That depends on the density of the metal. Cast steel is about 490 pounds/cubic foot.
Calculate the cross sectional area of the rebar, then multiply by the length to get volume.
A 1/2 inch diameter circle has a cross sectional area of 0.19634954084936207 square inches. Lets round it to 0.2 square inches, now the formula for calculating weight would be:
(((0.2 / 144) * linear feet) * 490) / 2000 = tons
What I did was first convert square inches to square feet: 0.2 / 144 = square feet
Then calculated cubic feet: square feet * linear feet = cubic feet
Now figured the pounds: cubic feet * density = pounds
Now the tons: pounds / 2000 = tons
That is for the 1/2 inch diameter cast steel rebar. Now do the same for the 3/8, or 5/8, or 7/16, etc.