Article — Cubic Feet Calculator
Cubic feet, explained: how to measure, convert, and use them
One cubic foot is the volume of a cube measuring one foot on every side. It equals 1728 cubic inches, 0.0283168 cubic meters, 28.3168 liters, and 7.48 US gallons. To find cubic feet for any rectangular space, multiply length by width by height with all three numbers in feet. Mixed units convert first.
The calculator at the top of this page handles the conversion automatically — type any dimension in ft, in, m, or cm, and the volume appears in five output units. The article below covers the formula, the conversions you'll actually use, and the mistakes that show up most often in shipping, concrete orders, and HVAC sizing.
What is a cubic foot?
A cubic foot is the volume of a cube one foot wide, one foot deep, and one foot tall. The unit symbol is ft³ or cu ft. It is the standard volume unit in the United States and parts of the UK for everything from refrigerators and freezers to lumber, shipping containers, and air conditioning capacity.
The cubic foot is not part of the SI metric system but it survives because the foot itself survives in US construction and consumer goods. A typical refrigerator is rated in cubic feet (18 to 25 ft³). A pickup truck bed holds 50 to 80 ft³ of cargo. A 20-foot shipping container has 1,172 ft³ of usable interior — the standard global unit of containerised trade.
The cubic foot has an exact metric definition. The international foot was fixed at 0.3048 meters in 1959 by the International Yard and Pound Agreement, so one cubic foot equals (0.3048)³ = 0.028316846592 m³ — exact to as many decimal places as you want.
The cubic feet formula
The cubic feet formula for a rectangular box is length × width × height, with every dimension in feet. A room that is 12 feet long, 10 feet wide, and 8 feet high holds 960 cubic feet of air. A small storage closet of 4 × 3 × 8 ft holds 96 cubic feet.
L × W × H (all ft) = ft³L × W × H (in) ÷ 1728 = ft³L × W × H (m) × 35.315 = ft³ft³ ÷ 27 = yd³For non-rectangular shapes the math changes. A cylinder uses πr²h in feet for cubic feet — a 55-gallon drum (22.5 in diameter × 34.5 in tall) is π × (0.9375 ft)² × 2.875 ft = 7.94 ft³. A sphere uses (4/3)πr³. For irregular objects, the displacement method works: submerge in a tank of known volume and measure displaced water. One US gallon of displaced water equals 0.1337 ft³.
Calculating cubic feet from inches
If your dimensions are in inches, multiply them together and divide by 1728. The number 1728 is 12³ — the count of one-inch cubes that fit in a one-foot cube. A shipping box measuring 24 × 18 × 18 inches contains 7776 cubic inches, which equals 7776 ÷ 1728 = 4.5 cubic feet.
This is the single most common cubic-feet calculation. Boxes, drawers, microwaves, and small storage items are usually measured in inches because they fit on a tape measure cleanly. Converting to cubic feet matters for shipping quotes, storage-unit sizing, and product specs.
If you have a 24 × 18 box, you have 432 square inches of footprint — not 432 cubic inches of volume. You also need the height. Sketching the third dimension before you start saves the most common cubic-feet mistake: skipping a multiplication and confusing area with volume.
Cubic feet to cubic yards and concrete
Twenty-seven cubic feet equals one cubic yard. The conversion exists because a yard is three feet, and 3³ = 27. Concrete, mulch, topsoil, and gravel are sold by the cubic yard at most US suppliers, even though home projects are usually measured in feet.
For a concrete slab, multiply length × width × thickness, all in feet. A 20 × 10 ft slab at 4 inches thick is 20 × 10 × 0.333 = 66.7 cubic feet, or 2.47 cubic yards. Concrete trucks deliver in increments of a quarter or full cubic yard, so you would order three cubic yards and accept the small over-pour as cheaper than running out mid-pour.
Always add 10 percent to a concrete order. The reasons: uneven sub-grade lets the slab eat extra, spillage during pumping costs a few cubic feet, and arriving short means a second truck callout that can exceed the cost of the over-pour by tenfold. Ten percent on a 2.5 yd³ order is 0.25 yd³ — about $30 of concrete to insulate against a $300 callout.
