Fire Glass Calculator
Find how much fire glass you need for a gas fire pit, fire table, or fireplace, by weight and volume, and how many bags to buy. Enter the shape and size of the area and the depth of the glass.
Add more areas, or subtract areas that need no glass.
- Choose US or Metric. This also sets the units of the results and the bag sizes: 25, 20, and 10 lb bags for US, and 9 and 4.5 kg bags for Metric.
- Pick the shape of the area and enter its inside dimensions. A round fire pit is a circle. For a fireplace, choose Trapezoid and enter the front width and the back width as the two parallel sides and the depth from front to back as the height. For a shape that is not listed, choose Known area and enter the area directly.
- Enter the Glass depth: the depth of the fire glass layer only, not the sand, pea gravel, or lava rock under it.
- Use Add section for an L-shaped table or a second fire pit. Set a section to Subtract for a part that needs no glass, such as a lava rock center. You can also enter a ring directly with Ring.
- The Density field shows a typical density for crushed fire glass. For glass beads or drops, or for lava rock, enter a density as described below. The reset link under the field brings back the typical value.
- The waste allowance starts at 0%. Raise it if you'll mound the glass over the burner or the bottom of the pit is uneven.
- If your bag is not listed, enter the weight printed on the bag in Custom bag size to add it to the results.
- Optionally, enter a price per bag, pound, or kilogram to estimate the cost.
The dimension and depth fields accept whole numbers, decimals, fractions,
and mixed numbers, such as 36, 2.5,
3/4, or 2 1/2.
How much fire glass you need
The volume of an even layer of glass is:
Volume = area × glass depth
Fire glass is sold by weight, which is:
Weight = volume × density
The calculator uses a typical density of 90 lb/ft³, or 1,442 kg/m³ (1 ft³ of fire glass weighs about 90 lb). With every dimension in inches, that works out to:
Pounds = length × width × depth ÷ 19.2
Pounds = diameter × diameter × depth ÷ 24.4 (round fire pit)
The waste allowance is added to the volume after subtracting any sections, and the calculator doesn't subtract the space taken by the burner or its pan.
The number of bags is:
Bags = total weight ÷ weight per bag, rounded up
Fire glass bags are labeled by weight, so the bags are counted by weight. The calculator shows these bag sizes:
| Bag | Sold by |
|---|---|
| 25 lb | Margo Garden Products |
| 20 lb | Margo Garden Products and MSI |
| 10 lb | American Fire Products, Starfire Direct, Margo Garden Products, and MSI |
| 9 kg | The 20 lb bag as labeled in Canada (VEVOR) |
| 4.5 kg | The 10 lb bag as labeled in Canada (VEVOR) |
The metric bags are the kilogram labels of the 10 lb and 20 lb bags sold in Canada. A 4.5 kg bag holds a little less than 10 lb (4.54 kg), so it sometimes takes one more 4.5 kg bag than 10 lb bags. If you buy glass by the kilogram, choose kg as the price unit.
Glass depth
Most fire pits and fireplaces use 2 to 4 in (5 to 10 cm) of glass, the range American Fire Products and Starfire Direct give in their fire glass calculators. How much glass goes over the burner differs by maker: American Fire Products says a burner should always be completely covered and suggests covering it with about 1/2 to 1 in of small lava rock before the glass, MSI suggests at least 2 in of glass over the burner, and Starfire Direct suggests no more than 1 in over a propane burner. Follow the instructions for your burner and fire pit.
Enter only the depth of the glass. The sand or pea gravel that fills the pit under the burner is a separate layer. To find how much of it you need, use the Gravel Calculator for pea gravel or the Sand Calculator for sand.
Density and type of glass
The typical value comes from Alterna, which says 90 pounds of its crushed FireGlitter glass fills about 1 ft³. The calculators from American Fire Products and Starfire Direct use 12 lb per gallon, about 89.8 lb/ft³. The values in maker guides range from about 89.8 to 96 lb/ft³ for crushed and reflective glass.
Round glass beads and drops pack more tightly and weigh more. Alterna gives 110 lb/ft³ for its FireDrops. For beads or drops, enter 110 lb/ft³ in Density to get a closer amount.
Lava rock is much lighter. American Fire Products' lava rock calculator uses half the weight of fire glass, about 45 lb/ft³. To find the weight of lava rock for a fire pit, enter 45 lb/ft³ as the density.
Shapes
For a fireplace, the front of the firebox is usually wider than the back. Enter it as a trapezoid: (front width + back width) ÷ 2 × depth from front to back gives the floor area, the same way the fireplace calculators from American Fire Products and Starfire Direct do.
For a fire bowl whose sides slope, measure the diameter halfway down the glass layer and enter it as a circle. For a bowl that narrows toward the bottom, the result is slightly less than the actual volume.
The round fire pit calculators from American Fire Products and Starfire Direct use 0.8 in place of π ÷ 4 (about 0.785), so this calculator shows about 1.6% less glass for a round pit than they do.
Examples
A round fire pit
A round fire pit 36 in across, filled 3 in deep, holds 1.77 ft³ (50.04 L) of glass. At 90 lb/ft³, that's 159 lb (72 kg): 7 bags of 25 lb, 8 bags of 20 lb, or 16 bags of 10 lb. At $35 per 10 lb bag, the cost is $560.00.
A fireplace
A fireplace 36 in wide at the front, 24 in wide at the back, and 15 in from front to back has a floor of (36 + 24) ÷ 2 × 15 = 450 in². Filled 3 in deep, it takes 70 lb of glass: 3 bags of 25 lb, 4 bags of 20 lb, or 8 bags of 10 lb.
A fire pit with a lava rock center
A ring 48 in across with a 24 in lava rock center, filled 2 in deep, takes 141 lb of glass: 6 bags of 25 lb, 8 bags of 20 lb, or 15 bags of 10 lb.
Metric dimensions
A round fire pit 90 cm across, filled 8 cm deep, holds 50.89 L of glass. At 1,442 kg/m³, that's 73 kg: 9 bags of 9 kg or 17 bags of 4.5 kg.
FAQ
About 159 lb, or 16 bags of 10 lb, for a round pit 36 in across filled 3 in deep. Filled 2 in deep, it takes about 106 lb, or 11 bags of 10 lb. Beads and drops weigh more, so enter their density to see the amount for them.
Usually 2 to 4 in. The glass should cover the burner by the amount its maker recommends, which differs between burners.
About 0.67 ft² at 2 in deep, or 1.33 ft² at 1 in deep, at 90 lb/ft³. A 10 lb bag holds about 0.11 ft³ (192 in³) of glass.
No. Fire glass is made for gas flames, and American Fire Products says it is not safe for wood-burning fires.
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