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対数計算機

The logarithm of a number to any base, including ln and log₁₀, with change-of-base steps, or the exponent x that solves bˣ = y, exact when rational.

更新日 検証済みの例：6

Type e for the natural logarithm
試す
結果
結果: 10
小数点以下の最大桁数：12；最も近い値へ、等距離ならゼロから遠い値へ
正確な値
10
Natural log of the number
6.931471805599
Base-10 log of the number
3.01029995664

log base 2 of 1024 is 10: 2 must be raised to the power 10 to give 1024.

y = log base 2 of x

05105001,0001,5002,000xy(1024, 10)
計算方法 S

  1. Change of base

    log⁡21,024=ln⁡1,024ln⁡2=6.931471805599450.693147180559945=10\log_{2} 1{,}024 = \frac{\ln 1{,}024}{\ln 2} = \frac{6.93147180559945}{0.693147180559945} = 10

  2. 正確な値

    210=1,024  ⇒  log⁡21,024=102^{10} = 1{,}024 \;\Rightarrow\; \log_{2} 1{,}024 = 10

  3. Check

    210=1,0242^{10} = 1{,}024

対数計算機について

The logarithm log_b x is the exponent that turns the base b into x: log₂ 1024 = 10 because 2¹⁰ = 1024. Any base can be computed from natural logarithms with the change-of-base rule, log_b x = ln x ÷ ln b, and the same rule solves an exponential equation bˣ = y, giving x = ln y ÷ ln b.

Common uses are counting doublings or halvings (base 2), orders of magnitude and the decibel and pH scales (base 10), and continuous growth and decay (base e ≈ 2.71828). Solving 3ˣ = 20 gives x = ln 20 ÷ ln 3 ≈ 2.7268.

The number must be positive, and the base positive and not 1. When the answer is a fraction it is also shown exactly, so log₄ 8 appears as 3/2 as well as 1.5. A base below 1 gives negative logarithms for numbers above 1: log₀.₅ 8 = −3.

計算例

log₂ 1024

Calculate
Log of x
Base b
2
Number x
1024
結果
10
正確な値
10

照合元：2¹⁰ = 1024

log₁₀ 0.001

Calculate
Log of x
Base b
10
Number x
0.001
結果
-3
正確な値
−3

照合元：10⁻³ = 0.001

ln 10

Calculate
Log of x
Base b
e
Number x
10
結果
2.302585092994

照合元：Python Decimal(10).ln() = 2.302585092994045684017991454684364…

log₄ 8

Calculate
Log of x
Base b
4
Number x
8
結果
1.5
正確な値
3/2

照合元：4^(3/2) = (√4)³ = 8

よくある質問

What is the difference between log and ln?

ln is the natural logarithm, with base e ≈ 2.718282, while log on most calculators means base 10. Some textbooks and programming languages use log for the natural logarithm instead; Python's math.log(10) returns 2.302585. The two differ by a constant factor, ln x = ln 10 × log₁₀ x ≈ 2.302585 × log₁₀ x, so ln 10 ≈ 2.302585 while log₁₀ 10 = 1.

How do you calculate a logarithm with a different base?

Use the change-of-base rule: log_b x = ln x ÷ ln b, or equally log₁₀ x ÷ log₁₀ b. For log₂ 1024 that is 6.931472 ÷ 0.693147 = 10. For log₄ 8 it gives 1.5, which is exact because 4^(3/2) = (√4)³ = 8. Any base works except 1, and the base must be positive.

How do you solve an exponential equation like 3ˣ = 20?

Take logarithms of both sides: x × ln 3 = ln 20, so x = ln 20 ÷ ln 3 ≈ 2.995732 ÷ 1.098612 ≈ 2.726833. The same method finds doubling times: money growing 7% a year doubles when 1.07ˣ = 2, at x = ln 2 ÷ ln 1.07 ≈ 10.24 years.

Why is the logarithm of 0 or a negative number undefined?

No real power of a positive base gives 0 or a negative number: 2ˣ is positive for every real x and only approaches 0 as x heads toward −∞. So log₂ 0 has no value and ln(−1) has no real value; in complex numbers ln(−1) = iπ. A base of 1 is excluded too, since 1ˣ = 1 for every x.

What is log base 2 used for?

log₂ x counts how many times 1 must be doubled to reach x. log₂ 1024 = 10, so 1,024 = 2¹⁰ and whole numbers from 0 to 1,023 fit in 10 bits. It also bounds halving processes: a binary search of 1,024 sorted items needs at most ⌊log₂ 1024⌋ + 1 = 11 comparisons.

「対数計算機」の精度はどのくらいですか？

精度は入力値と計算方法の前提に依存します。十進演算には有効数字50桁を使いますが、推定、数値計算手法、元データの精度はそれより低い場合があります。表示の丸め処理でこれらの制約がなくなるわけではありません。 独立した出典の解答と照合した計算例：6。 例えば、「log₂ 1024」は2¹⁰ = 1024と照合しています。

この計算方法の出典は何ですか？

NIST Digital Library of Mathematical Functions §4.2 — logarithms, change of base (4.2.E17); Khan Academy — Change of base formula for logarithms.

この計算機について

log⁡bx=ln⁡xln⁡bbx=y  ⟺  x=ln⁡yln⁡b\begin{gathered} \log_b x = \frac{\ln x}{\ln b} \\[10pt] b^{x} = y \iff x = \frac{\ln y}{\ln b} \end{gathered}

出典

  1. NIST Digital Library of Mathematical Functions §4.2 — logarithms, change of base (4.2.E17)
  2. Khan Academy — Change of base formula for logarithms

出典と照合済み

この計算機には、独立した出典の解答を使った計算例が 6 件あります。テストに組み込まれており、ここでも実行できます。

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