Fire Glass Calculator

1. Pick the pit shape, round or rectangle.
2. Enter the diameter, or the width and length, in inches.
3. Set the glass depth and any lava rock filler.
4. Add a spare allowance and read the pounds of glass.

Here is what each result line means:

Building the pit itself first? Size the ring with the concrete block calculator and the base with the gravel base calculator.

A fire glass calculator estimates how much fire glass a gas fire pit or fireplace needs to fill the burner area to a chosen depth. Fire glass is tempered glass that will not melt or discolour, used in place of logs or lava rock over a gas burner.

The weight is the fill volume times the glass density, which the common manufacturer formula folds into a simple divisor: 24 for round pits and 19 for rectangles. A lava rock layer underneath can cut the glass you need, since lava is cheaper and lighter.

For a round pit the glass is the diameter times itself times the depth, over 24. For a rectangle it is width times length times depth, over 19. Both give pounds when the sizes are in inches.

1. Measure the pit in inches and set the glass depth.
2. Use the round or rectangle formula for the pounds of glass.
3. Add a little spare so you do not run short.
4. If using a lava rock filler, size it for the filler depth below the glass.

Makers recommend at least 2 inches of glass, with 2 to 3 inches common. A burner pan takes up some room, so the formula is a fill-to-depth estimate and the real amount is often a touch less.

Suppose you have a 25 inch round pit filled to 2 inches of glass.

Calculation: 25 times 25 times 2 is 1,250; divided by 24 is about 52 pounds of fire glass, or around 58 pounds with 10 percent spare. A 1 inch lava rock layer underneath would cut the glass by filling the bottom of the pit.

The lesson: depth drives the weight, so a thin top layer of glass over lava rock is the economical fill.

Pit Size

The glass scales with the pit area, so a bigger diameter or longer trough needs a lot more. The area grows with the square of the diameter.

Depth

Weight is in direct proportion to depth. Going from 2 to 3 inches adds half as much again.

Lava Rock Filler

A lava rock base fills the bottom cheaply, so only the top couple of inches are glass. This can cut the glass cost sharply on a deep pit.

Burner Pan

A burner and its pan take up room the formula does not subtract, so the real glass is often a little less than the fill-to-depth figure.

The two fills are often combined.

A common approach is lava rock to within a couple of inches of the top, then fire glass over it. Both must be rated for fire and kept dry, since trapped water can cause popping.

New Gas Pits

Size the glass for a new fire pit or table before ordering.

Refills and Top-ups

Work out a refill after cleaning or a colour change.

Troughs and Linear Burners

Estimate a long rectangular fireplace or trough with the rectangle formula.

Budgeting with Lava Rock

Compare an all-glass fill with a lava-rock-plus-glass fill to cut cost.

1. Using Regular Glass

Only tempered fire glass is safe over a burner. Ordinary glass can shatter and throw shards. Buy glass rated for fire.

2. Too Deep Over the Burner

Piling glass too deep can smother the flame or trap gas. Follow the burner maker depth, usually around 2 inches over the burner.

3. Ignoring the Burner Pan

The pan takes up room, so the fill-to-depth formula slightly over-orders. That is fine as a safe margin, but do not be surprised by a leftover bag.

4. Wet Glass or Rock

Trapped moisture can pop when heated. Keep the fill dry and cover the pit when not in use.

5. Forgetting the Spare

Dye lots and sizes vary. A little spare avoids a second order that does not quite match.

The formula is the standard manufacturer estimate to a set depth, but the burner pan and glass size shift the real amount, so treat the result as a close figure and check the pit maker chart.

For a round pit, multiply the diameter by itself by the depth and divide by 24 for pounds. For a rectangle, use width times length times depth over 19. A 25 inch round pit at 2 inches deep takes about 52 pounds.

At least 2 inches, with 2 to 3 inches common. Keep it to the burner maker depth over the burner so the flame is not smothered.

Yes. Lava rock is a cheaper, lighter filler for the bottom of the pit, with a couple of inches of fire glass over it. It cuts the glass you need on a deep pit.

Yes. The pan takes up room the formula does not subtract, so the real glass is often a little less than the fill-to-depth figure. The extra makes a safe margin.

Yes. Fire glass is tempered to take heat without melting, discolouring or shattering. Never use ordinary glass over a burner.

Fire glass has a bulk weight around 90 pounds per cubic foot, which is what the 24 and 19 divisors are based on. Lava rock weighs roughly half as much for the same volume.

Use width times length times depth, in inches, divided by 19. A 48 by 12 inch trough at 3 inches deep is about 91 pounds.

Trapped water can turn to steam and pop when the pit heats up. Keep the glass dry and cover the pit when it is not in use.

Yes. Clean it of soot and debris and it can be reused. Top up any that is lost or broken, keeping to the same colour and size.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• Home Depot: Fire Glass assembly and coverage instructions (round divided by 24 and rectangle divided by 19 formulas; 2 in minimum depth).
• Fire Pit Surplus: Fire Glass Calculator (recommended glass depth by pit type).
• BIA Technical Note 10, Dimensioning and Estimating Brick Masonry (Brick Industry Association; volume estimating method).