Go to Google Maps and find the place you want to measure. Use “Layers” to switch to Satellite view. Then in the Layers menu select Measure. Now you can drop points around the border of the area to be measured! The total area in distance and square footage (meters) shows up!
It actually is pretty accurate, within inches when measuring across relatively short distances. Try it yourself by measuring a known quantity like a baseball field or football field. Accuracy will obviously degrade over much longer distances, but it is more than sufficient to measure your yard.