Half-life (t½)
t½ (also written t1/2 or T½) is the half-life: the time it takes for half of a radioactive substance to decay, or in chemistry for half of a reactant to be used up.
How to read it
OpenStax’s Chemistry 2e defines the half-life of a radioactive nuclide, t1/2, as the time required for half of the atoms in a sample to decay, and says each nuclide has its own constant half-life: cobalt-60, used to treat cancer, has a half-life of 5.27 years. OpenStax’s University Physics writes it T1/2, and gives carbon-14’s as 5730 years. The IUPAC Green Book uses t1/2 for radioactive decay and also in chemical kinetics, where it is the time for a concentration to fall to half its starting value, and notes that half-lives are commonly given in years, unit a.
Also searched as: half life symbol, half-life symbol, t1/2, t½, T½, t one half.
Half-life, decay constant and mean life
t½ is tied to the decay constant λ: OpenStax gives t½ = ln 2/λ, or about 0.693/λ. The mean life, written τ, is a different time, the average time a nucleus exists before it decays; the Green Book relates the two as t½ = τ ln 2.
Typing ½
The one-character fraction ½ is Unicode’s VULGAR FRACTION ONE HALF (U+00BD).
Read next
- Next in Physics and chemistry symbolsh-bar (ħ)ħ, said “h-bar”, is the reduced Planck constant: the Planck constant h divided by 2π. Its value is exactly 1.054 571 817… × 10⁻³⁴ joule seconds, and it appears in the basic equations of quantum physics.
- The half-life is ln 2 divided by the decay constant λ.LambdaLambda (Λ, λ) is the eleventh letter of the Greek alphabet. λ is wavelength in physics and chemistry, the decay constant of a radioactive substance, the regularisation rate in machine learning, and the mark of an anonymous function in programming.
More physics and chemistry symbols
The same shape elsewhere
Browse symbols drawn with letters and numbers.
Sources
- Chemistry 2e, 21.3 Radioactive Decay, OpenStax (Rice University), Radioactive Half-Lives
- University Physics Volume 3, 10.3 Radioactive Decay, OpenStax (Rice University), Radioactive Decay Law; Radioactive Dating
- Quantities, Units and Symbols in Physical Chemistry (the Green Book), IUPAC Green Book, 3rd edition, 2nd printing (2008), International Union of Pure and Applied Chemistry (IUPAC), Sec. 2.12 Chemical kinetics; Sec. 2.4 notes 18–19
- Latin-1 Supplement: Range 0080–00FF (code chart), The Unicode Standard, Version 18.0, Unicode Consortium, Latin-1 Supplement, 00BD
Researched, edited and approved by Rory Hansen before publishing. Last reviewed October 9, 2026. How we research
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