You need to specify what kind of ratio you're looking for. Here are some possibilities and how to calculate them:

1. Diameter Ratio:

* Mercury diameter: 4,880 km

* Earth diameter: 12,742 km

* Ratio: 4,880 km / 12,742 km = approximately 0.38

This means Mercury's diameter is about 38% of Earth's diameter.

2. Mass Ratio:

* Mercury mass: 3.30 × 10^23 kg

* Earth mass: 5.97 × 10^24 kg

* Ratio: 3.30 × 10^23 kg / 5.97 × 10^24 kg = approximately 0.055

This means Mercury's mass is about 5.5% of Earth's mass.

3. Surface Gravity Ratio:

* Mercury surface gravity: 3.7 m/s²

* Earth surface gravity: 9.81 m/s²

* Ratio: 3.7 m/s² / 9.81 m/s² = approximately 0.38

This means Mercury's surface gravity is about 38% of Earth's surface gravity.

4. Distance from the Sun Ratio:

* Mercury average distance from the Sun: 57.9 million km

* Earth average distance from the Sun: 149.6 million km

* Ratio: 57.9 million km / 149.6 million km = approximately 0.39

This means Mercury is about 39% as far from the Sun as Earth.

Please clarify what kind of ratio you are interested in so I can give you the most accurate answer.