How to Calculate Cubic Feet

The most common cubic feet question is not “what is the formula” — it is “did I do this right?” You measured a slab, multiplied three numbers, and ended up with an answer that does not match what your contractor quoted. Or you measured a moving truck and got a number that looks suspiciously small. Or you converted from inches and the result is off by a factor of ten — but which way?

This guide is the slow version of the answer. It walks through the math one step at a time, with eight worked examples covering every common shape and unit. By the end you will be able to calculate cubic feet manually, spot your own errors, and know exactly when to stop trusting a hand calculation and reach for a calculator instead. Use the table of contents to skip to your specific situation, or read top to bottom for the full walkthrough.

Cubic feet is a measure of three-dimensional volume. You calculate it by multiplying length × width × height when the shape is rectangular, or by using a matching geometric formula for cylinders, spheres, cones, and prisms. This guide walks through every step with worked examples and the conversion factors you will actually need.

What is a cubic foot?

A cubic foot (ft³) is the volume of a cube whose sides are each one foot long. It equals 1,728 cubic inches, about 0.0283 cubic meters, 28.3 liters, or 7.48 US gallons. In the United States it is the default unit for shipping, moving, refrigerator capacity, firewood, and HVAC airflow. Everywhere else you usually see cubic meters or liters instead.

The basic formula

That is it for a box, a room, or a shipping container. The catch is that all three measurements must be in the same unit before you multiply, and that unit must be feet if you want the answer in cubic feet. Mixing inches with feet is the single most common mistake.

Step-by-step process

1. Measure length, width, and height. Use a tape measure or ruler. Write each value down with its unit so you do not lose track later.
2. Convert everything to feet. See the conversion list below.
3. Multiply L × W × H. The product is your volume in cubic feet.
4. Round sensibly. Two decimal places is plenty for most projects.
5. Convert if needed. Cubic yards for landscaping, cubic meters for international shipping, gallons for tanks and pools.

Converting units to feet

• Inches → feet: divide by 12
• Yards → feet: multiply by 3
• Centimeters → feet: multiply by 0.0328084
• Meters → feet: multiply by 3.28084
• Millimeters → feet: multiply by 0.00328084

Worked examples

Rectangular box (in feet)

A storage box measures 5 ft long, 3 ft wide, and 2 ft tall.

That is the same as 0.85 cubic meters or 224.4 US gallons.

Cylinder (a water tank)

A tank has a 2 ft radius and a 6 ft height.

That holds about 564 US gallons.

Box measured in inches

A microwave is 21 in × 18 in × 13 in.

The shortcut here is that 12 × 12 × 12 = 1,728, so dividing the inch volume by 1,728 gives you cubic feet directly.

From meters and centimeters

A shipping crate is 1.2 m × 0.8 m × 0.6 m.

For mixed metric, convert each side first: 1.2 m × 3.28084 = 3.937 ft, 80 cm × 0.0328084 = 2.625 ft, 60 cm × 0.0328084 = 1.969 ft, then multiply.

Sphere (a basketball)

A regulation NBA basketball has a diameter of 9.55 inches, so the radius is 4.775 inches, or 0.398 feet.

That is the air volume inside the ball. Sphere volume scales with the cube of the radius, so a ball that is 10% larger holds 33% more air. This matters when comparing fitness equipment, water tanks, and decorative spheres.

Cone (a pile of mulch)

You dumped a load of mulch into a roughly conical pile that is 6 feet across at the base and about 3 feet tall at the peak.

That is just over one cubic yard (27 ft³). The cone formula has a (1/3) coefficient that catches everyone the first time — a cone holds exactly one-third of what a cylinder with the same radius and height would hold. Try the cone calculator to skip the manual math.

Triangular prism (a wedge ramp)

A skateboard launch ramp is 8 feet long, 4 feet wide at the back, and 2 feet tall at the back — tapering to zero at the front. The cross-section is a right triangle.

The trickiest part of a triangular prism is identifying which two dimensions are the cross-sectional triangle (base and triangle height) and which is the length the prism extrudes along. Once you have that, the math is quick. The triangular prism calculator labels each dimension to remove the guesswork.

Irregular shape (an L-shaped room)

An L-shaped sunroom is 14 feet long along its longest wall and 10 feet deep at its widest point, but a 4 ft × 3 ft notch is cut out of one corner where a structural pillar sits. The ceiling is 8 feet high.

Break the floor area into two rectangles. The full bounding box is 14 × 10 = 140 ft². Subtract the 4 × 3 = 12 ft² notch to get 128 ft² of actual floor. Then multiply by ceiling height:

For shapes with multiple notches, curves, or pillars, segment them into rectangles and add or subtract each piece. The irregular shape calculator lets you enter each segment separately and handles the addition.

