Logarithm Calculator

A log calculator computes the logarithm of any positive number with any base you choose. Whether you need log base 10 (common log), natural log (ln) using base e, log base 2 for computer science, or a custom base, this tool gives you an instant, precise answer. Logarithms answer the question: "To what power must I raise the base to get this number?"

This calculator also handles inverse logarithms (antilog), which convert a log value back to its original number, and demonstrates key properties of logarithms like the product rule, quotient rule, and power rule. These properties are fundamental in algebra, calculus, and every branch of science.

• Algebra and precalculus - Solving exponential equations by taking the log of both sides
• Chemistry - Calculating pH values, which are negative base-10 logarithms of hydrogen ion concentration
• Computer science - Analyzing algorithm complexity using log base 2 (binary logarithms)
• Finance - Computing compound interest periods and doubling time using the rule of 72
• Acoustics - Measuring sound intensity in decibels, which use a logarithmic scale
• Seismology - The Richter scale is logarithmic, where each whole number is a 10x increase in amplitude

• The concept is abstract - Logarithms are the inverse of exponentiation, and thinking "backwards" about powers is unintuitive for many students
• Multiple bases cause confusion - Switching between log base 10, natural log, and log base 2 requires careful attention to context
• Properties require memorization - The product, quotient, and power rules are easy to mix up, especially under exam pressure
• Domain restrictions trip people up - You cannot take the log of zero or a negative number, which creates issues when solving equations

The logarithm log_b(x) = y means that b^y = x. For example, log_10(1000) = 3 because 10^3 = 1000. To compute a log with an unusual base, use the change of base formula: log_b(x) = ln(x) / ln(b), or equivalently log(x) / log(b). This lets you calculate any logarithm using just natural log or common log.

The antilog (inverse log) reverses the process. If log_10(x) = 2.5, then x = 10^2.5 = 316.23. Antilogs convert logarithmic scale values back to their original numbers, which is essential when working with pH, decibels, or any logarithmic measurement.

Example

Find log base 5 of 125. Ask yourself: 5 raised to what power equals 125? Since 5^3 = 125, the answer is 3. Using the change of base formula: log(125) / log(5) = 2.0969 / 0.6990 = 3.0, confirming the result.

• log_10(100) = 2 - Because 10^2 = 100
• ln(e^5) = 5 - The natural log and e cancel out, a core identity
• log_2(256) = 8 - Because 2^8 = 256, fundamental in computing
• log_10(0.001) = -3 - Logs of numbers less than 1 are negative since 10^-3 = 0.001
• log_3(81) = 4 - Because 3^4 = 81, demonstrating custom base usage

• Handle any base instantly - No need to manually apply the change of base formula every time
• Verify homework solutions - Check your algebraic log simplifications against computed values
• Explore log properties interactively - See how the product, quotient, and power rules work with real numbers
• Compute antilogs easily - Convert from logarithmic scales back to standard numbers without confusion
• Build intuition - Experimenting with different bases and values helps you develop a feel for logarithmic relationships

What is the difference between log, ln, and log base 2?

log (or log_10) uses base 10 and is common in chemistry and general science. ln uses base e (approximately 2.718) and is used in calculus, physics, and continuous growth models. log_2 uses base 2 and is standard in computer science for analyzing binary data and algorithm complexity. They are all logarithms, just with different bases.

Why can't I take the logarithm of a negative number?

In the real number system, no positive base raised to any power produces a negative result. Since log_b(x) asks "b to what power equals x," and b^y is always positive for positive b, there is no real answer when x is negative. Complex logarithms exist but require imaginary numbers.

What is the change of base formula?

The change of base formula is log_b(x) = log_a(x) / log_a(b), where a can be any base you have available. Most often, you use log_10 or ln since those are on standard calculators. For example, log_7(49) = log(49) / log(7) = 1.6902 / 0.8451 = 2.

What is an antilog and when do I use it?

An antilog reverses a logarithm. If log_10(x) = n, then antilog(n) = 10^n = x. You use antilogs whenever you need to convert from a logarithmic scale back to the original value. For instance, if a solution has pH 4.5, the hydrogen ion concentration is 10^-4.5 = 3.16 x 10^-5 moles per liter.

What are the main properties of logarithms?

The three key properties are: Product Rule: log_b(xy) = log_b(x) + log_b(y). Quotient Rule: log_b(x/y) = log_b(x) - log_b(y). Power Rule: log_b(x^n) = n * log_b(x). These rules let you break complex logarithmic expressions into simpler pieces, which is essential for solving equations.

How are logarithms used in real life?

Logarithms appear everywhere. The Richter scale measures earthquake magnitude logarithmically. Decibels measure sound intensity on a log scale. pH is the negative log of hydrogen ion concentration. Compound interest calculations use logs to find doubling time. Even music uses logarithmic frequency relationships between notes.