Halflife
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Number of halflives elapsed 
Fraction remaining 
Percentage remaining 


0  ^{1}/_{1}  100  
1  ^{1}/_{2}  50  
2  ^{1}/_{4}  25  
3  ^{1}/_{8}  12  .5 
4  ^{1}/_{16}  6  .25 
5  ^{1}/_{32}  3  .125 
6  ^{1}/_{64}  1  .563 
7  ^{1}/_{128}  0  .781 
...  ...  ...  
n  ^{1}/_{2n}  100/(2^{n}) 
Halflife (t_{½}) is the time required for a quantity to fall to half its value as measured at the beginning of the time period. In physics, it is typically used to describe a property of radioactive decay, but may be used to describe any quantity which follows an exponential decay.
The original term, dating to Ernest Rutherford's discovery of the principle in 1907, was "halflife period", which was shortened to "halflife" in the early 1950s.
Halflife is used to describe a quantity undergoing exponential decay, and is constant over the lifetime of the decaying quantity. It is a characteristic unit for the exponential decay equation. The term "halflife" may generically be used to refer to any period of time in which a quantity falls by half, even if the decay is not exponential. For a general introduction and description of exponential decay, see exponential decay. For a general introduction and description of nonexponential decay, see rate law.
The converse of halflife is doubling time.
The table on the right shows the reduction of a quantity in terms of the number of halflives elapsed.
Probabilistic nature of halflife
A halflife usually describes the decay of discrete entities, such as radioactive atoms, which have unstable nuclei. In that case, it does not work to use the definition "halflife is the time required for exactly half of the entities to decay". For example, if there is just one radioactive atom with a halflife of one second, there will not be "onehalf of an atom" left after one second. There will be either zero atoms left or one atom left, depending on whether or not that atom happened to decay.
Instead, the halflife is defined in terms of probability. It is the time when the expected value of the number of entities that have decayed is equal to half the original number. For example, one can start with a single radioactive atom, wait its halflife, and then check whether or not it has decayed. Perhaps it did, but perhaps it did not. But if this experiment is repeated again and again, it will be seen that  on average  it decays within the halflife 50% of the time.
In some experiments (such as the synthesis of a superheavy element), there is in fact only one radioactive atom produced at a time, with its lifetime individually measured. In this case, statistical analysis is required to infer the halflife. In other cases, a very large number of identical radioactive atoms decay in the measured time range. In this case, the law of large numbers ensures that the number of atoms that actually decay is approximately equal to the number of atoms that are expected to decay. In other words, with a large enough number of decaying atoms, the probabilistic aspects of the process could be neglected.
There are various simple exercises that demonstrate probabilistic decay, for example involving flipping coins or running a statistical computer program. For example, the image on the right is a simulation of many identical atoms undergoing radioactive decay. Note that after one halflife there are not exactly onehalf of the atoms remaining, only approximately, because of the random variation in the process. However, with more atoms (right boxes), the overall decay is smoother and less randomlooking than with fewer atoms (left boxes), in accordance with the law of large numbers.
Formulas for halflife in exponential decay
An exponential decay process can be described by any of the following three equivalent formulas:
where

 N_{0} is the initial quantity of the substance that will decay (this quantity may be measured in grams, moles, number of atoms, etc.),
 N(t) is the quantity that still remains and has not yet decayed after a time t,
 t_{1/2} is the halflife of the decaying quantity,
 τ is a positive number called the mean lifetime of the decaying quantity,
 λ is a positive number called the decay constant of the decaying quantity.
The three parameters , , and λ are all directly related in the following way:
where ln(2) is the natural logarithm of 2 (approximately 0.693).

