For deck forming I need to know if 2 X 6 at 12 inch center is equivalent to 2 X 8 at 16 inch for a span of less than 8 foot ?
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The 2x8 on 16" centers is stronger than 2x6 on 12" centers
Here's a calculator to play with. http://www.awc.org/codes-standards/calculators-software/spancalc
Pure physics tells us that a beam is proprotional to its height squared, so the strength of a 2x6 is 36/64ths the strength of a 2x8. Factor in a 12 vs 16 inch pitch and you get (36/12) compared to (64/16) = 3 compared to 4. The 2x8s on a 16-inch pitch are about 1/3 stronger than the 2x6s on a 12-inch pitch.
Of course, in real life other factors come into play, but this a good first approximation.