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Whenever I've come across notation for specifying how much reinforcing steel is in a member, it is typically by giving the area of steel as a percent of the cross-sectional area of the member.

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For example in this picture, you could say that the reinforcing steel is 0.02 or 2%. You could also specify the reinforcing steel given a schedule, e.g. #8 @ 12" O.C.

I recently came across a piece of software that was asking for the reinforcing steel of a floor slab in units of mm^2/mm. I cannot figure out how they came across those units. How could I calculate the reinforcement in these units given I know how much steel is in the slab in the other conventions I listed earlier?

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Your second bar description is what you need to compare it to:

#8 @ 12" O.C.

A #8 bar is 0.79 in2. This gives us $\frac{0.79in^2}{12in}$ or in2 over in. This is the similar to your mm2/mm. (Obviously the units are not interchangeable!)

This method of describing reinforcing is useful if you don't care about the actual size of rebar used. It could be #8 at 12" or #6 at 6.625".

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  • $\begingroup$ mm2/mm is not going to be the same as in2/in, you'll be out by a factor or 25.4. 1 inch = 25.4 mm. 1 in2/in is (25.4)^2/25.4 mm2/mm = 25.4 mm2/mm $\endgroup$
    – Fred
    Mar 7, 2015 at 4:38

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