Calculating a planet's mass & gravitational pull

[dogscoff]

Help!

I'm putting together a spreadsheet to help me create realistic planets for SE4 story writing purposes.

I type in the diameter (km) and surface area covered by ocean (%), and it automatically gives me the planet's circumference (km), total surface area (km^2), land and water surface area(km^2), and compares all these values to Earth for easy reference.

What I'd like in addition is to be able to plug in a Mean Density value (gr./cm^3) for the planet and from that get an estimate of the planet's surface gravity.

Unfortunately I don't know the calculations necessary for this, despite lengthy google searches. Can anyone help?

I assume I'll need the planet's volume, but I should be able to get that from the diameter. I appreciate that any figures I come up with won't be utterly accurate (because I'm working with spheres and planets aren't quite spherical, and because surface gravity is also affected by rotation) but they'll be good enough for my purposes.