Calculating ‘How Much Fire Glass Do I Need? ‘ is not difficult, especially with the number of websites that have a fire glass calculator available.
However, what does tend to make it slightly more complicated is the addition of lava rock as a filler in the base of the fire pit tray.
Why Use a Lava Rock Filler
We could write a whole article on this subject, but the essential point is that small-sized lava rock in the base of the tray ( 1/4 to 1/2 inch ) forces the gas that leaves the burner to spread out over a larger area, creating a more natural-looking flame. Lava rock is also cheaper than fire glass, so this reduces the overall cost as well.
The only time we do not recommend this method is if you plan to use a clear fire glass in your fire feature. The dark color of the lava rock could show through and spoil the look of your fire glass. In this case, you will need to use only fire glass to get the desired look.
The average depth of fire pit trays from the base of the unit to flush with the top is about 2 inches. So this will be approximately the combined depth of the lava rock filler and the fire glass on top.
The average inside depth of a tray from most brands is about 2 inches
However, you are not limited to a ‘flat top’ finish with the glass level with the top of the tray. It is perfectly acceptable to mound the glass up form the sides of the tray to the center. In other words, the overall depth will be greater toward the center of the fire glass than at the edges.
This is accomplished just with the fire glass layer. The lava rock filler layer is a flat layer.
Lava Rock Filler Depth
The average height to the top of the burner in fire pit trays is a little less than 1 inch; this does vary a from brand to brand. The objective with this filler layer is to bury the burner completely with the lava rock. The lava rock layer needs to be a minimum of 1/4″ above the burner.
The Final Calculation
Now that we know where the two layers are divided ( just above the burner ) we can now make the final calculation of how much lava rock and fire glass is needed.
The critical factor at this point is to know that fire glass has about twice the density of lava rock or to look at it another way lava rock has about twice the coverage of fire glass. 10 lbs of lava rock has about twice the volume than 10 lbs of fire glass, which makes our calculation rather easy.
- Measure from the base of the tray to just over the top of the burner. (Let’s say that is 1 1/2 inches )
- Using a fire glass calculator enter the length and width ( or diameter for a round fire pit ) and a depth of 1 1/2inches for our example
- For our example, the result of that calculation is 20 lbs. Remember that this result is for fire glass, and we are doing the lava rock calculation. As we know lava rock has twice the volume per pound than fire glass, then we halve that result.
- We now know we need 10 lbs of lava rock.
- Now repeat the calculation but this time add the depth of fire glass you want on top of the lava rock layer. When you get the result from the calculator, then this is the amount of fire glass you need with no further adjustment.
Please note that if using Crossfire burners or Bullet Burners the jets on these types of burners should not be buried.