SAFE reports Area of reinforcement in unit lentgh2/length (mm2/m, cm2/m, and in2/ft).
For calculating Number of bars@spacing, do the following:
Select Bar Diameter, multiply its area with strip width & divide Strip width by required Area of steel.
Assume we get strip Area of Reinf. from SAFE, As = 323 mm2 /m
suppose T10 mm diameter bar, Ab = 78.5 mm2
Check the strip width from SAFE, Assume, Ws = 1 m
Spacing can be calculated in this way, S = Ab x Ws/As
Therefore, S = 78.5×1/323 = 0.243 m
Provide T10@225mm Spacing,
As = 78.5×1/0.225 = 348.88 mm2/m