Fire Glass and Lava Rock Calculator
Enter the inside size of the burner pan and the fill depth. The result is the volume the pan holds and either the pounds of fire glass to fill it, as a range, or the cubic feet of lava rock.
Result
Fire glass needed
74 to 79 lb
- Glass volume
- 1,413.7 cu in
- Estimate before rounding up
- 73.44 to 78.54 lb
- Fire Pit Essentials published formula, for reference
- 75.0 lb
Notes on This Result
- Burner makers cap glass over the burner. The Outdoor Plus and HPC both say no more than 0.5 in. of glass over a propane burner, because more can push gas back out of the air mixer. Your burner manual sets how much media can sit over the burner, and it wins over any estimate here.
- If your burner maker publishes a quantity of media for your exact pan, use that instead of this estimate. Firegear, Outdoor GreatRoom and HPC all publish charts.
- Use only media rated for fire. Non-porous material that holds moisture, such as gravel, pebbles and river rocks, can fracture when heated.
Calculation Details
Formula
Volume = pan area × fill depth (round: π/4 × diameter² × depth). Fire glass = volume × 0.0519 to 0.0556 lb per cubic inch, from HPC Fire and MSI published coverage.
Assumptions
- Enter the inside dimensions of the burner pan. If your burner sits in a pan inside a larger bowl or table, only the pan gets media. Manufacturers say not to fill the space around it.
- Fire glass weighs about 90 to 96 pounds per cubic foot of space filled, by the published figures from HPC Fire and MSI. The answer is a range for that reason. Glass shape and size change it a little.
- The burner's own volume is not taken out, so the estimate runs slightly high.
- Two inches is a common fill depth, but your burner manual sets how much media can sit over the burner. On propane, two manufacturers say no more than half an inch of glass over the burner.
Sources
HPC Fire Inspired (Hearth Products Controls), Tools page, Fire Glass Calculator (page dateModified 2023-02-09 in its own schema data): volume in cubic inches divided by 231 and multiplied by 12 lb per gallon, 'based on 2 inch depth'
Checked . View at hpcfire.com.
NIST Guide to the SI (SP 811), Appendix B.9 conversion factors: gallon (U.S.) = 3.785 412 E-03 m3; ratio to the cubic inch is 231
Checked . View at nist.gov.
MSI Surfaces, Fire Glass: Specifications and Care Instructions (manufacturer page): a 10-pound bag covers approximately 1 square foot x 1.25 inch deep
Checked . View at msisurfaces.com.
Arithmetic: 12 in. x 12 in. per square foot, as used in the dossier's reading of the MSI coverage figure (research/calc-sources/fire-glass-calculator.md section 4a)
Checked .
Fire Pit Essentials, Use and Care Guide, fire glass (PDF created 2018-03-15; a retailer CDN copy of the manufacturer's guide): width x length x depth divided by 19, or diameter x diameter x depth divided by 24, gives pounds. Shown as a reference only
Checked . View at images.thdstatic.com.
The Outdoor Plus, Match-Lit Fire Pit Insert Manual 2024, PDF p. 5, and Fire Bowl Installation Manual FB-ML-M (2026), PDF pp. 10 and 11. HPC EI Series manual 860-EI (2024), Section 7, PDF p. 20, says the same for propane with glass: applying more than 1/2 inch of glass over a propane burner may create back pressure and gas leakage from the air mixer
Checked . View at theoutdoorplus.com.
The Outdoor Plus, Fire Bowl Installation Manual, Match Lit (FB-ML-M), PDF p. 11
Checked . View at theoutdoorplus.com.
What the Result Means
Fire glass is given as a range because manufacturers publish different coverage figures. By HPC Fire's and MSI's figures it weighs about 90 to 96 pounds per cubic foot of space filled, and the shape and size of the glass move that a little. Order toward the top of the range if you would rather have some left over than run short.
Lava rock is given in cubic feet because it is sold by volume and its weight varies with the rock. We could not find a manufacturer weight per cubic foot, so we do not publish one.
Enter the inside dimensions of the burner pan, not the bowl or table around it. If the burner sits in a pan inside a larger bowl, only the pan gets media, and manufacturers say not to fill the space around it. The burner's own volume is not subtracted, so the estimate runs slightly high. Bowls with sloped sides, pans with raised centers and drop-in pans with stepped floors are not modeled. Measure the flat-bottomed part, or use the manufacturer's quantity.
The burner manual sets the depth over the burner, and it overrides this calculator. The Outdoor Plus and HPC both say no more than half an inch of glass over a propane burner. Too much media can starve the flame, damage the pilot or, on propane with glass, push gas back out of the air mixer. Two inches is a common fill depth, which is why it is the default, but the manual governs.
Use only media rated for fire. Non-porous material that holds moisture, such as gravel, pebbles and river rocks, can fracture when heated. Do not fill the enclosure around a valve box, because that space is there for ventilation and drainage.
After you change the media on a propane unit, the manufacturers call for a test with the media in place. If you are not sure how to run it, hire a qualified gas technician.
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