Fire Glass Calculator
Find how much fire glass a fire pit needs by volume and weight.
Five pit shapes, reflective or recycled glass, metric and imperial units.
Updated October 21, 2026
Introduction
Fire glass is sold by weight, but a fire pit is described by its size. This page is the bridge between the two: pick the shape of the pit, enter its inside measurements and the thickness of the glass layer you want, and it returns both the volume of glass that layer holds and the weight to order.
Three numbers do the work — the area of the pit floor, the depth of the layer and the density of the glass — and each of them is a place where an order goes wrong. An outside measurement instead of an inside one, a pit depth typed into a layer-depth box, or a density that came from a different product all change the answer by more than people expect. The page keeps the three steps visible so you can see which one moved the weight.
It is written for the two ways people buy glass. If you are fitting out a pit, start from its size and read the weight at the end. If you already have glass in the garage, type that weight instead: the same chain runs backwards and tells you how much pit it fills, down to the missing side of the tray.
5 pit shapes
Rectangle, square, triangle, round and trapezoid
6 length units per box
Millimetres to yards, chosen on each measurement
7 weight units
Milligrams to long tons, and four density units
Quick start
Choose the glass type, or type your own density
Reflective and recycled glass carry the two densities suppliers quote most often, 1.457 g/cm³ and 1.445 g/cm³. If your supplier publishes a density for the exact product you are buying, choose the custom option and enter it — that is the only way the weight will match their packing rather than a typical figure.
Choose the pit shape
Rectangle, square, triangle, round and trapezoid are covered. The measurement boxes change to the ones that shape needs, and only the boxes for the shape you picked are used in the calculation.
Enter the inside measurements
The boxes change to suit the shape: length and width for a rectangle, one side for a square, base and perpendicular height for a triangle, a diameter for a round pit, and front width, back width and spacing for a trapezoid. Measure the opening the glass sits in rather than the outside of the wall.
Enter the layer depth
This is the thickness of glass you are adding, not how deep the pit is. The box opens in millimetres because a cover layer is usually measured that way; it accepts centimetres, inches or feet just as readily. Enter the depth the burner instructions ask for.
Read the weight in the final group
The volume row gives the glass the layer holds, and the collapsible final group gives the weight to order — in kilograms, pounds, metric tons or long tons. Here is the same chain for a 60 cm by 60 cm tray under a 5 cm layer of reflective glass:
Take the 26.226 kg to the supplier. Three 10 kg bags give you 30 kg with a little spare; a single 25 kg bag is short by more than a kilogram.
The calculation
Every shape ends in the same two multiplications. Only the area formula changes, and that is the whole model: a prism of glass standing on the plan area of the pit.
The chain
The layer volume is the floor area times the layer thickness; the weight is that volume times the density of the glass. Both multiplications run on one set of base units — centimetres, square centimetres, cubic centimetres and grams — so a pit measured in inches and a density quoted in pounds per cubic foot meet in the same place with no rounding at the seam.
The five area formulas
| Shape | Formula | What the page asks for |
|---|---|---|
| Rectangular | Inside length and width | |
| Square | One inside side | |
| Triangular | Base and perpendicular height | |
| Round | Inside diameter | |
| Trapezoidal | Front, back and the spacing between them |
The trapezoid covers the tapered pits built into outdoor tables and fireplaces, where the opening is wider at the front than at the back. A rectangle is the same formula with, so if your tapered pit happens to be parallel-sided, either shape gives the same answer.
Solving in the other direction
Nothing in the chain is a dead end. Type a known area and the page solves the missing measurement; type a known volume and it solves the area or the layer depth; type a weight and it works back to the volume, then to the area, then to whichever measurement is still open.
The area formula reverses the same way for each shape: a square's side is, a circle's diameter is, and a triangle's base or height comes from twice the area divided by the other dimension.
The two preset densities
Reflective glass carries a coating, so the same volume of it weighs a little more than the plain recycled product: 1.457 against 1.445 g/cm³. On the 18,000 cm³ layer from the quick start that is 26.226 kg of reflective glass or 26.010 kg of recycled — a 216 g difference on a 26 kg order, less than one per cent. In the units a US supplier may quote, the two come to about 91 and 90 pounds per cubic foot.
Which unit your supplier quotes
Data sheets quote density as kilograms per cubic metre (1,457 for reflective glass), grams per cubic centimetre (1.457), or pounds per cubic foot (about 91). All three land on the same calculation; enter the figure in the unit the sheet uses and the weight stays the same. If a supplier publishes coverage instead — how much a bag covers at a stated depth — turn that into a density by dividing the bag weight by the volume it covers, and the weight here will agree with their instructions.
Comparing a different fill material, such as a lava-rock base beneath the glass? The rectangular box size to weight calculator applies the same volume-times-density chain to any material you can name a density for.
Worked examples
A round fire bowl measured in inches
Situation: An outdoor table bowl has a 30 in inside opening and the burner instructions ask for a 2 in layer of fire glass.
Inputs
Result
The imperial figures are what the page shows when the area and volume rows are read in square inches and cubic inches; the weight step uses the cubic-centimetre volume the page carries underneath, which is why it reads 33.7538 kg rather than a directly multiplied imperial figure.
