Area of a rhombus is base times height, or half the product of the diagonals. All four sides are equal; that is not enough to skip the height. A parallelogram uses the same base-times-height idea, in area of a parallelogram.
Diagonals or base and height
If the figure labels both diagonals, (d₁ × d₂) / 2 is fastest. Side length alone is not the area. Volume of a cube is a different leftover, in volume of a cube.
The short list is in ACT Math formulas.
The bottom line
bh, or (d₁d₂)/2. Equal sides do not make the area s².
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