About counting ms palte weight

You can calculate it from:

Weight = length x width x thickness x density (keep units consistent)

Where the density depends on the exact composition of your steel

but 0.23% carbon - mild steel has a density of 0.284 lbs/in³

Or you could try this calculator:

http://www.chapelsteel.com/weight-steel-plate.html

But I know nothing about that calculator or how accurate the density it uses is.

Exactly what type/grade of steel are you inquiring about?