I posted this on another website before I realized they'd patented the site! So I have moved it here.
The answer depends on your stride length, the length of each step.
Number of kilometers in 10,000 steps =
10,000 steps X Stride Length in kilometers
Stride length can be measured or calculated:
Women .413 * height
Men .415 * height
For example a women of 160cm (5'3") would have a stride length of 160 * .413 = 66.08cm = 0.0006608 km to walk 10,000 steps she must walk:
10,000 steps * 0.0006608 km stride length = 6.608 km
Of course you can also do the reverse. How many steps did you take if you know how far you've walked:
Number of steps = Number of kilometers walked / Stride length in kilometers
For example the same woman with stride length 0.0006608 km walks 4 kilometers how many steps has she taken:
4 km walked / 0.0006608 km stride length = 6053 steps