Help to meassure my tank

ok sorry about this stupid question but I'm not really sure if I have a 30 gl tank I don't trust my math lol.
Those are the measures

20 in long
17.5 in tall
9 inches deep

 

if you go to the bottom of this thread, there's a chart to measure..I got it as a 15 gallon almost... not a 30g

 

oh god well I guess I start everything wrong

 

I have a true 30 gallon long. It measures 36 (long) x 12 (deep) x 16 (tall). Yours comes out to 13.64 US gallons. Mine is 29.92 US gallons. I hope that helps.

 

WOW thank you now I have an idea how urs look like and u have twice the measures I got I tough I had a big tank cheesus