actually happens if a non-legal unit is used. This article goes as far as the prohibition itself — penalty amounts and procedure are in the schedules to the Enforcement Decree of the Measurement Act and with the Korean Agency for Technology and Standards.
- Sector-specific exceptions. Several customary units remain in use and we could not pin the exception scope to the text — the Agency's legal metrology desk answers by sector.
- The latest revisions to defined values. SI definitions change by General Conference on Weights and Measures decision — the Korea Research Institute of Standards and Science (KRISS) publishes the values in force domestically.
Written as of August 2026. Statutory quotations are the text as published by the National Law Information Center, the exact inch and yard values come from NIST, and anything we computed is marked as such. The pyeong, geun and don factors are customary values we could not verify in legislation — use legal units (sq m, g, kg) in transactions, contracts and official documents. This article is general information.
