Water Evaporation Rate Calculator and Formula

Calculate water’s average evaporation rate by entering the amount of water lost and the elapsed time; the calculator returns the loss per unit of time. This simple measurement is useful for comparing conditions, but it does not predict evaporation accurately from temperature or humidity alone.

Formula

The average evaporation rate is the volume of water lost divided by the time observed:

R = V / t

• R is the average evaporation rate, in liters per hour (L/h).
• V is the measured volume of water lost, in liters (L).
• t is elapsed time, in hours (h).

The daily-rate output converts the hourly average to a 24-hour equivalent: R_day = 24R, in liters per day (L/day). This assumes the measured average rate is used as a simple constant; actual evaporation commonly changes through the day.

Worked Example

Using the calculator’s defaults, suppose 1 L is lost over 24 h.

1. Hourly rate: R = V / t = 1 L / 24 h = 0.041666… L/h.
2. Rounded to three decimal places, the displayed evaporation rate is 0.042 L/h.
3. Daily equivalent: 0.041666… L/h × 24 h/day = 1 L/day, displayed as 1.00 L/day.

The daily result is calculated from the unrounded hourly value, not from the displayed 0.042, so rounding does not inflate it to 1.008 L/day.

How to Measure Water Loss

For a direct measurement, use a container or system whose water level or volume can be measured reliably. Record the starting amount, wait a known interval, and measure again. The difference is the apparent water loss. Keep the measurement method and conditions consistent when comparing one period with another.

• For a small open container, measure the water level with a ruler and use the container’s dimensions to estimate lost volume. A level change is not itself a volume unless the container’s cross-sectional area is known.
• For a tank with volume markings or a reliable level sensor, record the readings at the start and end of the interval.
• For a pool or pond, account for rainfall, splashing, drainage, leaks, and water added during the observation. Otherwise, the measured level change is not evaporation alone.

A useful comparison is a nearby control container sheltered from splashes and runoff but exposed to similar air conditions. It can help distinguish evaporation from water added or removed by other causes. This is not a perfect correction if the control has different exposure, shade, or temperature from the water being studied.

What Changes the Evaporation Rate?

Evaporation occurs when water molecules escape from the liquid surface into the air. Its rate depends on the conditions at that surface and in the surrounding air, so a measured rate is specific to the place and time of measurement.

• Temperature: Warmer water generally evaporates faster because more molecules have enough energy to leave the liquid.
• Humidity: Air already containing more water vapor usually allows less net evaporation than drier air, all else equal.
• Air movement: Wind or ventilation can carry moist air away from the surface and replace it with drier air.
• Exposed surface area: A larger water surface can lose more water overall. Rate per area is a different quantity from total volume lost per hour.
• Sunlight and surrounding surfaces: Solar heating and heat transferred from nearby materials can warm the water and affect evaporation.
• Water chemistry and pressure: Dissolved substances and atmospheric pressure can also influence evaporation, though they may matter less than everyday changes in temperature, humidity, and airflow.

Reading the Result and Choosing Units

The calculator reports volume per time, not a percentage and not a rate per square meter. For example, 0.5 L/h means an average loss of half a liter each hour during the measured interval. It does not mean that half the water remaining will disappear every hour.

Metric volume units are convenient for many measurements: 1 L equals 1,000 mL and 1 cubic meter equals 1,000 L. In US customary units, 1 US gallon is about 3.785 L, so a rate in L/h can be converted to gallons per hour by dividing by 3.785. Do not confuse US gallons with imperial gallons. Keep the input volume in liters and time in hours for this calculator; convert other units before entering them. For instance, 30 minutes is 0.5 h, not 30 h.

Rates per area are often useful when comparing surfaces of different sizes. To calculate one, divide the volume-loss rate by the exposed surface area. The result might be expressed as L/m²/h. A depth-loss rate, such as millimeters per day, is another form: for a broad, level surface, 1 mm of water depth over 1 m² equals 1 L. These are not the same as the calculator’s total-volume rate unless surface area is also known.

Common Measurement Errors and Limits

The calculation is straightforward, but the measurement may not isolate evaporation. A falling level could also reflect a leak, seepage, water use, or drainage; a rising level could reflect rain or refill. For open outdoor water, these effects can be comparable to evaporation. Record additions and removals, or choose a controlled measurement setup, before treating the difference as evaporated water.

Very short observation periods can produce misleading results because ruler readings, waves, splashes, and small volume-measurement errors become large relative to the amount lost. Longer intervals often make the average more stable, though conditions may change during them. Record the observation period and, when relevant, water temperature, air temperature, humidity, wind, and exposed area.

This calculator describes an observed average. It cannot infer a dependable rate from weather conditions alone, forecast a future rate, or separate evaporation from other losses without additional measurements. Do not assume that multiplying one day’s result by seven gives a reliable weekly estimate if weather, shade, or water use varies.

Frequently Asked Questions

How do I calculate evaporation rate from water-level change?
Convert the level change to a lost volume using the container’s shape or a level-to-volume chart, then divide that volume by the elapsed time. For a rectangular tank with a constant horizontal area, volume lost equals area multiplied by the drop in water depth.

What is a normal evaporation rate for water?
There is no single normal rate. Evaporation depends on exposed area, water and air temperature, humidity, airflow, sunlight, and other conditions. Measure the specific surface over a known interval rather than relying on a universal value.

How do I calculate evaporation per day?
Divide the measured volume loss by elapsed hours, then multiply the hourly rate by 24. This gives a 24-hour equivalent of the measured average, not necessarily the amount that will evaporate on a different day.

Does a lower water level prove evaporation?
No. Leaks, drainage, seepage, and water removal can also lower the level. Rain or refilling can conceal evaporation. Check for these effects or use a controlled comparison before attributing all level change to evaporation.

Can I calculate evaporation from temperature alone?
No. Temperature is only one influence. Humidity, airflow, surface area, solar exposure, and other factors also matter, so a direct measurement or a suitably detailed physical model is needed.

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in v | Water lost | L | 1 | 0
in t | Elapsed time | h | 24 | 0
out r | Evaporation rate | L/h | v / t | 3
out day | Daily rate | L/day | r * 24 | 2