Size to Weight Rectangular Cuboid Calculator

Pick up two objects the same size — one pine block and one steel block. The steel feels impossibly heavy by comparison. That difference is density: steel packs about 15 times more mass into the same space as pine. This calculator multiplies volume by density to give you weight, which is the same arithmetic a materials engineer uses to spec a lifting rig or a freight forwarder uses to quote a pallet.

The calculation has three steps. First, length times width times height gives you the volume in cubic centimetres (metric) or cubic inches (imperial). Second, that volume is converted to cubic metres, the standard unit for density. Third, volume in cubic metres times density in kg per cubic metre gives mass in kilograms. For imperial, the final step converts to pounds using the standard 2.20462 factor.

The density values in the preset list are standard industry reference values. Real materials vary slightly — a specific grade of stainless steel or a particular hardwood species will differ from the preset by a few percent. For engineering calculations where that margin matters, use the material’s datasheet density in the custom field rather than the preset.

Use this calculator when you need to plan transport for a cut block of material, estimate dead load for a structural element, verify a supplier’s weight claim on a material order, or check whether a two-person carry is safe before a delivery arrives. It is the right tool whenever you have a solid rectangular object and you know what it is made of.

Do not use this calculator for objects with complex shapes — cylinders, I-beams, tapered sections, or parts with holes and pockets. For those, you need a CAD tool that can integrate volume over an irregular geometry, or a dedicated shape calculator. This tool also should not be the final word on structural load calculations where a factor of safety is required — use it to generate a working estimate, then validate with your structural engineer.

This tool is also useful in reverse: if you know your maximum allowable weight (truck axle limit, shelf rating, floor loading specification) and you know the material, you can calculate what the maximum block dimensions should be by working backwards from the weight formula. The calculator does not do this reversal automatically, but the arithmetic is simple once you have the density and one or two fixed dimensions.

The most common mistake is entering dimensions in the wrong unit. A steel plate measured as 2,400 mm x 1,200 mm x 6 mm entered directly into a centimetre calculator produces a result 1,000 times too heavy. Always divide millimetre measurements by 10 before entering them, or switch your measurements to metres and reconsider whether centimetres is the right input scale for your specific block.

The second mistake is using a hollow object with a solid-block calculator. Box sections, pipes, I-beams, and hollow extrusions all have significantly less material than a solid block of the same outer dimensions. Using this tool on a structural hollow section will overestimate weight by 50-80%, which could lead to a lifting rig being underspecified for a lighter load — a less dangerous error than the reverse, but still a planning waste.

The third mistake is picking the wrong material preset for a non-standard alloy. Standard steel (7,850 kg/m³) and stainless steel (8,000 kg/m³) are close, but tool steel, duplex stainless, and titanium alloys all have distinctly different densities. When the specific material grade matters for structural or freight purposes, always pull the density from the material certificate or datasheet and enter it in the custom density field.

Weight = Length × Width × Height × Density

For metric inputs (dimensions in cm, density in kg/m³): Volume (cm³) = L × W × H Volume (m³) = Volume (cm³) ÷ 1,000,000 Weight (kg) = Volume (m³) × Density (kg/m³)

For imperial inputs (dimensions in inches, density in kg/m³): Volume (in³) = L × W × H Volume (m³) = Volume (in³) × 0.000016387064 Weight (kg) = Volume (m³) × Density (kg/m³) Weight (lb) = Weight (kg) × 2.20462

The density conversion factor from in³ to m³ is exact: 1 cubic inch = 16.387064 cubic centimetres, so 1 in³ = 0.000016387064 m³. Using kg/m³ as the universal density unit means you only need one density table regardless of whether the dimension inputs are metric or imperial.

The formula assumes uniform density throughout the block, which is true for most homogeneous metals but less accurate for timber (moisture content shifts apparent density by 10-25%), concrete (varies by mix and aggregate), and composites. For timber specifically, the density presets reflect air-dry values — green or wet timber can be 30-50% heavier. If you are planning a log delivery or freshly cut hardwood transport, enter a higher custom density than the preset to avoid underestimating your load.