If you are happy with an approximation then think about this: - At zoom level 9, your world has 2^9 by 2^9 tiles (that's 512x512 tiles). - The Earth's circumference is roughly 40,000 kilometres. Thus: - For the y axis, you have 20,000 km from North Pole to South Pole, divided by 512, that's roughly 39 km for the height per tile. (The 512x512 tiles don't exactly stretch all the way to the poles, only to the 85° line, but we can ignore that for this approximation.) This tile height is the same everywhere. - For the x axis it is not quite so easy because the number of kilometres used by the 512 tiles is different depending on where you are - 40,000 km at the Equator, 25,000 at the latitude of the Scilly Isles, and 20,000 at the latitude of Unst. So it's fair to use 22,500 as an approximation for the UK; divide that by 512 and you end up at a tile width of roughly 44km. Or very roughly: A zoom level 9 tile in the UK has 40x40km. (Double the widht/height width/height when you decrement the zoom level, halve the width/height if when you increment.)

If you are happy with an approximation then think about this: - At zoom level 9, your world has 2^9 by 2^9 tiles (that's 512x512 tiles). - The Earth's circumference is roughly 40,000 kilometres. Thus: - For the y axis, you have 20,000 km from North Pole to South Pole, divided by 512, that's roughly 39 km for the height per tile. (The 512x512 tiles don't exactly stretch all the way to the poles, only to the 85° line, but we can ignore that for this approximation.) This tile height is the same everywhere. - For the x axis it is not quite so easy because the number of kilometres used by the 512 tiles is different depending on where you are - 40,000 km at the Equator, 25,000 at the latitude of the Scilly Isles, and 20,000 at the latitude of Unst. So it's fair to use 22,500 as an approximation for the UK; divide that by 512 and you end up at a tile width of roughly 44km. Or very roughly: A zoom level 9 tile in the UK has 40x40km. (Double the widht/height when you decrement the zoom level, halve the width/height if you increment.)