Area counts how many unit squares cover a surface. A square metre is a square one metre on each side, so both dimensions carry the length conversion. That is why area factors are the square of length factors.
One metre is 100 centimetres, but one square metre is 10,000 square centimetres
A 1 m by 1 m square becomes 100 cm by 100 cm. Multiplying its sides gives 100 × 100 = 10,000 cm². Dividing a cm² value by only 100 therefore applies a length conversion to an area and produces the wrong result.
The exact inch definition creates exact square-inch factors
The international inch is exactly 0.0254 m. Squaring it gives exactly 0.00064516 m² per in². AreaCalc stores that relationship as a reduced rational number instead of rounding the factor before conversion.
Convert after the shape formula, then reverse-check
First calculate in the square of the input unit. Then multiply by the exact input-to-metre squared factor and divide by the output unit's square-metre factor. Reversing the two factors should recover the original area before display rounding.