Tank Volume Calculator

About Tank Volume Calculator

How Tank Volume Formulas Work

Each tank shape has its own volume formula. The calculator converts every input to a consistent base unit (cubic feet or cubic metres), then applies standard conversion factors to produce gallon and litre values.

Shape	Formula	Variables
Vertical cylinder	V = pi x r² x h	r = radius, h = height
Horizontal cylinder	V = pi x r² x L	r = radius, L = length
Rectangular	V = L x W x H	L = length, W = width, H = height
Sphere	V = (4/3) x pi x r³	r = radius

Worked example - vertical cylinder: A residential water storage tank is 4 feet in diameter and 6 feet tall. The radius is 2 feet. Volume = pi x 2² x 6 = pi x 4 x 6 = 75.40 cubic feet. Converting to US gallons: 75.40 x 7.48052 = 564.0 gallons. Converting to litres: 75.40 x 28.3168 = 2,135.1 litres. That lines up with a standard 550-gallon poly tank sold for rainwater collection.

Worked example - rectangular: An IBC tote measures 40 x 48 x 46 inches. Converting to feet: 3.333 x 4.0 x 3.833 = 51.11 cubic feet. In US gallons: 51.11 x 7.48052 = 382.3 gallons. The rated capacity of a standard IBC tote is 275 gallons because the internal dimensions are smaller than the outer frame, but the maths confirms the formula works correctly against the outer shell volume.

Volume Unit Conversion Reference

The key conversion factor is 1 cubic foot = 7.48052 US gallons, derived from the NIST definition of 1 US gallon = 231 cubic inches and 1 cubic foot = 1,728 cubic inches (1,728 / 231 = 7.48052). One US gallon of water weighs 8.34 pounds at 62 degrees F. Imperial gallons are about 20% larger than US gallons, based on the metric definition of 1 imperial gallon = 4.54609 litres.

From	To	Multiply By
1 cubic foot	US gallons	7.48052
1 cubic foot	Imperial gallons	6.22884
1 cubic foot	Litres	28.3168
1 US gallon	Litres	3.78541
1 imperial gallon	Litres	4.54609
1 imperial gallon	US gallons	1.20095
1 barrel (oil)	US gallons	42
1 cubic metre	Litres	1,000
1 US gallon of water	Pounds	8.34

Common Tank Sizes and Dimensions

Knowing standard tank dimensions makes it easier to verify your measurements or pick the right replacement. Propane tanks are only filled to 80% capacity to allow for gas expansion, so a 500-gallon propane tank actually holds about 400 gallons of usable propane.

Application	Shape	Typical Dimensions	Capacity
Home water heater	Vertical cylinder	20 in dia x 50 in tall	40-50 US gallons
Rain barrel	Vertical cylinder	23 in dia x 34 in tall	55 US gallons
120-gal propane tank	Horizontal cylinder	18 in dia x 4.5 ft long	120 gal (96 usable)
250-gal propane tank	Horizontal cylinder	30 in dia x 7.5 ft long	250 gal (200 usable)
Above-ground oil tank	Horizontal cylinder	2.5 ft dia x 5 ft long	275 US gallons
IBC tote	Rectangular	40 x 48 x 46 in (outer)	275 US gallons
500-gal propane tank	Horizontal cylinder	37.5 in dia x 10 ft long	500 gal (400 usable)
Residential water tank	Vertical cylinder	4 ft dia x 6 ft tall	~560 US gallons
Fish tank (large)	Rectangular	6 ft x 2 ft x 2 ft	~180 US gallons

How Does Partial Fill Work?

The fill percentage slider multiplies total volume by the fill fraction. For vertical cylinders and rectangular tanks, this is straightforward: 50% fill means exactly half the volume. But for horizontal cylinders, the relationship between liquid height and volume is not linear. A horizontal cylinder tank that appears half-full by sight line may hold more or less than 50% depending on where the liquid level sits relative to the radius.

The exact partial fill volume for a horizontal cylinder uses the circular segment formula: V = L x r² x (theta - sin(theta)) / 2, where theta = 2 x arccos((r - f) / r), f is the fill height from the bottom, L is the tank length, and r is the radius. This calculator uses a linear percentage approximation, which is accurate enough for planning purposes but slightly over- or under-estimates at mid-fill levels for horizontal tanks.

Fill Level	Volume (500 gal tank)	Typical Action
100%	500 gallons	Just delivered or filled
75%	375 gallons	Normal operating level
50%	250 gallons	Start planning a refill
25%	125 gallons	Schedule delivery soon
10%	50 gallons	Urgent - order immediately

Propane delivery companies typically recommend scheduling a refill when the gauge reads 20-25%. Running a propane tank below 10% risks losing pilot lights and may require a leak test before the provider will refill it.

