For Armour Stone Calculations like most natural stones, we use a rough calculation to get approximate amounts. You must first measure the length x height of the area in which you intend to use the Armour Stone. Once you have the length and height you multiple the two numbers together (Lenth x Height) and multiply the answer by 425 and divide the answer by 2000. This will give you an approximate amount of stones needed in tons.
20ft(Length) x 10ft (Height) = 200 (answer) x 425 = 85, 000 / 2000 (Assuming using standard sized stone) = 42.5 Tons = 42 Pieces of 3ft x 2ft x 2ft Armour Stone.
We would also be happy to help you calculate if you contact us and request a free quote.