Evaporation Calculation Formula: Methods and Limits

There is no single universal evaporation calculation formula: evaporation depends on temperature, humidity, wind, surface area, and the liquid’s properties. This calculator uses a basic mass-balance estimate: enter the starting and ending liquid volumes and the time elapsed to get the average evaporation rate in volume per unit time.

Formula

For a measured volume decrease over a known interval, the average evaporation rate is:

R = (V₁ − V₂) / t

• R is the average volume-loss rate, in liters per hour (L/h).
• V₁ is the starting liquid volume, in liters (L).
• V₂ is the ending liquid volume, in liters (L).
• t is elapsed time, in hours (h).

The second result converts the hourly average to a 24-hour equivalent: R_day = 24R, in liters per day (L/day). This is a unit conversion, not a prediction that the rate will remain constant throughout a day. The equation assumes the measured loss is evaporation; leaks, draining, splashing, or other withdrawals make that assumption false.

Worked Example

Using the calculator’s default values, the starting volume is 10 L, the ending volume is 8 L, and the elapsed time is 24 h.

1. Find the volume decrease: 10 L − 8 L = 2 L.
2. Divide by elapsed time: 2 L ÷ 24 h = 0.083333… L/h.
3. Rounded to three decimal places, the average rate is 0.083 L/h.
4. Convert that unrounded rate to a daily equivalent: (2 L ÷ 24 h) × 24 h/day = 2.000 L/day.

The daily result uses the unrounded rate before display rounding. Multiplying the shown 0.083 by 24 would give 1.992, a small difference caused only by rounding.

What This Estimate Tells You

This calculation is useful when you can measure liquid level or volume at two times—for example, to describe water loss from a container or to compare evaporation under two sets of conditions. It reports an average over the interval. It does not reveal how quickly evaporation varied from hour to hour, nor does it isolate the physical process from other causes of liquid loss.

For a tank or pool with a constant horizontal surface area, a level drop can be converted to volume loss. If the area is in square meters and the water level falls by meters, multiply area by the level decrease to get cubic meters. One cubic meter equals 1,000 liters. In US customary units, area in square feet multiplied by level decrease in feet gives cubic feet; one cubic foot is about 7.4805 US gallons. Keep measurements in compatible units before dividing by time.

For instance, a level drop alone is not a volume unless the container’s geometry is known. A one-centimeter drop in a wide pool represents much more water than the same drop in a narrow bucket. In a vessel with sloped sides or an irregular shape, surface area may change with level, so a simple area-times-depth estimate may be inaccurate.

Physical Evaporation and More Detailed Methods

The calculator’s mass-balance formula works from observed loss. Predicting evaporation before it happens requires additional information and a model suited to the situation. Water molecules leave a liquid surface, and the rate generally increases when the vapor-pressure difference between the surface and surrounding air is larger. Warm liquid, drier air, and air movement often increase evaporation; actual effects depend on the setting and conditions.

Engineering and environmental calculations may use empirical or physical models that include variables such as surface temperature, air temperature, relative humidity, wind speed, atmospheric pressure, and exposed area. Some methods estimate a depth of water evaporated per day, commonly in millimeters per day or inches per day. Multiplying depth by surface area gives volume loss, provided units are converted consistently. A model may also require coefficients calibrated for a particular surface, climate, or measurement method; its output should not be treated as a universal constant.

For a measured mass decrease, use the analogous mass rate, (m₁ − m₂)/t, such as grams per hour or kilograms per day. To convert evaporated water mass to volume, water density is needed; density varies slightly with temperature. Do not use a volume formula with mass values or mix liters, gallons, and cubic feet without conversion.

Measurement Steps and Common Errors

1. Choose a measurement method and record the starting volume, level, or mass and the exact time.
2. At the later time, measure the same quantity with the same instrument and record elapsed time in hours for this calculator.
3. Check whether anything besides evaporation could have changed the amount: leaks, rain, refilling, spills, sampling, or use.
4. Calculate the decrease and divide by elapsed time. Keep extra digits during the calculation, then round the displayed result sensibly.

A negative calculated rate means the ending volume exceeded the starting volume. That can happen after rain or refilling, or because of measurement error; it is not a positive evaporation loss. This calculator allows that signed result rather than silently changing it. Zero elapsed time is invalid because division by zero has no defined result, and the calculator requires a positive time.

Measurement quality matters. Read a liquid level from the same reference point, use an appropriate calibrated measuring device, and avoid comparing measurements taken under different conditions if the goal is a fair comparison. For an open container, changes in temperature can affect volume and readings. Covering or moving the container, changing airflow, or changing the exposed surface can also make one interval unlike another.

Interpreting Results and Limits

Use the hourly result to describe the observed interval, and the daily equivalent only as a convenient rescaling. A short measurement period may not represent a full day because temperature, sunlight, humidity, and wind change over time. For a longer-term estimate, collect observations across representative conditions or use a model designed for the location and application.

Also distinguish evaporation from boiling. Evaporation can occur at the liquid surface below the boiling point; boiling involves vapor formation throughout the liquid. A volume-loss observation alone cannot tell you which process occurred, and this formula does not calculate heat energy, vapor concentration, or the time required for a liquid to disappear.

Frequently Asked Questions

What is the simplest formula for evaporation rate?
For measured liquid loss, divide the starting amount minus the ending amount by elapsed time: R = (V₁ − V₂)/t. This gives an average rate in the volume unit per time unit.

How do you calculate evaporation per day?
First calculate a rate per hour, then multiply by 24 to express it in amount per day. Treat this as a conversion of the measured average, not proof that evaporation stays constant for 24 hours.

Can I calculate evaporation from a water-level drop?
Yes, if you know the water surface area and can convert the level decrease into lost volume. For a vertical-sided container, volume loss is surface area multiplied by the level drop; irregular or sloped containers need a geometry-specific conversion.

What factors affect evaporation rate?
Important factors include liquid and air temperature, relative humidity, wind or air movement, exposed surface area, and atmospheric pressure. Their combined effect depends on the surface and surrounding conditions.

Why is my calculated evaporation rate negative?
A negative result means the measured ending amount was greater than the starting amount. Rain, refilling, a leak flowing inward, or inconsistent measurements may explain the increase; check the observations before interpreting it as evaporation.

Related

• Saturation Pressure Calculator and Formula
• Atmospheric Pressure by Elevation: Formula and Calculator
• Evaporation Rate Formula and How to Calculate It

in v1 | Starting volume | L | 10 | 0
in v2 | Ending volume | L | 8 | 0
in t | Elapsed time | h | 24 | 0
out rate | Average evaporation rate | L/h | (v1 - v2) / t | 3
out day | Average evaporation per day | L/day | rate * 24 | 3