# Determining capacity of a tank

Discussion in 'Boat Design' started by ted655, May 31, 2007.

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### ted655Senior Member

I want to know how many gallons of diesel my tank holds.
The formula I have says L" X W" X H"= cu. in., divided by 231.
When I apply this to known capacities of tanks in a catalog, I don't come close. Even allowing for strap indents & room for vapor expansion I'm always "almost" twice their ammouint.
Anyone have a method of doing this , using simple math?
Thanks

2. ### lazeyjackGuest

lx hxw, in metres,
say .6x.6x.6= .216 cu/m obviosly is 216 litres
or in feet, 2x2x2 =8 cu ft
6.25 gals in cu feet, =48 plus,
are you pulling our legs?

3. ### lazeyjackGuest

oh and that is imp gals, you will need to x .8 for US gals

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### charmcSenior Member

Ted,

For me it's simpler to work from cubic feet. 1 cu ft = 7.48 US gal. 1728 cu inches = 1 cu ft.

So a tank 12" W x 24" H x 48" L = 13,824 cu inches, divided by 1728 = 8 cu ft, x 7.48 = 59.84 US gal. "max" capacity. 13,824 divided by 231 = 59.84, same answer.

Working, or usable capacity can be as low as 70% of total volume, since there needs to be vapor headspace and the fuel suction tube is off the bottom of the tank. A conservative marine tank supplier will factor in an allowance for the suction tube's being fully submerged even during pitch and roll movements, so working capacity will always be significantly less than total volume.

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### alan whiteSenior Member

Inches high times inches wide times inches deep, divide that by 1728, you get cubic feet.
One cubic foot of water weighs 62 lbs. One pound of water is a pint, eight pints to a gallon, 62 divided by 8 makes 7 3/4.
7 3/4 gallons to the cubic foot.

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### alan whiteSenior Member

And Charlie's gotten to it while I was writing. He had the actual weight of water, a bit less than 62 lbs per cubic foot.

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### alan whiteSenior Member

I divided 1728 by 231, your figure, and got 7.48. So the figure gives the weight of one gallon. Read the formula again, I like that 231 figure, easy to remember.

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### alan whiteSenior Member

yeah--- okay. You take the cubic inches, divide by 231, and you get the weight if water, not the gallonage.

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### charmcSenior Member

Hey Alan, I was working strictly with volume. 7.48 US gallons of water at "standard" temp will fill 1 cubic foot volume.

One gallon of water = 8.33 pounds (I know, Stu, it's easier in metric ) and 7.48 gallons x 8.33 pounds = 62 pounds.

Same, same, but I figured volume. My point is that working capacity will be less than volume.

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### ted655Senior Member

Yes, I was serious, even if it is fall off the log for you .
Thanks, I have more than 1 way now. Here is a sample of what I encountered'
Sold as a 29 gal tank. 25.75 X 23 X 8 = 4738 cu. in. / 231 = 20.51 gal. (not close enough for me), where's the other 9+ gals?
14.5 X 14 X 23 = 4669 / 231 = 20.21 gal. (sold as 12 gal tank) again a 8+ difference, this time the other way. Here is another conversion that isn't close either.
To Convert Multiply by To Obtain
cu.in. ------- 0.00433 .---------- gal.
Any how, thanks to all, I'll be able to figure my tanks now, so I know how much \$\$\$ to borrow to fill them. \$3.48 gal. on the Ohio river as of yesterday, ouch!!

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### alan whiteSenior Member

You can tell I'm not much of a mathematition. Obviously, 16 fluid ounces doesn't weigh quite 16 oz either, and a gallon doesn't weigh 8 lbs. exactly.
And therein lies the rub--- I'll use 8.33 from now on.

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### PoidaSenior Member

X + Y = Z

Where:
X = Syphon out existing fuel
Y = Filling up at Gas Station
Z = Amount indicated on bowser

What a genius

poida

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### Raggi_ThorNav.arch/Designer/Builder

I am glad we converted to the SI a hundred years ago, or so

14. ### lazeyjackGuest

sorry ted, look mate try workin litres, 1000 l in a cu /m
its so simple, get a tape measure, measure it in mm, 100mm, = tenth of metre and so on. so 100 =.1m, .2 m =200mm, so its easy

.5x .7x.3 the result=litres 105
3.8 l in one usa gal
dont worry I,m absolutely useless at cad , but can do math , simple in my head, I also have many woman and boats in my head too)

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### ted655Senior Member

.
So simple a xaveman thought of it.

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