Among the extra features on a good pedometer should be one that reports the physical distance you've covered. However, all a pedometer can really do is count steps. It estimates distance by multiplying the steps you took by an average stride distance you've entered. It's very hard to get your average stride length right by actually measuring your steps. But by comparing the pedometer's estimate to a known distance, you can home in on a correct figure.
M/F Find your home on Gmaps Pedometer. Gmaps Pedometer (link below) is a handy overlay to Google maps that allows you to mark out a path on the map and calculates the total distance of that path. Above the map screen will be a box marked "Jump to." Enter your address and click "Go." The map should change to center on your home. Zoom in or out as needed until you've oriented yourself.
M/F Look for a good walking course. Study the map of the area around your house and look for a good route to walk. Use straight streets if you can - curving roads will make it very hard for Gmaps Pedometer to give you an accurate measurement. And, if possible, try to stick to level ground so your stride length will be as uniform as possible.
M/F Plot your route on the map. On the left side of the Gmaps Pedometer screen is a box with buttons and outputs. Click the "Start Recording" button, then double click on your house. A red marker point will appear. From now on, whenever you double click on the map, a new red marker will be added, along with a line connecting it to the preceding marker. And the total distance from the first marker to the last will be displayed in either English or Metric units.Plot out your entire route, using as few markers as possible. For example, if you're planning to walk five blocks from your house and turn right, place one marker at your house and the second at the intersection where you'll turn. Be careful and as precise as you can be when placing points. Zooming in closer will make it easier to place your marks exactly where you want to go. Note that Gmaps Pedometer just measures distances. It doesn't know what's between the markers you put down. If you put two markers on diagonally opposite corners of a block, for instance, it will happily plot out a route for you that goes straight through people's houses. If you do make a mistake, there's an "Undo Last Point" button that will let you back up and fix it.
M/F Finish your route and note the total distance. Design your route as a loop that will bring you back to your house and place the last marker right on top of the first one. Ideally the total distance for your route will be a mile or more. The longer the route, the more steps you'll take to finish it, and the closer your average stride length will be to a real average. Jot this distance down - or edit your route to bring it to a perfect round number you can easily remember, like two miles exactly. You're now done with Gmaps Pedometer.
M/F Enter an average stride length into your pedometer. Pedometers typically offer a pretty wide range of possible stride lengths, from a foot up to four feet. Give it your best guess or, if you really have no idea, pick a number somewhere in the middle of the range, like two and a half feet.
M/F Go walking. Now you've got a hypothesis (your guess of your average stride length) and a real number to test it against (the distance you got from Gmaps Pedometer). If you walk the same route you plotted, you'll know how far you actually went. Compare that to how far your pedometer thinks you went. If they agree, you're lucky! But your pedometer will probably be off. If your pedometer thinks you went farther than Gmaps Pedometer claims, the stride length you gave it is too long. Re-enter a smaller number. On the other hand, if the pedometer thinks you didn't go as far as Gmaps Pedometer, then the stride length you gave it is too short. Nudge the number up a bit and try again. As you repeat the same walk, you'll gradually home in on the right number. When your pedometer agrees with Gmaps Pedometer, you've got it right. Now you can walk other routes, or just let the distance accumulate as you walk around during the day, and you'll be able to trust the distance figure your pedometer gives you.