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!
I used it just now to figure out how much dirt to order to cover my yard (re-seeding with grass) so that’s accurate enough for me I’m not doing rocket science here.