How big are the planets?
The planets fall into two very different size groups. The four inner, rocky planets are small: Earth is the largest of them, and Mercury is not much bigger than our Moon. The four outer giants are enormous by comparison — Jupiter is about 11 times as wide as Earth.
That split comes from where each planet formed. Beyond the frost line, ice provided extra building material, letting planets grow big enough to capture huge amounts of gas. Read the full story in Step 1: Gathering the materials.
Width versus volume
Comparing widths can be misleading. Volume grows with the cube of the diameter, so a planet twice as wide has eight times the volume. Jupiter is about 11 times wider than Earth, which means more than 1,300 Earths could fit inside it. The Sun is about 109 times wider than Earth — over a million Earths would fit inside.
Size is not the same as mass
Saturn is almost as wide as Jupiter but has less than a third of its mass, because it is made of lighter material and is less compressed. Uranus is wider than Neptune, yet Neptune is heavier. Use the weight calculator to see how gravity differs.
Data
Diameters are equatorial diameters from the NASA Planetary Fact Sheet. Giant planets are slightly flattened by their fast rotation, so they are a little narrower from pole to pole.