steel quantity of box culvert

attached is a cross section of a 20 meter long box culvert. I need to know the steel quantity required for it.
As per my calculations, dia 12mm = 2,916 kg, dia 14mm = 193 kg and dia 16mm = 6,984 kg. Please help to verify.
 

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  • cross section.JPG
    cross section.JPG
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