Cubic feet to gallons and liters
One cubic foot equals 7.48052 US gallons, or 6.23 imperial UK gallons, or 28.3168 liters. The US-versus-imperial gallon gap is 20 percent, which is the most common cause of fish-tank sizing errors when an American hobbyist follows a British care guide.
For water-volume work, 1 ft³ of fresh water weighs 62.43 pounds — useful for sizing pumps, drainage, or rooftop loads. A 1000-gallon storage tank holds 133.7 ft³ of water and 8330 pounds of water (4.2 short tons). Saltwater is about 2.5 percent denser, which matters for aquarium structural load calculations.
Cubic feet for shipping and storage
Cubic feet is the working unit of US logistics. Shipping containers, moving trucks, freight forwarders, and self-storage units all quote in ft³. The 20-foot ocean container is the world's standard unit of containerised trade — a "TEU" or twenty-foot equivalent unit — with 1,172 ft³ of usable interior. A 40-foot container holds 2,390 ft³, and a 40-foot high-cube container holds 2,694 ft³.
For self-storage, the most common units are 5×5 (200 ft³), 5×10 (400 ft³), 10×10 (800 ft³), and 10×20 (1,600 ft³), assuming 8-foot ceilings. A 10×10 unit holds a 1–2 bedroom apartment. A 10×20 unit holds a full 3-bedroom house.
- 20-ft container = 1,172 ft³ (33.2 m³) — the standard TEU
- 40-ft container = 2,390 ft³ (67.7 m³) — the workhorse of trans-Pacific trade
- 26-ft moving truck = 1,700 ft³ — full 4-bedroom house
- 16-ft moving truck = 800 ft³ — 2-bedroom apartment
- 10x10 storage = 800 ft³ — 1-2 bedroom apartment contents
- Pickup bed = 50 to 80 ft³ depending on size and bed length
- Cargo van = 250 to 400 ft³ — common Amazon and trade vehicle
UPS, FedEx, and DHL bill on the higher of actual weight or dimensional weight. The formula for dim weight in inches is L × W × H ÷ 139 = lbs. A 24 × 24 × 24 inch box has 8 cubic feet of volume and 99 lbs of dimensional weight — even if it physically weighs 5 lbs. A right-sized box can cut shipping costs by 50 to 70 percent on lightweight items.
Cubic feet in HVAC and airflow (CFM)
CFM stands for cubic feet per minute — the standard US measure of airflow. An HVAC supply rated at 400 CFM delivers 400 cubic feet of air into the room every minute. Range hoods, bathroom fans, and air purifiers all quote CFM.
To find air changes per hour, multiply CFM by 60 and divide by room volume in cubic feet. A 400 CFM bathroom fan in a 320 ft³ bathroom (8 × 5 × 8 ft) gives (400 × 60) ÷ 320 = 75 air changes per hour — well above the 8-per-hour code minimum. For a kitchen range hood over a gas cooktop, the rule of thumb is 100 CFM per linear foot of cooktop.
Common cubic-feet mistakes
The mistakes that show up over and over are predictable and worth knowing in advance.
- Mixing units — 24 × 18 × 18 inches is not 24 × 18 × 18 ft³. Either keep everything in feet or divide the inch result by 1728.
- 3 vs 27 for yards — converting feet to cubic yards needs ÷ 27, not ÷ 3. The cube of the linear factor is the volume factor.
- US vs imperial gallon — a 50-gallon British aquarium guide gives a 60-US-gallon American tank. The 20 percent gap matters for fish stocking.
- Skipping the third dimension — 12 × 10 is square feet, not cubic feet. Volume always needs three measurements.
- Cylinders treated as boxes — a tank is L × W × H only if it is rectangular. Round tanks use πr²h, which is about 78.5 percent of the bounding-box volume.
- No allowance for waste — concrete, mulch, and gravel orders should be 10 percent over the calculated volume.