Evaporation Rate Formula and How to Calculate It

The basic evaporation rate formula is the amount of liquid that disappears divided by the elapsed time: E = Δm/Δt for mass, or E = ΔV/Δt for volume. Enter the amount lost and the time interval to calculate an average rate; the result is meaningful only when the measurement units and conditions are stated.

Formula

For a measured loss of liquid mass, the average evaporation rate is:

E = Δm / Δt

• E is the average mass evaporation rate, in kilograms per hour (kg/h).
• Δm is the mass lost during the measurement, in kilograms (kg).
• Δt is the elapsed time, in hours (h).

The calculator also converts the result to kilograms per day by multiplying kg/h by 24 hours per day. That second output is a unit conversion, not a prediction that the measured conditions will remain unchanged for a full day.

Worked Example

Using the calculator’s defaults, a measured 1 kg of liquid evaporates over 2 hours:

1. Average rate: E = 1 kg ÷ 2 h = 0.500 kg/h.
2. Equivalent daily rate: 0.500 kg/h × 24 h/day = 12.000 kg/day.

The daily figure describes the same measured rate expressed over a 24-hour period. It does not account for changes in temperature, humidity, airflow, sunlight, or exposed surface area.

Mass, Volume, and Surface-Area Rates

“Evaporation rate” can refer to several different quantities. Mass per time is useful when weighing a container or calculating water loss by weight. Volume per time is often convenient for tanks and pools. A surface-area rate, such as kg/(m²·h) or mm/day, lets you compare evaporation across surfaces of different sizes.

For a volume measurement, use E_V = ΔV/Δt. For a rate normalized by exposed area, use E_A = Δm/(A × Δt), where A is the exposed liquid surface area. State the quantity and units whenever reporting a result; “0.5 per hour” is incomplete because it does not say whether the amount is mass, volume, or depth.

Mass and volume rates can be converted if the liquid density is known: mass = density × volume. For water near ordinary room conditions, 1 litre has a mass close to 1 kilogram, but this is an approximation and should not be assumed for other liquids or conditions. In US customary units, a mass rate might be lb/h, while a volume rate might be gal/h; use a defined conversion rather than mixing these directly.

How to Measure Evaporation

The simplest measurement compares the liquid’s amount at the beginning and end of a timed interval. Keep track of how the amount is measured and whether other processes could also change it. A practical measurement sequence is:

1. Choose the quantity to track: mass, volume, or liquid depth.
2. Record the starting amount and the start time.
3. After a known interval, measure the remaining amount using the same method.
4. Subtract the final amount from the initial amount, then divide by the elapsed time.
5. Record relevant conditions, including exposed area, temperature, humidity, and airflow.

For a container on a scale, mass loss is often straightforward, provided the scale and container remain stable. For an open tank, a falling level may reflect evaporation, leakage, water use, or rainfall as well as evaporation. Isolate or estimate those other gains and losses before treating the entire change as evaporation.

What Changes the Rate?

The simple formula calculates an observed average; it does not explain or predict the physical process. Evaporation generally increases when more liquid is exposed, the liquid is warmer, the surrounding air is less humid, or moving air removes water vapour from above the surface. The liquid’s properties and the local environment also matter.

Because these conditions vary, a rate measured at one time may not hold later. A pool’s daily loss on a warm, windy day can differ from its loss on a cool, humid day. If conditions change during a measurement, the formula still gives the average over that interval, but it cannot show when the rate was higher or lower. Shorter timed measurements can reveal variation, though measurement noise may become more important.

Common Errors and Limits

• Mixing units: Convert minutes to hours, grams to kilograms, or gallons to litres before calculating, or use consistent units throughout. For example, 30 minutes is 0.5 hours, not 30 hours.
• Confusing rate and total loss: A rate has “per time” in its units. Total evaporation over a period is rate multiplied by time only if that rate is representative of the period.
• Ignoring other water changes: Leaks, splashes, drainage, rain, and withdrawals can mimic or hide evaporation.
• Overstating precision: The answer cannot be more reliable than the measurements. A scale reading rounded to the nearest 0.1 kg does not support a result reported to many decimal places.
• Assuming a universal rate: There is no single evaporation rate for all liquids or settings. A measured rate applies to its particular conditions and method.

This calculation is an empirical average, not a substitute for a physical evaporation model. Predictive engineering calculations may require temperature, humidity, air velocity, pressure, surface geometry, and fluid properties. The formula alone cannot provide those missing inputs or determine a future rate.

Frequently Asked Questions

What is the formula for evaporation rate?
Divide the amount evaporated by elapsed time: E = Δm/Δt for mass, or E = ΔV/Δt for volume. Include the units in the result.

How do I calculate evaporation per day?
Measure the amount lost over a known interval and divide by that interval. If the result is per hour, multiply by 24 to express the same average as a daily equivalent; changing conditions may make an actual day’s loss different.

How do I calculate a pool’s evaporation rate?
Measure the water-level drop over a timed interval while accounting for rainfall, water added, leaks, and water use. A depth rate such as mm/day or inches/day is useful for a broad, level pool; multiply depth change by surface area to estimate volume lost.

Is evaporation rate the same as drying time?
No. Evaporation rate measures liquid loss per unit time. Drying time also depends on the starting amount, the point at which something counts as dry, and how conditions change during drying.

Can I compare rates measured with different units?
Yes, after converting them to the same basis—for example, kg/h to g/min or gal/day to L/day. For a fair physical comparison, also consider exposed surface area and environmental conditions.

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in m | Mass evaporated | kg | 1 | 0
in t | Elapsed time | h | 2 | 0.000001
out rate | Average evaporation rate | kg/h | m / t | 3
out perday | Equivalent daily rate | kg/day | rate * 24 | 3