Formulas for every shape

For irregular shapes, divide the object into rectangular sections, calculate each one, then add the cubic feet together. Our irregular shape calculator does this automatically.

Which shape and which formula?

Most cubic feet problems boil down to identifying the right shape, then plugging numbers into the matching formula. Use this decision flow:

• Are all six faces flat rectangles? It is a rectangular cuboid. Multiply length × width × height. Boxes, rooms, fridges, shipping containers, swimming pools (rectangular ones), garden beds, and most furniture fall here.
• Is it round with two flat ends? It is a cylinder. π × radius² × height. Tanks, pipes, drums, silos, columns, log sections, mugs, and most cans.
• Is it round and pointed at one end? It is a cone. (1/3) × π × radius² × height. Mulch piles, sand piles, hoppers, ice cream cones (the cone, not the scoop), and party hats.
• Is it a perfect ball? It is a sphere. (4/3) × π × radius³. Sports balls, spherical tanks, and decorative globes.
• Does it have a triangular cross-section all the way through? It is a triangular prism. ½ × base × triangle height × prism length. Wedge ramps, attic spaces (sometimes), and some hopper bottoms.
• Does it have curves, notches, or pieces missing? Use the irregular shape calculator. Break the shape into rectangular pieces, calculate each, and add or subtract.

When in doubt about a real object, draw it from the side and from the top. Whichever simple shape the side and top profiles match is your answer.

Cubic feet vs other volume units

Cubic yards are most useful for landscaping materials. Gallons are the right unit for tanks and pools. Cubic meters apply to international freight and most non-US engineering work.

When to use cubic feet — and when not to

Cubic feet is the right unit for residential and light-commercial work in the United States. It shows up on appliance labels, freight quotes, bagged-material packaging, and HVAC sizing tables. For these jobs, it is the unit your supplier already speaks.

Outside of those cases, cubic feet is often the wrong default:

• Bulk landscaping orders — soil, mulch, gravel, and compost are sold by the cubic yard. Calculate in cubic feet, then divide by 27. See the conversion page or the mulch and soil calculators.
• International shipping — most carriers and customs authorities outside North America use cubic meters or CBM. See cubic feet to CBM.
• Tank capacity — gallons (US) and liters dominate this domain. Use the gallons or liters converters.
• Lumber sales — board feet is the standard for hardwood and dimensional lumber. See cubic feet to board feet.
• Engineering, scientific, or medical calculations — SI units (cubic meters, liters, milliliters) are standard. Cubic feet looks unprofessional in those contexts even when the math is correct.

Knowing when to convert away from cubic feet is just as valuable as knowing when to use it. The converter index lists every common conversion.

Practical uses

Cubic feet shows up in more household and trade situations than most people realize. Here is where it matters most:

Moving and storage

Moving truck rentals publish cubic feet capacity for every truck size: a 10-foot truck holds roughly 380 ft³, a 15-footer 760 ft³, a 20-footer about 1,015 ft³, and a 26-footer about 1,700 ft³. Self-storage units use the same unit — a 5×5 unit is about 200 ft³, a 10×10 about 800 ft³, and a 10×20 about 1,600 ft³.

Estimating your belongings in cubic feet before booking saves money. A medium box is about 3 ft³, a king-size mattress about 50 ft³, a sofa about 35 ft³. Use the moving calculator to add up an entire household, or the storage unit calculator to right-size a unit.

Landscaping and gardening

Mulch, soil, gravel, sand, and compost are all sold by the cubic yard in bulk and by the cubic foot in bags. To order accurately you measure your bed in cubic feet, then divide by 27 for cubic yards or by the bag size for bag count. The most common mistake is forgetting to convert depth (usually inches) to feet — see the common mistakes guide.

Tools: mulch, soil, gravel, sand, raised garden beds.

Concrete and construction

Bagged concrete lists yield in cubic feet on the package: an 80-lb bag yields about 0.6 ft³, a 60-lb bag 0.45 ft³, and a 40-lb bag 0.30 ft³. Calculate your slab or footing volume in cubic feet, then divide by the bag yield to get the bag count. For larger pours, divide cubic feet by 27 to get cubic yards (the unit ready-mix trucks use). The concrete calculator handles all three at once.

HVAC and ventilation

Heating, cooling, and ventilation are sized by room volume. The standard formula: CFM = (room ft³ × air changes per hour) ÷ 60. A bedroom needs 4–6 ACH, a kitchen 6–8 ACH, a bathroom 8–10 ACH. The HVAC airflow calculator computes required CFM for any room volume.

Appliances

Refrigerators, freezers, ovens, microwaves, and clothes dryers all quote interior capacity in cubic feet. A standard refrigerator is 18–22 ft³, a chest freezer 5–25 ft³, an oven cavity 4–6 ft³, a microwave 0.7–2.2 ft³. Knowing how to compare your existing appliance to a new one (fridge, freezer, oven, microwave) prevents surprises at delivery.

