This applet computes probabilities and percentiles for the F-distribution $X \sim F_{(\nu_{1},\nu_{2})}$.
Directions:
To compute a left-tail probability, select $P(X \lt x)$ from the drop-down box, enter a numeric $x$ value in the blue (left) box and press the "Compute" button. The probability $P(X \lt x)$ will appear in the pink (right) box. Select $P(X \gt x)$ from the drop-down box for a right-tail probability.
To determine a percentile, enter the percentile (e.g. use 0.8 for the 80th percentile) in the pink (right) box, select $P(X \lt x)$ from the drop-down box, and press the "Compute" button. The percentile $x$ will appear in the blue (left) box.
On the graph, the $x$ value appears in blue while the probability is shaded in pink.