Size to Weight Calculator (Rectangular Box)

Free Online Size to Weight Calculator (Rectangular Box)

This size to weight calculator estimates how much a rectangular box, block, plate, sheet, or crate weighs, using only its length, width, height, and the material it's made from. Instead of putting a real object on a scale, you enter the dimensions and pick a material — steel, aluminum, wood, plastic, glass, concrete, and more — and the calculator does the rest, including an option for hollow objects with a set wall thickness instead of a solid block.

This is the same math shipping companies, metal suppliers, and engineers use every day: volume multiplied by density gives weight. Knowing the weight ahead of time helps with shipping cost estimates, structural load calculations, lifting equipment planning, and material ordering.

How Size to Weight Calculation Works

Every material has a density — a fixed relationship between how much space it takes up and how heavy it is, usually measured in kilograms per cubic meter (kg/m³) or pounds per cubic foot. Steel is dense and heavy for its size; cardboard and foam are light for their size. Once you know an object's volume and its material's density, multiplying the two together gives you the weight, without ever touching a scale.

For a solid rectangular box, volume is simply length × width × height. For a hollow box — like a metal enclosure, a shipping crate, or a plastic container with walls — the calculator subtracts the empty inner cavity from the outer volume first, so you get the weight of the material itself, not the weight of an imaginary solid block the same size.

Size to Weight Formula

Here's the math running behind this calculator:

• Outer Volume = Length × Width × Height
• For a hollow box: Inner Volume = (Length − 2×Wall) × (Width − 2×Wall) × (Height − 2×Wall)
• Net Volume = Outer Volume − Inner Volume (Net Volume = Outer Volume for a solid box)
• Weight = Net Volume × Material Density
• Total Weight = Weight per Box × Quantity

How to Use This Calculator

Measure the outer length, width, and height of the box and enter them, choosing whichever unit you measured in — millimeters, centimeters, meters, inches, or feet. Select the material from the dropdown, or choose 'Custom Density' if you know the exact density of your material and want to enter it directly. If the object is hollow, like a sheet metal box or a plastic bin, turn on the hollow toggle and enter the wall thickness; leave it off for a solid block, bar, or plate. Set the quantity and your preferred weight unit (pounds or kilograms), and add a price per unit weight if you want an estimated material cost. The result updates instantly, with a full breakdown in the step-by-step panel.

Worked Example

Say you have a solid mild steel plate that's 12 inches long, 8 inches wide, and 6 inches high. First, convert to meters: 12 in = 0.3048 m, 8 in = 0.2032 m, 6 in = 0.1524 m. Multiply for volume: 0.3048 × 0.2032 × 0.1524 ≈ 0.00944 cubic meters. Mild steel has a density of about 7,850 kg/m³, so weight = 0.00944 × 7,850 ≈ 74.1 kg, which converts to roughly 163 pounds. If you needed 3 of these plates, the total weight would be about 489 pounds.

Common Material Densities

Density is the number that makes this whole calculation work, and it varies enormously between materials. Here are some commonly used reference densities in kilograms per cubic meter:

• Steel (mild): ~7,850 kg/m³ — Aluminum: ~2,700 kg/m³ — Copper: ~8,960 kg/m³
• Stainless steel: ~8,000 kg/m³ — Brass: ~8,500 kg/m³ — Cast iron: ~7,200 kg/m³
• Glass: ~2,500 kg/m³ — Concrete: ~2,400 kg/m³ — Granite: ~2,700 kg/m³
• Pine wood: ~500 kg/m³ — Oak wood: ~750 kg/m³ — Plywood: ~600 kg/m³ — MDF: ~750 kg/m³
• HDPE plastic: ~950 kg/m³ — PVC: ~1,400 kg/m³ — Acrylic: ~1,190 kg/m³ — ABS: ~1,050 kg/m³
• Corrugated cardboard: ~130 kg/m³ — much lighter than solid wood or plastic because of its air-filled fluting.
• These are typical reference values for solid, void-free material. Actual density can shift with alloy grade, wood species and moisture content, or manufacturing process — check your supplier's spec sheet for precise work.

Solid vs. Hollow Objects

A solid block, bar, or plate is straightforward: its entire volume is made of material, so outer volume times density gives the exact weight. A hollow object — a welded steel enclosure, an injection-molded plastic case, a wooden crate, a shipping box — only has material in its walls, with empty space in the middle. Calculating that as if it were solid would overestimate the real weight, sometimes by a large margin.

To handle this correctly, this calculator lets you enter a wall thickness. It shrinks the outer dimensions inward by that thickness on every side to get the inner cavity's volume, then subtracts that cavity from the outer volume. What's left is the actual volume of material used, which is what should be multiplied by density.

Where This Calculator Is Used

Size-to-weight estimates like this come up constantly across trades and industries:

• Metal fabricators and machine shops estimating the weight of steel, aluminum, or brass stock before cutting or shipping it.
• Shipping and logistics teams calculating dimensional and actual weight for freight quotes.
• Woodworkers and furniture makers estimating how heavy a finished piece will be before it's built.
• Engineers and product designers doing early-stage weight estimates for enclosures, brackets, and structural parts.
• Movers and riggers planning lifting equipment for heavy crates, machine bases, or stone slabs.
• Hobbyists and makers estimating plastic or acrylic part weight for 3D printing or laser-cut projects.

Tips for a More Accurate Estimate

This calculator gives a solid estimate, but a few things can improve accuracy further. Always measure the actual outer dimensions rather than a nominal or rounded size, since even small measurement errors compound through a volume calculation. If you know the exact alloy, grade, or wood species, look up its specific density rather than relying on a general category average, since density can vary noticeably within a material family (different steel alloys, for example, or wet versus kiln-dried lumber). For hollow objects with uneven wall thickness, use the thinnest wall dimension for a conservative (lower) estimate, or calculate a weighted average thickness for a closer match to reality.