How many bags of concrete do I need for a 10×10 slab at 4 inch thick. At 4 inch thick, for a 10×10 slab, you will need either seventy four 60- pounds or fifty six 80- pounds or 112- 40- pounds bags of concrete. Thus, either 74- 60lb or 56- 80lb or 112- 40lb bags of concrete will be required for a 10×10 slab at 4″ thick ...