Shipping and freight

Domestic US carriers (UPS, FedEx, USPS) calculate dimensional weight from cubic inches, but international and freight carriers quote cubic feet or CBM (cubic meters). Shipping pallets are sold by cubic feet of capacity, and warehouse leases are often priced per cubic foot per month. The shipping calculator covers dimensional-weight math.

Specialty applications

Sauna heater wattage scales with cubic feet (1 kW per 50 ft³ is the rule of thumb — see the sauna heater calculator). Subwoofer enclosure design hinges on internal cubic feet (the subwoofer box calculator accounts for driver displacement and bracing). Greenhouse heating is sized by cubic feet (greenhouse calculator). Pool gallons come from cubic feet × 7.48 (pool calculator).

Common mistakes to avoid

• Mixing inches with feet inside the same multiplication.
• Using diameter when the formula calls for radius (radius is half the diameter).
• Forgetting to cube the conversion factor when going meters → feet (it is meters first, then convert once).
• Confusing square feet with cubic feet.
• Rounding too early. Keep at least three decimal places during the calculation, round only at the end.

Read the full list in common cubic feet mistakes.

Pro tips for accuracy

• Measure twice, calculate once. Tape measures stretch and walls are rarely perfectly square.
• For odd shapes, take the average of two measurements per dimension.
• Add 5–10% extra to material orders (mulch, soil, concrete) to cover overage and waste.
• Subtract the volume of doors, windows, and built-ins from room totals when sizing HVAC.
• For shipping, measure to the largest box that contains the item, not the item itself.
• When converting between units mid-calculation, write the conversion factor next to the number you used it on. Going back to debug a wrong answer is much faster when each step is labeled.
• For materials sold by weight (gravel, sand) but measured by volume, calculate cubic feet first, then multiply by density. Doing it in either order works mathematically, but the volume-first order is easier to sanity-check.
• Use a phone calculator with parentheses for any formula that has division or subtraction in it — the cone formula is a common place where order-of-operations errors creep in.

Glossary

Cubic foot (ft³)

The volume of a cube with sides of one foot each. 1 ft³ = 1,728 in³ = 0.0283168 m³ = 28.3168 L = 7.48052 US gallons. Sometimes written as “cu ft”, “CF”, or “cuft” on packaging.

Cubic yard (yd³)

27 cubic feet. The standard unit for ordering bulk landscaping and concrete in the United States. See conversion details.

Cubic meter (m³)

Approximately 35.3147 cubic feet. The SI unit of volume and the international standard for freight and engineering. Also called a “CBM” in shipping contexts.

Volume

The amount of three-dimensional space an object or container occupies. Volume is measured in cubic units (ft³, m³, in³). Different from area (square units) and length (linear units).

Density

Mass per unit volume. Usually given in pounds per cubic foot (lb/ft³) for US materials or kilograms per cubic meter (kg/m³) elsewhere. Multiply cubic feet by density to get weight.

Cuboid

A rectangular box — six rectangular faces, all corners square. Most rooms, shipping containers, and household items are cuboids. Volume = length × width × height.

Radius

Distance from the center of a circle or sphere to its edge. Half of the diameter. Critical for cylinder, cone, and sphere formulas — using the diameter instead of the radius is the most common error in volume calculations. See common mistakes.

π (pi)

The mathematical constant approximately equal to 3.14159. Appears in any volume formula involving curves (cylinders, cones, spheres). Modern calculators use the full value internally; using 3.14 introduces roughly a 0.05% error.

Air changes per hour (ACH)

How many times the entire volume of air in a room is replaced per hour. Used in HVAC sizing. CFM = (room cubic feet × ACH) ÷ 60. See the HVAC airflow calculator.

Net vs gross volume

Gross is the outer or bounding volume of an item. Net is the usable interior volume. Important when sizing a moving truck (use gross to fit boxes, net to fit furniture) and when comparing appliance specs.

Bulking factor / swell

The extra volume loose material occupies compared to its in-place state. Excavated dirt swells about 25%. Mulch can settle 20–30%. See the dirt and mulch calculators for typical values.

What to do next

You now have the complete toolkit: the formulas, the unit conversions, the decision tree, the worked examples, and the glossary. Three reasonable next steps depending on your situation:

• If you have a project ready to size, jump straight to the main calculator — it handles all six shapes, all four input units, and converts to cubic yards, gallons, liters, and CBM in one screen.
• If your project is a specific material, use a topic-specific calculator that already knows the typical depths, bag sizes, and density factors: concrete, soil, mulch, gravel, moving, or shipping.
• If you want a unit-conversion shortcut, the unit conversion chart lists every common factor in a printable single-page reference.

And if you spot a calculation that does not look right, this guide will still be here. The math does not change.

Frequently asked questions