Click "show" to see a detailed derivation of the relationship between halflife, decay time, and decay constant. Start with the three equations We want to find a relationship between , , and λ, such that these three equations describe exactly the same exponential decay process. Comparing the equations, we find the following condition:
Next, we'll take the natural logarithm of each of these quantities.
Using the properties of logarithms, this simplifies to the following:
Since the natural logarithm of e is 1, we get:
Canceling the factor of t and plugging in , the eventual result is:
By plugging in and manipulating these relationships, we get all of the following equivalent descriptions of exponential decay, in terms of the halflife:
Regardless of how it's written, we can plug into the formula to get
 as expected (this is the definition of "initial quantity")
 as expected (this is the definition of halflife)
 , i.e. amount approaches zero as t approaches infinity as expected (the longer we wait, the less remains).
Decay by two or more processes
Some quantities decay by two exponentialdecay processes simultaneously. In this case, the actual halflife T_{1/2} can be related to the halflives t_{1} and t_{2} that the quantity would have if each of the decay processes acted in isolation:
For three or more processes, the analogous formula is:
For a proof of these formulas, see Decay by two or more processes.
Examples
There is a halflife describing any exponentialdecay process. For example:
 The current flowing through an RC circuit or RL circuit decays with a halflife of or , respectively. For this example, the term half time might be used instead of "half life", but they mean the same thing.
 In a firstorder chemical reaction, the halflife of the reactant is , where λ is the reaction rate constant.
 In radioactive decay, the halflife is the length of time after which there is a 50% chance that an atom will have undergone nuclear decay. It varies depending on the atom type and isotope, and is usually determined experimentally. See List of nuclides.
the half life of a species is the time it takes for the concentration of the substance to fall to half of its initial value
Halflife in nonexponential decay
The decay of many physical quantities is not exponential—for example, the evaporation of water from a puddle, or (often) the chemical reaction of a molecule. In such cases, the halflife is defined the same way as before: as the time elapsed before half of the original quantity has decayed. However, unlike in an exponential decay, the halflife depends on the initial quantity, and the prospective halflife will change over time as the quantity decays.
As an example, the radioactive decay of carbon14 is exponential with a halflife of 5730 years. A quantity of carbon14 will decay to half of its original amount (on average) after 5730 years, regardless of how big or small the original quantity was. After another 5730 years, onequarter of the original will remain. On the other hand, the time it will take a puddle to halfevaporate depends on how deep the puddle is. Perhaps a puddle of a certain size will evaporate down to half its original volume in one day. But on the second day, there is no reason to expect that onequarter of the puddle will remain; in fact, it will probably be much less than that. This is an example where the halflife reduces as time goes on. (In other nonexponential decays, it can increase instead.)
The decay of a mixture of two or more materials which each decay exponentially, but with different halflives, is not exponential. Mathematically, the sum of two exponential functions is not a single exponential function. A common example of such a situation is the waste of nuclear power stations, which is a mix of substances with vastly different halflives. Consider a sample containing a rapidly decaying element A, with a halflife of 1 second, and a slowly decaying element B, with a halflife of one year. After a few seconds, almost all atoms of the element A have decayed after repeated halving of the initial total number of atoms; but very few of the atoms of element B will have decayed yet as only a tiny fraction of a halflife has elapsed. Thus, the mixture taken as a whole does not decay by halves.
Halflife in biology and pharmacology
A biological halflife or elimination halflife is the time it takes for a substance (drug, radioactive nuclide, or other) to lose onehalf of its pharmacologic, physiologic, or radiological activity. In a medical context, the halflife may also describe the time that it takes for the concentration in blood plasma of a substance to reach onehalf of its steadystate value (the "plasma halflife").
The relationship between the biological and plasma halflives of a substance can be complex, due to factors including accumulation in tissues, active metabolites, and receptor interactions.
While a radioactive isotope decays almost perfectly according to socalled "first order kinetics" where the rate constant is a fixed number, the elimination of a substance from a living organism usually follows more complex chemical kinetics.
For example, the biological halflife of water in a human being is about 7 to 14 days, though this can be altered by his/her behaviour. The biological halflife of cesium in human beings is between one and four months. This can be shortened by feeding the person prussian blue, which acts as a solid ion exchanger that absorbs the cesium while releasing potassium ions in their place.