A cubic foot of water weighs about 62.4 pounds at room temperature. That number is stable enough to use as a reliable rule of thumb for most everyday purposes, from sizing an aquarium stand to estimating the load on a deck after a rainstorm. But it is not fixed. The actual weight shifts with temperature, dissolved minerals, and even altitude, and understanding where those shifts matter can save you from engineering mistakes or wasted money.
Where the 62.4-Pound Figure Comes From
Water’s density peaks at roughly 39.2 °F (3.98 °C). At that temperature, one cubic foot of pure water weighs about 62.43 pounds. As the water warms or cools from that point, it expands slightly and becomes less dense, so the weight per cubic foot drops a little. At a typical indoor temperature of around 70 °F (21 °C), a cubic foot of water weighs closer to 62.3 pounds. At near-boiling temperatures it dips further, to roughly 60 pounds per cubic foot.
The reason 62.4 gets used as the standard approximation is that it falls right in the middle of the range most people encounter in daily life. Tap water in a house, water in a swimming pool, water filling a basement after a flood: all of these sit between about 50 °F and 80 °F, where the weight per cubic foot stays between roughly 62.2 and 62.4 pounds. For back-of-the-envelope calculations, 62.4 is close enough that the rounding error is smaller than the uncertainty you already have about the volume you’re estimating.
How Temperature Changes the Weight
Temperature is the biggest variable most people will run into. Water behaves unusually compared to most liquids: instead of being densest when it is coldest, it reaches peak density a few degrees above freezing and then actually gets lighter as it cools further toward 32 °F. This quirk is why ice floats and why lakes freeze from the top down rather than the bottom up.
For practical weight calculations, the temperature effect plays out like this:
- Near freezing (32–39 °F): about 62.4 lb/ft³, essentially at peak density.
- Cool tap water (50–60 °F): about 62.4 lb/ft³, negligible change.
- Room temperature (68–77 °F): about 62.2–62.3 lb/ft³.
- Hot water heater output (120 °F): about 61.7 lb/ft³.
- Near boiling (200 °F): about 60.1 lb/ft³.
If you are filling a hot tub and want to know how much weight your deck needs to support, using 62.4 will slightly overestimate the load because the water is warm. That’s actually the safer direction to be wrong, so the standard figure works fine. But if you’re running hot-water lines in an industrial boiler system, the difference between 62 and 60 pounds per cubic foot across hundreds of cubic feet starts to matter for pipe stress calculations and pump sizing.
Saltwater, Hard Water, and Other Real-World Liquids
Pure water is a laboratory concept. The water in your house, your pool, or the ocean has stuff dissolved in it, and dissolved substances make water heavier per cubic foot.
Seawater is the most dramatic everyday example. Average ocean water contains about 3.5 percent dissolved salts by weight, which pushes its density up to about 64 pounds per cubic foot. That is roughly two pounds more per cubic foot than fresh water, and in a large volume the difference adds up fast. A 10,000-gallon saltwater aquarium weighs around 2,700 pounds more than a freshwater tank of the same size, purely because of the salt.
Municipal tap water falls much closer to the pure-water figure. Most city water supplies have total dissolved solids somewhere between 100 and 500 parts per million, which changes the weight per cubic foot by less than a tenth of a pound. You would never notice the difference without laboratory instruments. Hard water with elevated calcium and magnesium content sits at the upper end of that range but still rounds to 62.4 for any practical purpose.
Even the gases dissolved in water have a measurable effect, though it is vanishingly small. At 32 °F, absorbing the oxygen, nitrogen, and argon naturally present in the atmosphere lowers water’s density by about 3 parts per million, while dissolved carbon dioxide raises it by roughly 0.3 parts per million.1Journal of Physical and Chemical Reference Data. Effects of isotopic composition, temperature, pressure, and dissolved gases on the density of liquid water Those shifts are far too small to matter outside of precision laboratory work, but they illustrate how sensitive water-density measurements become when you push toward high accuracy.
