I suppose you need to know the radius of the earth offhand. Or you can easily derive the radius if you remember that a meter was initially defined as 1/10,000,

I suppose you need to know the radius of the earth offhand. Or you can easily derive the radius if you remember that a meter was initially defined as 1/10,000,000 of the distance from the equator to the North Pole. So you can easily calculate the Earth's circumference, and from there the radius: 10,000,000 * 4 / 1000 / 3.1416 / 2 = 6366 km. Wikipedia gives 6371 km as the mean radius.