Length times width times height. The formula is trivial. What catches people is that cubic feet is used for four completely unrelated jobs, and each one measures a slightly different box.
Cubic feet equals length times width times height, all in feet. A room 12 ft by 15 ft with an 8 ft ceiling is 1,440 cubic feet.
The trap is the same one that catches people on area, only worse, because the error is cubed rather than squared. A ceiling of 8 ft 6 in is 8.5 ft, not 8.6. Get that wrong on all three dimensions and the compounding is significant.
Set the tool to decimal feet before you start and the conversion disappears. Most laser measures also have a volume mode that takes three readings and does the multiplication for you, which removes the arithmetic step entirely. It is one of the few modes on these tools that genuinely earns its place. Using the modes properly is covered in the main guide.
Cubic inches to cubic feet: divide by 1,728. Cubic feet to cubic yards: divide by 27. Cubic metres to cubic feet: multiply by 35.315. Cubic feet to US gallons: multiply by 7.481.
Heating, cooling and ventilation. You want the volume of air the equipment has to move or condition, so measure the open space. Include anything continuous with the room, exclude a closet behind a closed door. Sloped ceilings matter here: use the average height rather than the peak, or you will oversize the equipment and pay for it twice, once at purchase and again in short cycling.
Storage and moving. You want the volume your possessions occupy, which is measured item by item rather than as a room. Measure each large item as its box and add. Do not measure the room. A room measurement tells you nothing useful about how much storage unit you need.
Shipping and freight. You want the box that contains the item, including packaging, at its widest point in each dimension. Carriers charge on dimensional weight, calculated from that outer box rather than actual weight, so the protruding handle counts even though it is mostly air.
Concrete, soil, mulch and gravel. You want the volume of the void being filled, and then you almost always convert to cubic yards because that is how it is sold. Depth is the dimension people get wrong, because it is quoted in inches while everything else is in feet. Four inches of depth is 0.333 ft, not 4.
Length and width are easy. Height is where the errors come in, because people estimate it or use a standard figure.
Standard ceiling heights are a convention, not a guarantee. Eight feet is common in newer US construction, nine and ten feet in some builds, and older properties vary widely. Measuring it takes five seconds with a laser measure pointed at the ceiling, and there is no reason to guess.
Two things to watch. Measure floor to ceiling, not floor to the underside of a beam or a bulkhead, unless the bulkhead runs the length of the room, in which case treat it as a separate smaller box. And on a sloped ceiling, measure at the lowest point and the highest point and average them for volume purposes.
For a stairwell void or a double height space where you cannot reach the ceiling, use the indirect mode. Point at the ceiling, then the floor, and the tool solves the height. Indirect measurement is covered here, and it is exactly the situation the mode exists for.
Split them, the same way you would for area. Measure each rectangular section as its own box, multiply each out, add the results. An L-shaped room with a consistent ceiling height is two boxes.
A quicker route for a room with a uniform ceiling: work out the floor area first using whatever method suits the shape, then multiply the whole area by the height in one step. That is fewer operations and fewer chances to lose a number. Awkward floor shapes are covered here.
Volume numbers get large quickly and it is easy to be an order of magnitude out without noticing.
A rough benchmark: an ordinary bedroom runs somewhere around 1,000 to 1,500 cubic feet. A large living room with a high ceiling might reach 3,000. If your figure has come out at 15,000 for a bedroom, you have multiplied something twice or mixed a unit.
For material sold by the cubic yard, divide by 27 and check that too. A patio base four inches deep across 200 square feet is about 67 cubic feet, which is 2.5 cubic yards. If your answer is 25 yards, look again at the depth conversion, because that is where it almost always goes wrong.
Not sure where to begin? The tools we point most people at first are gathered in our main picks.