Why Precision Gets Surprisingly Complicated
If 62.4 is good enough for plumbing and pool decks, who needs more precision? The answer is metrologists, the scientists who maintain measurement standards. The kilogram was historically defined in relation to water (one liter of water at maximum density was supposed to weigh exactly one kilogram), and even though that definition has since been replaced, precise water-density values still underpin calibration work in everything from pharmaceutical manufacturing to fuel trading.
Two international bodies maintain standard formulations for the density of liquid water. The formulation recommended for metrology by the International Committee for Weights and Measures in 2001 and the thermodynamic property formulation adopted by the International Association for the Properties of Water and Steam in 1995 give consistent results within the temperature and pressure range most calibration labs use.2Metrologia. Density of water: roles of the CIPM and IAPWS standards For those working at the level of parts-per-million accuracy, even the isotopic composition of the water matters: water molecules containing heavier oxygen-18 or deuterium are slightly denser than those with the more common lighter isotopes. Distilled water from different geographic sources can differ measurably in isotopic makeup. None of this touches the answer for someone wondering how much a fish tank weighs, but it is a fascinating rabbit hole if you want to understand how the simple-sounding question “how much does water weigh?” becomes genuinely hard at high precision.
Pressure and Altitude
Water is often described as incompressible, and for everyday purposes that description is close enough. But it is not literally true. Squeeze water hard enough and it does get denser. At the bottom of the Mariana Trench, where pressure exceeds 15,000 pounds per square inch, seawater is roughly 5 percent denser than it is at the surface. A cubic foot of water down there weighs a few pounds more than at sea level.
At everyday pressures the effect is negligible. Moving from sea level to Denver (about 5,280 feet elevation) changes atmospheric pressure by roughly 17 percent, but because liquid water resists compression so effectively, the change in density is too small to measure with anything short of a research-grade instrument. If you are hauling water in a truck from the coast to a mountain town, the water weighs the same when it arrives.
Where pressure does matter is in deep wells and hydraulic systems. Engineers designing offshore drilling equipment or deep-water pipelines account for the slight increase in water density at depth because the cumulative pressure on long vertical columns of fluid affects pump performance and structural loads.
Ice Versus Liquid Water
When water freezes, it expands by about 9 percent. That means a cubic foot of ice weighs less than a cubic foot of liquid water, coming in at roughly 57.2 pounds instead of 62.4. This expansion is what makes ice float and what bursts pipes in winter: the water needs more room as a solid than it did as a liquid, and if it is trapped inside rigid plumbing, something has to give.
For weight-bearing calculations, the ice-versus-water distinction matters in a few scenarios. A flat roof collecting snow and ice can hold less total water-equivalent weight per cubic foot than you might assume if you are thinking in terms of liquid. On the other hand, the volume of ice is larger for the same mass of water, so it takes up more space. A chest freezer packed with frozen water bottles contains less total weight than a chest of the same interior volume filled with liquid water, even though it looks completely full.
Quick Conversions for Common Volumes
The 62.4-pounds-per-cubic-foot figure is handy on its own, but people rarely think in cubic feet. Here’s how it translates to volumes you are more likely to encounter:
- One gallon: about 8.34 pounds. (A cubic foot holds 7.48 gallons, so 62.4 ÷ 7.48 ≈ 8.34.)
- Five-gallon bucket: about 41.7 pounds of water, plus the weight of the bucket.
- 55-gallon drum: about 459 pounds when full.
- Standard bathtub (roughly 80 gallons): about 667 pounds of water alone.
- One cubic yard (27 cubic feet): about 1,685 pounds.
These round numbers are all for fresh water near room temperature. For saltwater, add roughly 3 percent. For very hot water, subtract a percent or two.
Practical Situations Where the Weight Matters
Knowing that water weighs 62.4 pounds per cubic foot is the kind of fact that sits quietly in the background until you actually need it. Here are some common situations where it suddenly becomes the most important number in the room.
