Water Weight Calculator

Weight is volume times density

Water has no fixed weight per gallon, because its density changes with temperature and pressure. The whole calculation is one line: weight in pounds equals volume in liters multiplied by the density in kilograms per liter, divided by 0.45359237. The density figures come from the USGS water density table, which lists grams per cubic centimeter at each step of the thermometer, and one cubic centimeter is one gram, so each number doubles as kilograms per liter.

Two constants do the rest of the work. One US gallon is 3.785411784 liters, and one pound is 0.45359237 kilograms. Those are exact definitions, so the only judgment in the calculation is which density row to use.

At 39.2 F (4 C) the density peaks at 1.00000 kg per liter, which is where the familiar figure of 8.345 lb per gallon comes from. Round that and you get the 8.34 lb shortcut that shows up on freight tables and shipping labels. The calculator starts at 70 F, a typical indoor temperature, and steps through the density table as you change the temperature.

The kilogram comes first in that chain for a reason. Density tables, including the USGS one, publish mass per volume in metric units, so liters and kilograms are the natural pair, and pounds are a conversion at the end. Do the division by 0.45359237 once, at the last step, and rounding errors stay small. Working in pounds the whole way means converting the density first, which adds a step and a chance to slip.

How density moves with temperature

Water is densest at 39.2 F and gets lighter as it heats up or as it freezes. The changes are small, under half a percent across the range most people care about, but they are enough to matter when you fill a large tank or fill a truck:

The spread from freezing to boiling is 8.34 lb down to 8.00 lb per gallon, a drop of about 4 percent. For most household tasks the 8.34 lb shortcut is fine, but a hot tub, a boiler or a cooling tower deserves the right number. The NIST guide to the International System of Units lays out the liter and kilogram definitions that the conversion depends on, so the constants above are not opinions.

Evaporation is the one path that moves water out without a drain. A humidifier that uses a gallon a day releases 8.33 lb of vapor into the house at 70 F, and the mass does not vanish; it condenses on cold window glass, on brick and on the underside of the roof deck. A swamp cooler can move several times that much, which is why the supply line and the roof both matter.

Gallons to pounds and kilograms

Here is the everyday table, with everything at 70 F and a density of 0.99802 kg per liter:

Multiply the 10 gallon row and you have a number most people have felt: a full ten gallon bucket of water is 83.29 lb, which is heavier than most people expect and heavier than a 50 pound bag of flour. Double it for two buckets, and add the weight of the buckets themselves. The same table works for gallons of anything, but only if the liquid is as light as water; diesel, paint and brine all carry extra mass per gallon.

That small gap between 8.33 and 8.34 is the reason shipping paperwork uses one reference number and this calculator uses the real density. Freight scales do not care about the temperature of your bucket; a tank you are filling does, because the same volume can take 4 percent more mass when it is cold. Use the shortcut for a guess and the table when the number has to hold up on a permit, a label or a load rating.

Worked example: ten gallons at 70 F

Take the default job, 10 gallons of water at 70 F, and walk through it:

• Volume in liters: 10 x 3.785411784 = 37.854 L
• Density at 70 F from the USGS table: 0.99802 kg per liter
• Mass in kilograms: 37.854 x 0.99802 = 37.78 kg
• Mass in pounds: 37.78 / 0.45359237 = 83.29 lb

Check it against the per-gallon number: one gallon at 70 F is 8.33 lb, and 10 of those is 83.29 lb. The two paths agree, which is a good sign the arithmetic is right.

Worked example: hot water and cold water

Now compare the same volume at two temperatures. Take 100 gallons at 32 F and 100 gallons at 140 F:

• At 32 F: 378.541 L x 0.99987 = 378.49 kg = 834.43 lb
• At 140 F: 378.541 L x 0.98338 = 372.25 kg = 820.67 lb

The hot side carries 13.76 lb less mass in the same tank, which is what you feel when you fill a hot water heater at 140 F instead of a cold one at 60 F. A tank that starts cold and heats up loses a little mass out the overflow as the water expands, so the scale under a boiler reads a touch lower after a long run. For a fuel oil tank the density falls the same way, and the gauge on a nearly full tank can read the same while the mass drops.

Other volume units

The same formula handles every volume unit once you convert to liters. All of these are at 70 F:

One cubic foot is the unit that shows up on construction drawings and in pool and aquarium volume work, and 62.30 lb at 70 F is the number to plan around. The USGS facts about water lists the same 62.416 lb per cubic foot at 32 F and 61.998 lb at 212 F, which brackets the range this table covers.

Kitchen shelves follow the same arithmetic. A one gallon jug is 8.33 lb, a 12 quart stockpot filled to the rim holds 24.99 lb of water, and a 21 quart canner holds 43.73 lb before a single jar goes in. Nobody lifts those by the handle when they are full, and the shelf they sit on should be rated for the total, not the pot.

Real loads: tubs, tanks and drums

Once you know the weight of water per gallon, real objects are easy. These four examples show how much weight a container adds to whatever it holds:

The hot tub row is the one that makes people stop. A 300 gallon tub at 100 F holds 2,486.55 lb of water, over a ton, and that load sits on whatever the tub sits on. Water heaters have the same problem: a 100 gallon tank at 140 F puts 820.67 lb on the floor, so the drain and the floor structure have to be planned for it. Add the container weight to every line before you decide anything about a stand or a joist.

A pool top-off is the same math at a bigger scale. One inch of water over 800 square feet is 66.67 cubic feet, which is 1,887.88 liters, so it weighs 1,884.15 kg or 4,153.8 lb at 70 F. That is why a garden hose runs for hours to raise a pool an inch, and why a leak of an inch a day is a real water bill rather than a rounding error.

Rain barrels and cisterns are the same story with a downspout attached. A full 50 gallon barrel holds 416.44 lb of water at 70 F, and four of them along one wall put 1,665.76 lb on a deck or a concrete pad. Check the joists or the slab before the first rain, not after, because a barrel that seemed light when empty is a small car once it fills.

Where people go wrong

• Leaving out the container. A 55 gallon drum holds 458.09 lb of water, and the empty steel drum adds around 60 lb. Ask a seller for both figures.
• Borrowing water density for another liquid. Fuel oil and diesel run a bit under 7.5 lb per gallon, and a brine mix can be well past 9 lb. The tables above cover fresh water alone.
• Flat-rating temperature on a large fill. The 32 F to 140 F gap is 13.76 lb per 100 gallons, enough to push a trailer past its rating.
• Confusing fluid ounces with weight ounces. A fluid ounce of water is 0.07 lb, while a weight ounce is 0.0625 lb. The fluid ounce is the one printed on the jug.
• Swapping imperial for US gallons. The US gallon is 3.785 liters; the imperial gallon is 4.546, so an imperial gallon of water is 10 lb. This page works in US gallons.

A well pressure tank is the quiet version of the same load. An 80 gallon tank filled at 60 F carries 667.01 lb of water inside a basement corner, and a 30 gallon tank carries 250.13 lb. Nothing about that changes when the pump cycles; the mass sits there year round.