Calculateur de logarithme

Logarithm Calculator

Calculate logarithms for any positive number using common log (base 10), natural log (base e), or any custom base you enter. This tool instantly computes the exponent to which the base must be raised to produce your input value. Logarithms appear throughout mathematics, science, engineering, and finance whenever calculations involve exponential growth, very large numbers, or data that spans many orders of magnitude.

How Logarithms Are Calculated

The logarithm base b of a positive number x is the exponent y such that b^y = x, written as log_b(x) = y.

Key cases:

• log10(1000) = 3 because 10^3 = 1000
• ln(e) = 1 because e^1 = e (where e ≈ 2.71828)
• log2(64) = 6 because 2^6 = 64
• log_b(1) = 0 for any base, because b^0 = 1

The change of base formula converts any logarithm to common or natural log: log_b(x) = log10(x) / log10(b). This is how calculators handle non-standard bases internally.

Worked Examples

Example 1 — doubling time: A savings account earns 5% continuous interest. How many years to double? t = ln(2) / 0.05 = 0.6931 / 0.05 = 13.86 years.

Example 2 — earthquake magnitude: The Richter scale uses base-10 logarithms. An earthquake measuring 6.0 releases 10^(1.5 x 6.0) = 10^9 units of energy. A magnitude-7.0 earthquake releases 10^10.5 units — about 31.6 times more than a 6.0.

Example 3 — bits of information: A system can represent 1,024 distinct values. The number of bits required = log2(1024) = 10. Verify: 2^10 = 1024. So 10-bit encoding handles 1,024 possible states.

Reference Table — Logarithm Values Across Common Bases

When to Use This Calculator

• When solving exponential equations where the variable is in the exponent, such as 3^x = 243 (x = log3(243) = 5)
• When calculating doubling time or half-life using the formula t = ln(2) / r
• When converting between logarithmic scales — decibels, pH, Richter magnitudes — and underlying linear values
• When analyzing data that spans multiple orders of magnitude and needs a log scale to be readable
• When working with information theory, binary encoding, or computer science problems that require log base 2

Common Mistakes

1. Attempting log of zero or a negative number — log(0) is undefined (approaches negative infinity as x approaches 0), and logarithms of negative numbers require complex arithmetic. This calculator only accepts positive inputs.
2. Mixing up common log and natural log — log10(100) = 2, but ln(100) ≈ 4.605. These are different functions. In most scientific contexts, “log” without a subscript means log10; in mathematics and many physics equations, “log” often means ln.
3. Misapplying the product rule — log(A + B) does not equal log(A) + log(B). Only multiplication under the log splits: log(A x B) = log(A) + log(B). Adding values before taking the log gives a completely different result than adding their logs.
4. Forgetting that the base must be positive and not equal to 1 — log base 1 is undefined because 1^y = 1 for any y, making it impossible to uniquely solve for an exponent.

Real-World Applications

Logarithms are built into the measurement scales of everyday life. The decibel scale for sound intensity is logarithmic — a 30 dB difference represents a 1,000-fold change in sound power, not a 30-fold change. The pH scale for acidity uses -log10 of the hydrogen ion concentration: a pH of 3 (vinegar) is 10 times more acidic than a pH of 4. The Richter scale for earthquakes works similarly, so a 7.0 earthquake releases about 31.6 times the energy of a 6.0. In finance, the continuous compounding formula A = P x e^(rt) is solved for time using natural log: t = ln(A/P) / r. In information theory, Shannon entropy — a measure of how much information is in a message — is computed using log base 2, where each bit of information corresponds to a factor of 2 in possible states.

Tips

• The Rule of 72 offers a quick mental estimate for doubling time: divide 72 by the annual growth rate (72 / 6% = 12 years)
• When graphing exponential data, applying a log scale to the y-axis converts the curve into a straight line, making trends far easier to identify
• log(A x B) = log(A) + log(B) — this is how analog slide rules performed multiplication before electronic calculators existed
• In computer science, log2 tells you how many bits are needed: log2(65,536) = 16, so 65,536 values require 16-bit storage
• Negative logarithms just mean the input is between 0 and 1: log10(0.001) = -3 because 10^(-3) = 0.001
• The natural log and base-10 log are proportional: ln(x) = log10(x) x 2.302585, so you can convert between them with a fixed multiplier