water displacement of sand and rock

[KLN]

Does anyone have a rough formula for water displacement of sand and rock by weight? I have about 40 pounds of sand and 60 pounds of rock in a 55, and want to know how much water I have. 45 gallons, perhaps?

[XXFirefighter]

I got 55 pounds of live sand and 90 pounds of life rock and 45 gallons filled my 55 PERFECT !

[Biggen]

Saltwater weighs approx 8.5 lbs per gallon. So if you have 100lbs of rock and sand in a 55 gallon, you are displacing approx 11 gallons of water. So you have roughly 43 gallons of water in the tank.

This actually isn't exactly right because displacement refers to volume and NOT weight but its a good estimate.

[MadManMac]

The problem with that calculation is that you are using the weight of the water, not thh weight of the sand and rock. Sand and rock is far more dense than water and as such would displace less water by weight. You need to know the approximate size or volume of the sand and rock to measure its displacement, the weight is really irrelevant.

[Biggen]

The problem with that calculation is that you are using the weight of the water, not thh weight of the sand and rock. Sand and rock is far more dense than water and as such would displace less water by weight. You need to know the approximate size or volume of the sand and rock to measure its displacement, the weight is really irrelevant.

That is why I quantified my statement with:

This actually isn't exactly right because displacement refers to volume and NOT weight but its a good estimate

[KLN]

I was guessing the sand and rock to be twice the density of saltwater for the calculation, but wondered if anyone had a better guestimate.

Just found this tidbit: water weighs 8.3452641 pounds per gallon. Saltwater would then weight that times 1.025 or 8.55 pounds per gallon, I reckon.

So if I have 100 pounds of stuff twice the density of water, it would displace 50 pounds of water, or about 5.8 gallons of water. I would have guessed it was more than that, like someone said above

[BillyZ]

the sand will be pretty easy to figure out and should be rather consistent from tank to tank. The LR, however, will very greatly depending on the type of rock, FL aquacultered, to tonga and even from piece to piece. The easiest way to tell how much your LR is taking up is to measure the amount of change in water level in your tank when you add the LR.

How to determine density Sand:
1)Take a graduated container with a known volume of water (a large glass pyrex measuring cup with 2 cups of water would work well)

2)Weigh out a known amount of dry sand, say 8 ounces (1/2 pound).

3)Add that sand to the measuring cup and note the rise in water volume.

4)Your density is then 8 ounces per (the change in the level of the measuring cup)

NOTE: 16 ounces = 1 pound and 16 cups = 1 gallon.

Example: If 8 ounces took up 1 cup (just speculating) then the density of dry sand would be 8 pounds per gallon. so a 50 pound bag of sand would then take up 6.25 gallons of volume. So as soon as someone has the opportunity to apply this to some dry sand you'll have your answer for the sand )