Square Kilometers to Barns (km² to b)

1 square kilometer equals 1×10³⁴ barns exactly.

1 square kilometer = 1×10³⁴ barns

How to convert square kilometers to barns

To convert square kilometers to barns, multiply the number of square kilometers by 1×10³⁴. This factor is exact. To go the other way, multiply barns by 1×10⁻³⁴.

barns = square kilometers × 1×10³⁴

Example: 10 square kilometers = 1×10³⁵ barns.

About these units

Square Kilometer (km²)

Metric (SI)Current unit

One million square meters (100 hectares). Used for the area of cities, countries and lakes.

Metric land measure took a surprisingly long time to settle on the kilometer. The meter was defined in 1799, yet people initially preferred the myriameter, 10,000 meters, and the French phrase myriamètres par heure was still in print in 1802. The Netherlands adopted the kilometer in 1817 under the name mijl. Once the kilometer was established, the square kilometer followed naturally as the unit for areas too large to count comfortably in hectares or square meters.

One square kilometer is 1,000,000 square meters, or 100 hectares, about 0.3861 square miles or 247.1 acres. A useful picture comes from history: many medieval European cities were enclosed by walls around roughly a square kilometer, with Delft at about 0.9 km² and Lucca at about 1.09 km². A standard 18-hole golf course of 6,000 meters needs about 0.8 km². The symbol km² means the square of a kilometer, not a thousand square meters. The planned Square Kilometre Array radio telescope takes its name from the collecting area it aims for.

1 square kilometer = 1,000,000 square meters (exactly).

Sources: Wikipedia: Square kilometre; Wikipedia: Kilometres per hour

Barn (b)

ScientificCurrent unit

Exactly 10⁻²⁸ square meters. Used in nuclear and particle physics for reaction cross-sections; the name is a joke about a target 'as big as a barn'.

Nuclear physicists coined this unit during the Second World War. Marshall Holloway and Charles Baker, working at Purdue on Manhattan Project problems, wanted a short name for the cross section of a nucleus and passed over names such as Oppenheimer and Bethe. They settled on a joke about a target that was really as big as a barn, and the word first appeared in print in a secret Los Alamos report of June 1943.

One barn is 10 to the power of minus 28 square meters, or 100 square femtometers, roughly the cross-sectional area of a uranium nucleus. NIST lists the conversion as exact. The barn was never an SI unit, though the BIPM acknowledged it in the eighth SI Brochure because particle physics relies on it. Prefixed forms dominate today: the femtobarn is the usual choice in high-energy physics, and the inverse femtobarn measures integrated luminosity. Informal names exist too, such as outhouse for a microbarn and shed for a yoctobarn.

1 barn = 1×10⁻²⁸ square meters (exactly).

Sources: Wikipedia: Barn (unit); NIST SP 811, Appendix B.8 conversion factors

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