7706# approximately.

BTW, your post encouraged me to repeat the drawing in Solidworks.

My one gave me 7626.748lb for the mass.

Double check your units.

Cheers.

Hmmm... not sure what happened there... Definitely made a mistake somewhere along the lines... I'll have to re-create the model to see what happened, My models are always right on +-~5% for weight, and such.. I must have put in a wrong value.. or maybe as you said, I need to check my units. Sorry...

I get approx 4050 Lbs. I verified it using Solidworks, specifying standard mild steel (.28 lbs per inch³), creating a model of the tank, and using the mass properties function.. both figures were within 30lbs of each other.

Find the area of plate by multiplying 5*pi*15 to find the number of sq ft. Multiply this by 30.6 (weight of 3/4" plate per sq ft). Do the same for the bottom plate, (area formula) but multiply by 25.5 (weight of 5/8" plate per sq ft). Add the two together for total weight.

1. Find area of cylinder:

A=Π D where Π = 3.14; A = area and D = diameter

A = 3.14 x 5 x 15 = 235.5 sf. Multiply by 30.6 (weight of ¾" steel plate per sf)

Answer = 7206 #

1. Find area of bottom:

A = Π D² ÷ 4

A = 3.14 x 25 ÷ 4 = 19.625 sf. Multiply by 25.5 (weight of 5/8" steel plate)

Answer = 500.4375#

1. Add 7206 + 500 = 7706 #

You can follow the same, but using cu ft, multiply by 490 you get the same answer.

I do get 497lbs on the .625" plate... lets see about the cylinder... I'll triple check it.

3562.97 Lbs for the cylinder... This is where we differ.

The formula for finding the surface area of a tank, open at the top and closed at the bottom is: total A = ΠD x H (area of cylinder) plus Πr² (area of bottom)

Where Π = 3.14; D = diameter of tank; r = radius; H = height of tank.

Tank diameter = 5'; Height of tank = 15'; radius = 2.5'

Area of cylinder = 3.14 x 5 x 15 = 235.5 s/f

Weight of ¾" steel plate = 30.6# per s/f

Weight of cylinder = 235.5 x 30.6 = 7206.3#

Area of tank bottom = 3.14 x (2.5)² = 19.625 sf

Weight of 5/8" steel plate = 25.5# per s/f

Weight of bottom = 19.625 x 25.5 = 500.4375 #

Adding the two together 7206.3 + 500.4375 = 7706.7375#

You may also do it by converting s/f of plate to c/f.To do so, multiply 235.5 x ¾ and divide by 12 = 14.6875 c/f.

To that, take 19.625 and multiply that by 5/8 and divide by 12 to get c/f. or 19.625 x .625 ÷ 12 = 1.02 c/f. Add those together and get 15.71 c/f. Multiply 15.71 x 490 = 7698# .

The answers you get using s/f or c/f are close enough as some rounding of numbers is done.

Density = 0.28 pounds per cubic inch

Mass = 4048.57 pounds

Volume = 14367.17 cubic inches

Surface area = 72972.82inches^2

In SI:
5'= 1,524m dia.
15' = 4,572m lenght
3/4" thk = 0,01905m wall
5/8" bottom = 0,015875m

Bottom area pi*d^2/4 = 1,824147 m^2
Bottom volume = 0,02895833 m^3

Bottom internal area:
internal dia. = 1,524-2*0,01905 = 1,4859m
Area = 1,73407 m^2

Difference between outer and inner area: 0,0900673 m^2
ultiplied by lenght minus bottom thickness:
wall volume = 0,41035788 m^3

Total material volume: 0,4393162 m^3

490 lb = 222,260276 kg
1 cuft = 0,0283169 m^3

meaning a density of around 7849 kg/m^3 what is a good value for steel.

With this, the total mass would be 3448,2 kg, or around 7600 lb.
Point to Mr.ronseto this time! Hehehe.
The little difference may arise from considering the bottom inside the walls or not.

After all, do not trust softwares blindly. Always do a sketch calculation before going on.
And, if in doubt, go SI.I know inches and feet are usefull, but when it comes to equations, they call for so many constants to multiply...