evaporation rate calculator

Use measured liquid loss over a known surface area and time period to estimate evaporation rate, mass flux, and depth loss.

Evaporation Flux = (Initial Volume − Final Volume) × Density / (Area × Time)

Initial Volume (L)

Final Volume (L)

Surface Area (m²)

Elapsed Time (hours)

Liquid Density (kg/L)

What this evaporation rate calculator measures

This tool helps you estimate how quickly a liquid is evaporating from an exposed surface. It is useful for water tanks, open process vessels, cooling ponds, lab trays, and even backyard pool checks. Instead of guessing, you can calculate measurable values from before-and-after readings.

The calculator reports several outputs so you can use the one that best fits your work:

Volumetric evaporation rate in liters per hour (L/h)
Mass evaporation rate in kilograms per hour (kg/h)
Evaporation flux in kg/m²·h
Equivalent depth loss in mm/h and mm/day

How the formula works

Step 1: Find total liquid lost

Subtract final volume from initial volume. If you started with 120 L and ended with 117.5 L, the total loss is 2.5 L.

Step 2: Divide by elapsed time

2.5 L over 24 hours gives a volumetric evaporation rate of 0.104 L/h.

Step 3: Convert to mass loss (optional but common)

Multiply volume loss by density. For water at about 1 kg/L, 2.5 L lost is 2.5 kg lost.

Step 4: Normalize by area

Mass flux compares evaporation across different container sizes:

Flux (kg/m²·h) = Mass loss (kg) / [Area (m²) × Time (h)]

This is the preferred way to compare evaporation performance between systems.

Factors that influence evaporation

Temperature: Warmer liquid and warmer air generally increase evaporation.
Humidity: Dry air accepts more vapor; humid air slows evaporation.
Air movement: Wind or forced airflow can significantly increase rate.
Surface area: Larger exposed area increases total evaporation loss.
Liquid properties: Different liquids have different vapor pressures and densities.
Pressure: Ambient pressure changes can alter phase transfer behavior.

Practical use cases

Pool and reservoir management

Estimate daily water replacement needs and compare expected loss with measured meter use.

Industrial operations

Track open-tank loss, improve process control, and estimate solvent usage and emissions.

Laboratory experiments

Measure evaporation under controlled conditions and compare trials objectively using flux.

Tips for better accuracy

Use consistent measurement times (for example, always 24-hour intervals).
Keep units consistent: liters, square meters, hours, and kg/L.
Avoid readings right after splashing, filling, or draining events.
Record weather variables if outdoors (temperature, humidity, wind).
Use longer test windows when evaporation is very small.

Important limitations

This calculator assumes observed volume loss is due only to evaporation. In real settings, leaks, splashing, rainfall, inflow, or instrument error may affect readings. For critical engineering design, pair this quick estimate with a detailed mass-transfer model and site measurements.