Septic Tank Sizing by Number of Bedrooms

Septic tanks are sized based on the number of bedrooms in a home, not the number of bathrooms. The Uniform Plumbing Code sets a minimum tank size of 750 gallons, though most US jurisdictions require at least 1,000 gallons for new installations. The general rule is to add 250 gallons for each bedroom beyond three.

Bedrooms	Minimum Tank Size (gallons)	Estimated Daily Flow (GPD)
1-2	1,000	300-400
3	1,000	450
4	1,250	600
5	1,500	750
6	1,750	900

State and county regulations vary. Always check with your local health department before sizing a septic system. Pennsylvania, for example, requires a minimum daily flow of 400 GPD for a single-family dwelling, with an additional 100 GPD per bedroom beyond three.

Water Storage Planning

The EPA estimates the average American uses 82 gallons of water per day at home, with a typical family of four using over 300 gallons daily. About 70% of that goes to indoor use (showers, toilets, laundry, dishwashing) and 30% to outdoor use (irrigation, car washing). For off-grid or emergency storage, the numbers break down like this:

Use	Daily Per Person	Family of 4 (weekly)
Drinking and cooking	1-2 gallons	28-56 gallons
Basic hygiene	5-10 gallons	140-280 gallons
Full household use	50-100 gallons	1,400-2,800 gallons
Emergency backup (3 days)	1 gallon minimum	12 gallons minimum

For emergency preparedness, FEMA recommends storing at least 1 gallon per person per day for a minimum of 3 days. A family of four should keep at least 12 gallons on hand, though 14 days of supply (56 gallons) is a more practical target for areas prone to extended power outages or natural disasters. Many families store water in 55-gallon drums or stacked 5-gallon containers, both of which can be verified using this calculator.

Rainwater Collection and Tank Sizing

Rainwater harvesting tanks are sized based on roof area and local rainfall. The standard formula is: gallons collected = roof area (sq ft) x rainfall (inches) x 0.623. A 1,500 square foot roof in an area that gets 1 inch of rain collects roughly 935 gallons per storm. Most residential rainwater systems use tanks between 500 and 5,000 gallons, with 2,500-3,000 gallons being a common choice for homes that supplement but do not fully replace their municipal water supply.

Worked example: A home in the Dallas-Fort Worth area has a 2,000 square foot roof. Average annual rainfall is about 37 inches. Potential annual collection = 2,000 x 37 x 0.623 = 46,102 gallons. That does not all arrive at once, though. The wettest month (May, roughly 5 inches) would yield about 6,230 gallons, so a 5,000-gallon tank would capture most of a single heavy month. The square footage calculator can help measure roof area for this calculation.

Choosing a Tank Shape

Shape	Advantages	Common Uses
Vertical cylinder	Small footprint, easy to read level gauges, gravity-fed pressure	Water storage, water heaters, rain collection
Horizontal cylinder	Low profile, stable on trailers, fits under structures	Fuel storage, propane, transport tanks
Rectangular	Space-efficient, stackable, fits tight spaces	Aquariums, IBC totes, custom-built tanks
Sphere	Maximum volume per surface area, even pressure distribution	LPG storage, industrial pressure vessels

Vertical cylinders are the most common residential choice because they take up less ground space and produce better gravity-fed water pressure. Horizontal cylinders are standard for propane and fuel oil because they can sit low to the ground or be buried. Spherical tanks are rare in residential settings and mostly appear in industrial LPG and chemical storage where pressure containment matters.

Tank Weight When Full

Water weighs 8.34 pounds per US gallon, so a full tank can be surprisingly heavy. A 500-gallon water tank weighs about 4,170 pounds (nearly 2 tonnes) when full, plus the weight of the tank itself. Fuel oil is slightly lighter at about 7.2 pounds per gallon, and propane is lighter still at roughly 4.2 pounds per gallon. Always confirm that the floor, stand, or pad under the tank can handle the total loaded weight before filling.

Liquid	Weight per US Gallon	500-Gallon Tank Weight (liquid only)
Water	8.34 lbs	4,170 lbs (1,892 kg)
Diesel fuel	7.1 lbs	3,550 lbs (1,610 kg)
Heating oil	7.2 lbs	3,600 lbs (1,633 kg)
Propane (liquid)	4.2 lbs	2,100 lbs (953 kg)
Gasoline	6.1 lbs	3,050 lbs (1,383 kg)

For bulk material estimates (gravel, mulch, soil), the cubic yards calculator converts area measurements to cubic yards. For general geometric volume calculations covering cones, pyramids, and other shapes, the volume calculator handles those. All calculations run entirely in your browser with no data sent to any server.

Sources

• EPA WaterSense - Statistics and Facts
• EPA - How We Use Water
• UpCodes - Septic Tank Capacity and Sizing (Uniform Plumbing Code)
• AmeriGas - Propane Tank Sizes Guide
• WaterCache - Rainwater Harvesting Calculator and Formulas
• Inch Calculator - Cubic Feet to Gallons (NIST definitions)