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Weibull Distribution Calculator

Calculate Weibull CDF, PDF, quantiles, mean, variance, and survival probabilities from scale and shape parameters.

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What the Weibull Distribution Models

The Weibull distribution models positive continuous data such as failure times, wind speeds, and material strengths. Its shape parameter controls skewness and tail behavior. Related tools: Exponential Distribution Calculator and Normal Distribution Calculator.

Weibull Formulas

$$F(x) = 1 - e^{-(x/\lambda)^k}, \quad x \ge 0$$

$$f(x) = \frac{k}{\lambda}\left(\frac{x}{\lambda}\right)^{k-1} e^{-(x/\lambda)^k}$$

$$Q(p) = \lambda[-\ln(1-p)]^{1/k}$$

$$\mu = \lambda \Gamma\left(1 + \frac{1}{k}\right)$$

Example

With scale $\lambda = 10$, shape $k = 2$, and $x = 6$, we get $P(X \le 6) = 1 - e^{-(0.6)^2} \approx 0.3025$.

Frequently Asked Questions

What do λ and k represent?

λ is the scale parameter. It stretches or compresses the distribution. k is the shape parameter. Values near 1 give an exponential-like shape, while larger k values produce a more bell-shaped curve.

When is the Weibull used in reliability?

It is common for modeling component lifetimes because it can represent increasing, constant, or decreasing failure rates through different shape values.

What is the survival function?

Survival is $P(X > x) = e^{-(x/\lambda)^k}$. It is the complement of the CDF.

Can x be zero?

Yes. At $x = 0$, the CDF is 0 and the PDF depends on k. For $k = 1$, the PDF at zero is $1/\lambda$.