The Water Meter Reads Cubic Feet, but Your Bill Comes in Gallons
Most US municipal water meters measure consumption in cubic feet. The meter's register —the odometer-like dial on top —counts in cubic feet. The utility's billing system multiplies by 7.48 to convert to gallons. Your water bill says "Usage: 6,000 gallons." The meter read 802 cubic feet. The multiplication happened in the billing software. Some utilities bill directly in cubic feet —typically in older Northeastern and Midwestern systems where the water infrastructure predates widespread gallon-based billing. A "unit" on a Philadelphia water bill is 100 cubic feet (748 gallons). The 7.48 multiplier is invisible to the customer but present in every water bill in the country, because the meter measures volume in cubic feet and the customer thinks about water in gallons. The conversion is performed roughly 100 million times per day across the US water utility industry. No one notices. The 7.48 is embedded in the firmware of every water meter register manufactured by Badger, Neptune, Sensus, and Master Meter —the four companies that make essentially all US water meters. The firmware multiplies the reed-switch pulses from the measuring chamber by a calibration factor that includes the 7.48 conversion when the register is set to display gallons. For more on the utility side and the 231 in³ definition that makes 7.48 necessary, see gallons to cubic feet.
Common Tank Sizes: Cubic Feet to Gallons
| Tank Dimensions | Cubic Feet | Gallons | Weight When Full (lb) | Application |
|---|---|---|---|---|
| 1×1×1 ft box | 1 | 7.48 | 62 | The definition. A cubic foot of water fits in a milk crate and weighs as much as a bag of concrete. |
| 55-gal drum (standard) | 7.35 | 55 | 460 | The universal industrial liquid container. Roughly 2 ft diameter × 3 ft tall. |
| IBC tote (275 gal) | 36.8 | 275 | 2,300 | 48×40×46 in. The white plastic cube in a metal cage. Rainwater storage, farm use, emergency water supply. |
| Residential septic tank | 134 | 1,000 | 8,345 | Roughly 5×8×3.3 ft. Septic tanks are sized in gallons but dimensioned in feet. The conversion is built into the formwork. |
| Swimming pool (small in-ground) | 2,000 | 15,000 | 125,000 | 15×30 ft, avg 4.5 ft deep. Roughly 62 tons of water —enough to fill 3 concrete trucks. |
| Municipal water tower (1M gal) | 133,681 | 1,000,000 | 8,345,000 | Roughly 50 ft diameter, 70 ft tall. 4,172 tons. The 7.48 factor at million-gallon scale. |
gal = ft³ × 7.48051948 | gal = (L × W × H in feet) × 7.48
Rainwater Harvesting: How the 0.623 Factor Turns Roof Inches Into Gallons
One inch of rain falling on one square foot of roof produces exactly 1/12 of a cubic foot of water —0.08333 cubic feet. Multiply by 7.48: 0.623 gallons per square foot per inch of rain. A 2,000-square-foot house in Chicago (36 inches of annual rainfall) catches 2,000 × 36 × 0.623 = 44,856 gallons per year. In a dry year with 25 inches of rain: 31,150 gallons. The difference —13,700 gallons —is the reason rainwater harvesting systems are sized for the dry year, not the average year. The 0.623 factor is derived entirely from the 7.48 gallons per cubic foot and the 12 inches per foot: 7.48 ÷ 12 = 0.623. No measurement, no standard, no treaty —pure arithmetic from the gallon definition and the inch-foot relationship. For the building-code side of cistern sizing and the structural implications of 8.345 lb per gallon sitting on a roof or deck, see gallons to cubic feet. For metric rainwater calculations (liters per square meter per millimeter of rain), see gallons to liters.
Frequently Asked Questions
How many gallons does a standard bathtub hold?
A standard US bathtub holds about 40—0 gallons when filled to the overflow drain. The tub's interior volume is roughly 6— cubic feet. 6 × 7.48 = 45 gallons; 8 × 7.48 = 60 gallons. The typical bath uses 30—0 gallons —most people don't fill the tub to the overflow. A soaking tub (deeper, wider) holds 80—00 gallons (10—3 cubic feet). The average American shower (8 minutes at 2.1 gpm) uses about 17 gallons —roughly one-third of a bathtub fill. The cubic-foot measurement is the tub's physical capacity. The gallon measurement is what the water heater must deliver. The two numbers are related by 7.48 but the water heater doesn't care about cubic feet —it cares about gallons per minute at the required temperature rise. The tank holds cubic feet. The heater sustains gallons per minute.
How do I calculate the gallon capacity of a cylindrical tank?
For a cylindrical tank: volume in cubic feet = π × (radius in feet)² × height in feet. Then multiply by 7.48 for gallons. Example: a cylindrical tank 4 feet in diameter and 6 feet tall. Radius = 2 ft. Volume = π × 2² × 6 = 75.4 cubic feet. Gallons = 75.4 × 7.48 = 564 gallons. The radius must be in feet, not inches. A 24-inch-diameter tank has a radius of 1 foot, not 12. This is the same inches-to-feet error that haunts concrete calculations (cubic yards to cubic feet), applied to round tanks instead of rectangular slabs. For the tank-weight check, multiply gallons by 8.345 for the water weight in pounds. For metric tank sizing in liters: gallons to liters.
Engineering Context
1 ft³ = 1,728 in³ ÷ 231 in³/gal = 7.48051948 gal. The conversion is exact given the gallon's definition but produces a repeating decimal (denominator = 77, numerator = 576 after simplification; denominator does not divide evenly into 10^n for any n). In water-resources engineering, the cubic-foot-to-gallon conversion is used for: water meter registers (ft³ →gal for billing), storage tank sizing (tank dimensions in feet →capacity in gallons), fire flow calculations (NFPA required fire flow in gpm from hydrants, pipe diameters in inches, flow velocity in ft/s —all requiring conversions through the 7.48 factor), and wastewater treatment (MGD —million gallons per day —is the unit, but treatment basin volumes are in cubic feet, and detention time is in hours; the conversion chain is ft³ →gal →MGD →detention time in hours). The 7.48 is the most-multiplied constant in US water engineering. For adjacent converters: gal to ft³, gal to L, ft³ to m³, yd³ to ft³, and the Volume Guide.