Fire Glass Calculator

Background.

Fire glass is bought by weight but needed by volume. You measure a burner pan or a decorative bed in length, width and depth, while the supplier prices the glass per kilogram or per pound, and the bridge between the two is bulk density: how many kilograms of loose pieces fill one cubic metre, air gaps included.

That figure sits well below the density of glass itself. Solid soda-lime glass weighs about 2,500 kg per cubic metre, but a heap of cubes, nuggets or beads traps air between the pieces. The default of 1,450 kg/m³ used here assumes the glass occupies 58% of the space and air the other 42%. Round beads, flat diamonds and crushed chips of mixed sizes all pack differently, so the density is a field you can change rather than a constant baked into the answer.

The arithmetic is short. Length times width times depth gives the volume the glass will fill once it has settled. A percentage is added on top for settling, spills and an uneven base. The result, multiplied by bulk density, is the order in tonnes. Because each step is a plain multiplication, a density that is 10% too low gives an order that is 10% too small.

Two things sit outside the estimate. It assumes a flat rectangle of uniform depth, so round and irregular pits need a small adjustment, described below. And it has no opinion on how much glass may safely cover a gas burner; that depth comes from the appliance manufacturer's instructions, not from a quantity calculation.

What is fire glass calculator?

The quantity estimated is the mass of loose fire glass needed to fill a rectangular space to a chosen depth after it settles. It depends on three things: the geometric volume of that space, a percentage added for settling and handling losses, and the loose bulk density of the particular glass. Bulk density is the mass of a known container volume filled with the pieces, air spaces and all — the same idea that ASTM C29 standardises for construction aggregates. The answer is given in metric tonnes, alongside the two volumes it is built from.

How to use this calculator.

1. 01Measure the inside of the pan, pit or bed in metres and enter Length and Width. For a round pit, enter the diameter in both fields and adjust the result as explained in the questions below.
2. 02Enter Compacted depth as the depth of glass you want once it has settled, in metres: 5 cm is 0.05 and 3 inches is about 0.076. If the lower part of the pit is filled with lava rock or another base, count only the glass layer on top.
3. 03Replace the 1,450 kg/m³ default with the supplier's bulk density if they publish one. If not, fill a 10-litre bucket level with the actual glass, weigh it, and multiply the kilograms by 100.
4. 04Set Waste / compaction allowance. Keep the 8% default for a level, enclosed pan; raise it where the base is uneven or glass can spill over open edges.
5. 05Read the order mass in tonnes. Multiply by 1,000 for kilograms or by about 2,205 for pounds, then round up to whole bags.

The formula.

Order mass (t) = L × W × D × (1 + w ÷ 100) × ρ ÷ 1,000. L, W and D are the length, width and settled depth in metres, so their product is the compacted volume in cubic metres. The letter w is the allowance percentage: 8% becomes a factor of 1.08, which lifts the compacted volume to the order volume. ρ is the loose bulk density in kilograms per cubic metre, and dividing by 1,000 turns kilograms into metric tonnes. Every term is a straight multiplier, so the answer is exactly proportional to each: double the depth and the order doubles. The two inputs most likely to be wrong are density, which varies from one glass shape and size to another, and depth, where a couple of centimetres across a large area adds up to a lot of glass. No shape factor is applied, so the space is treated as a box with a flat floor.

A worked example.

Frequently asked questions.

References& sources.

1. [1]ASTM International. ASTM C29/C29M. Bulk density and voids in aggregate (PAYWALL). Retrieved 2026-08-06. access: gated.