Barns — the sub-atomic area unit with a WWII origin
A barn is 10⁻²⁸ m² — the effective 'size' of a uranium nucleus from a neutron's point of view. The joke stuck; the unit outlived the classified project that coined it.
During the Manhattan Project, physicists needed an area unit for nuclear cross sections that would not accidentally reveal the size scale in classified telegrams. They chose 'barn' as a wry code — the nucleus was 'as big as a barn' compared to the tiny cross sections expected. The joke unit became standard and still appears in every reactor calculation today.
The exact factors
1 barn (b) = 10⁻²⁸ m² = 10⁻²⁴ cm² (exact) > 1 millibarn (mb) = 10⁻³¹ m² > 1 microbarn (µb) = 10⁻³⁴ m² > 1 outhouse = 10⁻⁶ b (real joke unit, occasionally used) > 1 shed = 10⁻²⁴ b (also real)
What a cross section actually is
A cross section is not a geometric area. It is the effective target area a particle presents for a given interaction. A U-235 nucleus has a geometric radius of about 7.4 fm — a physical area of 172 fm² = 1.72 b. But its thermal neutron fission cross section is 585 b — 340× bigger, because the interaction probability is dominated by nuclear resonances, not by geometry.
Worked example — a reactor calculation
A thermal neutron traversing 1 cm of U-235 metal (density 19.1 g/cm³, atomic mass 235) encounters 4.89 × 10²² atoms/cm² per cm path. With a fission cross section of 585 b = 5.85 × 10⁻²² cm², the mean interaction rate per neutron per cm is 4.89 × 10²² × 5.85 × 10⁻²² = 28.6. In other words, a thermal neutron in bulk U-235 fissions within a fraction of a millimetre — which is exactly why enriched uranium goes critical so quickly.
The five-second recap
1 barn = 10⁻²⁴ cm². It is not a real area — it is a probability dressed up in units. And yes, 'outhouse' and 'shed' are real units, in one paper each.
Try the converters mentioned in this article
FAQ
Why not just use m²?+
Cross sections in m² are numbers like 10⁻²⁴, which are painful to compare. Barns produce nice single- and triple-digit values.
What is a millibarn used for?+
Particle physics reaction rates. LHC cross sections are quoted in fb (femtobarn) or pb (picobarn) — the collider produces so many collisions that even 10⁻¹⁵ b interactions are visible.
Is barn an SI unit?+
No, but it is 'accepted for use with the SI' by the CIPM specifically because nuclear physics refused to abandon it.
This article was written by The Turbo Unit Converter engineering desk (Nuclear & particle physics) and last reviewed on 2026-06-28 against NIST SP 811 and the BIPM SI Brochure. Read our full editorial policy.