実用

Half Life Calculator

Half-Life Calculator

Initial Quantity (N₀)

Half-life (any time unit)

Elapsed Time

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Detailed Guide Coming Soon

We're working on a comprehensive educational guide for the Half Life Calculator in your language. The content below is shown in English.

とは何か Half Life Calculator?

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Half-life is the time required for half of a substance to undergo decay or be eliminated. It applies to radioactive decay, drug metabolism, and chemical reactions. After n half-lives, the remaining fraction is (1/2)ⁿ.

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方法 Half Life Calculator

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1Remaining quantity: N(t) = N₀ × (1/2)^(t/t½)
2N₀ = initial amount, t = elapsed time, t½ = half-life
3Equivalently: N(t) = N₀ × e^(−λt) where λ = ln(2)/t½
4After 10 half-lives: only ~0.1% of the original remains

解いた例

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例 1

入力:Carbon-14 (t½=5,730yr), 1,000 atoms, 11,460 years

結果:250 atoms remaining

2 half-lives: 1000→500→250

例 2

入力:Drug half-life 8 hours, dose 500mg, after 24 hours

結果:62.5 mg remaining

3 half-lives: 500→250→125→62.5

Notable Half-Lives

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Substance	Half-life	Application
Carbon-14	5,730 years	Archaeological dating
Uranium-238	4.47 billion years	Geological dating
Iodine-131	8.02 days	Medical imaging / therapy
Caffeine (in body)	5–6 hours	Drug metabolism
Alcohol	~1 drink/hour	BAC reduction
Aspirin	15–20 minutes	Drug metabolism

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プロのヒント

In medicine, it takes about 4–5 half-lives for a drug to reach steady state in the body, and 4–5 half-lives to be substantially eliminated. This guides dosing frequency.

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ご存知でしたか？

Radiocarbon dating (carbon-14) can only date materials up to ~50,000 years old — about 9 half-lives. Beyond that, too little C-14 remains to measure accurately. For older materials, potassium-40 (half-life 1.25 billion years) is used.

参考文献

›Half-life — Wikipedia

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Reviewed October 2026

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Half Life Calculator