Aquariums are the classic example. A modest 55-gallon freshwater tank holds about 459 pounds of water, plus the glass, gravel, and stand. A 150-gallon reef tank pushes past 1,250 pounds of water alone, and the saltwater adds another 40-odd pounds on top of that. Most residential floors are designed to handle about 40 pounds per square foot of live load. A large aquarium concentrating all that weight on a few square feet of floor can exceed the rating, especially on upper stories. People who skip this calculation sometimes end up with a tank in the basement, not by choice.
Water beds had their heyday in the 1970s and 80s, and one reason they faded from popularity is weight. A king-size water bed holds roughly 200 gallons, which translates to about 1,670 pounds of water. Plenty of older homes and apartments could not handle that load on a bedroom floor without reinforcement.
Rooftop loads are another area where the number matters. A flat commercial roof can accumulate standing water during a heavy rain. Just one inch of water spread across a 1,000-square-foot roof weighs about 5,200 pounds. Add clogged drains and the weight climbs fast, which is why ponding water is a leading cause of commercial roof collapse.
Landscaping and pool installation require the same awareness. An in-ground pool holding 20,000 gallons of water contains roughly 167,000 pounds, or about 83 tons, pressing outward against the pool walls and downward into the soil. Contractors account for soil type, water table level, and drainage to make sure the ground can handle the load without shifting.
How This Number Compares to Other Common Liquids
Water is denser than many liquids you might encounter around the house and lighter than some others. Knowing where water sits on the scale helps when you are estimating loads for something other than a fish tank.
- Gasoline: about 46 pounds per cubic foot, roughly 75 percent as heavy as water.
- Milk: about 64 pounds per cubic foot, slightly heavier than water because of dissolved sugars and proteins.
- Honey: about 89 pounds per cubic foot, nearly half again as heavy as water.
- Rubbing alcohol (isopropanol): about 49 pounds per cubic foot.
- Mercury: about 845 pounds per cubic foot, more than 13 times heavier than water.
These comparisons explain some everyday observations. Gasoline floats on water during a spill not because gasoline is “lighter” in some vague sense but because each cubic foot of it weighs about 16 pounds less. Oil-based salad dressing separates from the vinegar-water layer below it for the same reason.
The Metric Side of the Same Question
If you work in metric units, the equivalent question has a tidier answer. One cubic meter of pure water at maximum density weighs almost exactly 1,000 kilograms, or one metric ton. That neatness is not a coincidence: the metric system was originally designed with water’s density as a defining relationship. One liter of water weighs one kilogram, one milliliter weighs one gram, and so on. The relationship is not perfectly exact under modern definitions, but the discrepancy is tiny enough to ignore outside of high-precision calibration.
Converting between the systems, one cubic foot equals about 28.317 liters, and 62.4 pounds converts to about 28.3 kilograms. The numbers line up cleanly because the metric system was built around water in the first place. If you ever need to convert between cubic feet and liters for a project, multiplying by 28.3 and treating the weight in kilograms as numerically identical to the volume in liters will get you within rounding error of the right answer for fresh water at normal temperatures.
When Steam and Vapor Change the Picture
Once water transitions from liquid to vapor, the density drops dramatically. At atmospheric pressure and 212 °F, steam occupies roughly 1,600 times the volume of the liquid water that produced it. A cubic foot of steam at atmospheric pressure weighs only about 0.037 pounds, a tiny fraction of the 62.4 pounds that same volume held as a liquid. This massive volume change is the principle behind steam engines, steam turbines, and pressure cookers. It also explains why steam burns are so dangerous: a small amount of water carries an enormous amount of energy when it vaporizes, and that energy releases instantly when the steam contacts your skin and condenses back to liquid.
In industrial settings, superheated steam at high pressures is denser than atmospheric steam and can weigh several pounds per cubic foot, bridging part of the gap between vapor and liquid. Power plants and chemical processing facilities work with steam at these elevated pressures routinely, and accurate density values at those conditions are essential for sizing pipes, valves, and safety relief systems.