How this instrument works
A cubic foot is a familiar unit for anything sized in feet — aquarium tanks, planter boxes, shipping crates, storage bins — and it converts to exactly 28,316.846592 millilitres by the modern definition of the foot. Pounds, though, measure weight, and weight is not something geometry alone can tell you; a cubic foot of empty space and a cubic foot packed with lead obviously don't weigh the same, so the calculator needs one more piece of information before it can answer.
That missing piece is the Density field, and its default of 1.00 g/mL (water) is a starting point, never a universal answer. Potting soil is lighter than garden dirt because it's formulated with peat, bark, and perlite for drainage — it commonly runs from about 0.5 to 0.75 g/mL dry, rising toward 1.0 g/mL or more when fully saturated with water; dry sand sits closer to 1.5–1.7, gasoline is lighter than water near 0.74, and diesel fuel lands around 0.85. The field stays open on purpose so you can enter whichever of those — or something else entirely — actually matches what you're weighing.
- Type your volume into Volume (cubic feet) — the field starts at 5, a typical small aquarium or planter size.
- Set Density (g/mL) to whatever you're weighing; 1.00 (water) is only the starting point, not the answer.
- Mass (lb) updates instantly beneath both fields as you type.
- For loose materials like soil or gravel, measure the density as the material is actually sitting — compacted and loose versions of the same material report meaningfully different numbers.
Worked example — a 5 cubic-foot tank of water, then something lighter
A small 5 cubic-foot aquarium or water tank, left at the default Density of 1.00, gives Mass (lb) = 312.139802881 — close to the familiar rule of thumb that water runs about 62.4 pounds per cubic foot, five times over.
Refill that same 5 cubic-foot space with a lighter liquid closer to 0.80 g/mL, in the range of kerosene or a light fuel oil, and Mass (lb) falls to 249.711842305 — roughly 62 pounds less for identical volume. The tank didn't shrink; the contents simply became less dense.
Questions
Why isn't cubic-feet-to-pounds a single fixed number?
Because pounds measure mass and cubic feet measure space, and the only link between them is density — a property that depends entirely on the material filling that space. The cubic-feet-to-millilitre step is pure geometry and always exact; the millilitre-to-pound step depends on what you're actually weighing, so it can't be baked into one universal factor.
How do I find the density of my material?
For liquids and chemicals, check the product's safety data sheet or spec sheet, which almost always lists density directly. For soils, sand, or aggregate, published bulk-density reference tables give a reasonable starting range, though the safest approach for anything you're actually hauling or billing is to weigh a known volume yourself and divide.
Is the '62.4 pounds per cubic foot' rule for water exact?
It's close but not perfectly exact at every temperature. Water's density peaks near 4°C and eases down slightly as it warms, so real values run from about 62.3 to 62.43 pounds per cubic foot across ordinary temperatures. The commonly quoted 62.4 figure is a practical rounding that works fine for everyday use.
Does this work for loose or granular material like soil and mulch?
Yes, as long as you supply an accurate bulk density for that material's actual state. Loose, fluffy soil or mulch can weigh 15–25% less per cubic foot than the same material once compacted, so a density figure from a generic table may need adjusting for how your material is actually sitting.
Is g/mL the same thing as g/cm³ for this field?
Yes — a millilitre and a cubic centimetre are identical volumes, so g/mL and g/cm³ are interchangeable. If a source instead gives you a density in kg/m³, divide by 1000 first; 1,600 kg/m³ becomes 1.6 g/mL before you enter it.
Can I run this in reverse, from a known weight to a volume?
Yes, by rearranging the formula: cubic feet equal pounds divided by the product of 62.428 and density. That's useful for questions like how much space a known weight of gravel or soil will actually take up once delivered.