In practice: Order 33.75 kg, rounded up to whole bags. The same glass reads as 74.4145 lb if your supplier quotes in pounds.
Working back from a bag you already own
Situation: There is a 30 kg bag of reflective glass left from an earlier job, and the tray being filled is 60 cm wide. How long a pit will that layer cover?
Inputs
Result
The bag fills a tray about 1.37 m long at a 2.5 cm layer. Nothing about the density changes on this path: the page treats the glass density as the one figure it already knows, and reshapes the pit instead.
In practice: Type your real width and the page returns the length that uses the bag exactly. If your tray is shorter, the same glass sits deeper than 2.5 cm — still fine, as long as it stays well under the depth the page warns about.
Getting the layer right
The arithmetic is exact; the number in the depth box is the estimate. It is also the strongest lever in the whole page, so it is worth a moment before you order.
For the same 60 cm square tray, the layer depth moves the order as follows, with reflective glass at 1.457 g/cm³:
| Layer depth | Volume | Weight to order |
|---|---|---|
| 2.5 cm | 9,000 cm³ | 13.113 kg |
| 5 cm | 18,000 cm³ | 26.226 kg |
| 7.5 cm | 27,000 cm³ | 39.339 kg |
| 10 cm | 36,000 cm³ | 52.452 kg |
Doubling the depth doubles the weight, exactly, because the density and the area are unchanged. Nothing else on the page moves a weight that hard — not the shape, and not the choice between the two preloaded glass types.
Depth is the layer, not the pit
The depth box asks how much glass you are adding. Enter 400 in millimetres and that is 40 cm of glass sitting in the tray, not a pit 40 cm deep. For a cover above a burner, that is already a very deep bed, and above 50 cm the box turns red for exactly this reason.
Measure the inside of the pit
A 70 cm outside square with 5 cm walls is a 60 cm inside square. The inside footprint is 3,600 cm² instead of 4,900 cm², so measuring the outside would order about 36% more glass than the tray can hold at the same depth.
Buy whole bags, keep the spare
Divide the weight by the bag size and round up. 26.226 kg is three 10 kg bags; the 3.8 kg left over is a top-up layer later on. Ordering to the exact hundred grams usually means a second delivery instead of a spare bag.
Area grows with the square
At the same 5 cm layer, a 60 cm square pit takes 26.226 kg and a 120 cm square pit takes 104.904 kg — four times as much, because both sides doubled. A slightly wider pit is not a slightly bigger order.
Why the depth box turns red above 50 cm
A glass layer half a metre deep buries the burner and its air openings far below the surface, so no flame reaches the glass on top. In practice that reading almost always means the pit's own depth has been typed into the layer-depth box. The page keeps the number and the weight it implies on screen — you can see what the mistake costs — and flags the input instead of pretending the result is usable.
Questions
Why did a measurement change when I typed the weight?
Because the weight is an input as well as an answer. Type it and the page has one free variable left to solve for, so it reshapes the pit: the volume follows from the weight and the density, the area follows from the volume and the layer depth, and whichever measurement you left open takes the remaining value. The glass density never moves on that path — a preset stays a preset.
What if the pit is not one of the five shapes?
Work out its plan area separately and type it into the Area box. An L-shaped or irregular pit is usually a sum of rectangles: measure each one, add them up, and the rest of the page — layer volume, weight, and even the layer depth — works exactly as before. The square footage calculator handles the rectangle areas in square feet if that is how your drawing is measured.
Should I type the area, or the side measurements?
Type whichever you trust. From the sides, the page computes the area; from an area taken off a drawing or a supplier's spec sheet, it solves the missing side instead. Typing both is fine as long as they agree — the moment they do not, one of them is wrong, and the order follows the number you typed last.
Does the layer depth include the burner?
The page counts the whole layer from the floor of the tray upwards. It has no idea where the burner, its fittings or its air gaps sit inside that volume, so enter the depth exactly as the burner instructions define it. If those instructions give a minimum and a maximum above the burner, the minimum is the figure to buy to, and the maximum tells you how much spare glass you can keep back.
How far to trust it
- A layer is modelled as a flat prism. The page multiplies the plan area by one thickness, so a raked, mounded or sloped layer uses more glass than the figure and a thin scatter across a wide tray uses less. Fill to a level surface if you want the number to be the quantity.
- One density covers the whole layer. The preloaded numbers are typical values; two products of the same nominal type can pack differently, and smaller pieces pack more densely than large ones. Where a supplier quotes a density, entering it is what makes the resulting weight match their bags.
- A measurement error is a quantity error. On a small tray, a centimetre on each side is a few per cent of the area, and the layer depth multiplies straight through any mistake in it. Measure the inside twice before ordering a whole batch.
- Glass quantity only. This page sizes the media. It says nothing about burner capacity, gas supply, clearances, heat output or the structure that holds the pit; those come from the burner and pit manufacturer, and from whatever the installation has to comply with where you are.
- Use glass meant for fire pits, and follow the instructions. The page assumes certified fire-pit glass installed as its maker directs. It cannot tell you whether a substitute material is safe in a burner, so treat the quantity here as an estimate for planning and take the installation and clearance requirements from the equipment you are actually fitting.
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