Water Weight Calculator, Charts, and Explanation

Water Weight Calculator (by Unit of Volume & Temperature)

Note that water is at maximum density (or weight per unit of volume) at 39.2°F/4.0°C. The weight of water is usually given for water at maximum density by default, but we are using room temperature (70°F) as the default setting.

If you select a temperature other than 39.2°F/4.0°C, the weight values calculated will likely be slightly different than water weight values found elsewhere.

As water temperature deviates from 39.2°F (4.0°C), it expands slightly, reducing its density. This means that at higher temperatures, a given volume of water will weigh slightly less.

This calculator assumes pure freshwater. If dealing with saltwater, density (and thus weight) will be higher due to dissolved salts.

Water Weight Calculator (by Area and Depth)

How to Calculate the Weight of Water

To find the weight of water, you need to pick which unit of volume you want to use to measure the water, and then figure out how many of these units your quantity of water contains. All of the most common units of volume are included in Table 1 below.

Once you know the type of unit and the number of units, refer to the weight of your unit given in Table 1, and multiply the weight for one of those units by the number of units you have.

Next, you need to adjust the weight to account for the temperature of the water.

The weights given in Table 1 are for water at maximum density, which water achieves at 39.2°F (or 4°C). The density of water changes with temperature, meaning the weight of a fixed volume of water (such as a gallon) also varies slightly with temperature.

For small quantities of water, the effect of temperature is negligible for practical purposes. For large volumes, especially in scientific, engineering, or industrial applications, adjusting for temperature is important to ensure accuracy.

Table 2 below shows water density by temperature, and the second column in the table shows the percentage of maximum density for each temperature. You can select your temperature and see what percentage of maximum density it is.

Since the weight value from Table 1 is based on maximum density, you can adjust for temperature by multiplying that value by the percentage value from Table 2, which represents how close the actual density is to maximum density at your selected temperature.

While temperature is one key factor affecting water density, extreme altitude or pressure can also have minor effects. If precision is critical, consult specialized density tables that account for these variables.

Example:

How much does 100 gallons of water weigh at room temperature?

Step 1:

According to Table 1, a gallon of water weighs 8.34540 pounds at maximum density.

Step 2:

8.34540 × 100 = 834.54 pounds

100 gallons of water weighs 834.54 pounds at maximum density.

Step 3:

According to Table 2, the density of water at room temperature (70°F) is 99.802% of the density of water at 39.2°F (maximum density).

Multiply 834.54 pounds by 0.99802 to get 832.8876108 pounds, which we would round off to 832.89 pounds.

The final weight is rounded to two decimal places, but the level of rounding can be adjusted depending on the required precision.

So, 100 gallons of water weighs 832.89 pounds at room temperature.

Weight of Water by Unit of Volume and Temperature

Weight of Water and Temperature

At room temperature (I’m assuming room temperature to be 70°F or 21°C), the density of water is approximately 0.00198 g/cm³ lower than its maximum density at 39.2°F (4.0°C) (see the next table below).

You can figure that the weight of water at room temperature for any specific volume will be approximately 99.8 percent of the weight of the same volume of water at 39.2°F (4.0°C). For example, a cubic foot of water at room temperature will weigh 62.3044 lbs., which is approximately 99.8% of the 62.4280 lbs. that a cubic foot of water weighs at 39.2°F (4.0°C).

As water warms beyond 39.2°F (4.0°C), it expands slightly, decreasing its density and reducing its weight per unit volume. Conversely, cooling water causes slight contraction until it nears freezing, at which point it expands again.

These tables provide values for pure water; real-world water sources may contain dissolved minerals or impurities that slightly alter density and weight. In industrial or scientific applications, additional adjustments may be necessary to account for these factors.

I’m aware that a mole of water is a specific number of water molecules and not a unit of volume, but I thought I’d include it anyway. A mole of water refers to Avogadro’s number of water molecules (approximately 6.022 × 10²³ molecules), which corresponds to 18.015 grams of pure water. At 39.2°F (4.0°C), where water is at maximum density (1 g/cm³), one mole of water has a volume of approximately 18.015 mL (or cm³).

Note: A fluid ounce is a unit of volume, not weight. The weight of a fluid ounce of water varies depending on temperature and density.

Table 1: Weight of Water by Unit of Volume

This table gives the weight of water per unit for all of the common measurement units used to measure liquid volume. The weight values provided are for pure water at maximum density, which occurs at 39.2°F (4.0°C) under standard atmospheric pressure (1 atm).

Table 2: Density of Water by Temperature

Water Weight per Inch per Square Foot (for Ponding Water)

The table below provides the weight of water per inch in pounds for various areas/square footages. This should allow you to quickly and accurately estimate the weight of any amount of standing water on your low-slope roof. (I obviously wrote this section before I added the calculators to this web page).

One cubic inch of water weighs approximately 0.036127 pounds at maximum density (39.2°F / 4.0°C). At room temperature (70°F / 21°C), it will weigh slightly less due to its slightly reduced density. Since one square foot of water one inch deep contains 144 cubic inches of water, its weight is 144 × 0.036127 = 5.202288 pounds at maximum density. Everything else in the table has been calculated from this.

This table should give you an idea of how much weight a rain load could be imposing on your roof due to areas of ponding water or clogged drains. For practical roofing calculations, this slight variation in weight due to temperature is usually negligible, but for large roof areas or engineering assessments, it may be worth considering.

Keep in mind that prolonged ponding water can significantly increase roof stress, leading to structural damage or failure if drainage issues are not addressed. Always compare estimated water weight with the roof’s design load capacity to ensure safety. Also, see my roof drainage article for more about ponding water.

Weight of water values for areas or depths not covered in the table should be easy to figure out simply by adding together values for areas or depths that are listed.

Table 3: Weight of Water per Inch per Square Foot