Pond Water Evaporation Rate Calculator by Size

Estimate a pond’s water loss from evaporation by entering its surface area and the evaporation depth over a chosen period. The calculator returns the volume lost in gallons or liters; it does not predict the local evaporation rate, which must be measured or obtained from a suitable weather or water-resource source.

Formula

Evaporated volume equals the pond’s horizontal surface area multiplied by the depth of water evaporated. In the calculator’s metric inputs, 1 square metre multiplied by 1 millimetre equals 1 litre:

• Water lost (L) = A × E
• Water lost (US gal) = A × E × 0.264172

A is the pond’s horizontal water-surface area in square metres (m²). E is the evaporation depth over the period being considered, in millimetres (mm). The US-gallon result converts litres using 1 litre ≈ 0.264172 US gallons. These are volume estimates, not a rate unless you also specify the time interval represented by E.

Worked Example

Using the calculator’s default values, the pond surface area is 10 m² and the evaporation depth is 5 mm:

1. Metric volume: 10 m² × 5 mm = 50 L.
2. US gallons: 50 L × 0.264172 = 13.2086 US gal.
3. Rounded to two decimal places, the estimate is 50.00 L, or 13.21 US gal.

This example means 5 mm of water depth has evaporated from the pond’s surface during the period used for that depth. It does not mean the pond loses 5 mm every day unless the input depth is a daily value.

How to Use Pond Size and Evaporation Depth

First find the area of the water surface, not the pond’s bottom area or its total liner area. For a rectangular pond, multiply the average water-surface length by its width. For an approximately circular pond, area is π times the square of its radius. For an irregular outline, split the surface into simple shapes and add their areas, or estimate it from a scaled drawing.

Next obtain an evaporation depth for the period you care about. It may come from a local weather or water-resource source, a properly maintained evaporation pan with an appropriate adjustment, or a direct pond-level observation corrected for rainfall and water additions. Enter that depth in millimetres. For an estimate over several days, use a depth representing the whole period; if you have a daily depth, multiply it by the number of days only when that daily value is appropriate for those conditions.

The answer scales directly with both inputs. Doubling the surface area doubles estimated loss; doubling the evaporation depth does the same. For a US customary workflow, convert area to square metres and depth to millimetres before using this calculator: 1 square foot = 0.092903 m², and 1 inch = 25.4 mm. Alternatively, calculate cubic feet from square feet times feet of depth, then convert cubic feet to US gallons using about 7.48052 gallons per cubic foot.

What Changes a Pond’s Evaporation

Pond size alone cannot reveal an evaporation rate. Size determines how much volume corresponds to a given depth loss, while weather and pond conditions determine the depth lost. Relevant influences include air temperature, humidity, wind, solar energy, water temperature, shade, and exposure. A broad, shallow pond can lose a substantial volume because it has a large exposed surface, even if its total stored water is modest.

Evaporation changes over time, so a single rate is best treated as an estimate for its stated location and period, not a permanent pond property. A seasonal average can conceal hot, dry, windy spells and cooler, humid periods. For planning, calculate separate periods when useful data are available rather than assuming one daily figure applies unchanged throughout a year.

Interpreting the Result and Its Limits

The calculator reports the volume represented by the specified depth loss. To express it as an average over time, divide that volume by the period length. For example, if 50 litres represents five days, the arithmetic average is 10 litres per day. This average does not show how loss varied within those five days.

The calculation assumes a roughly level water surface and a meaningful average depth of evaporation across that surface. It does not account for seepage through the pond floor or liner, leaks, overflow, splashing, water removed by pumps, or water added by rain and filling. Those processes can also change water level, but they are not evaporation and should not be folded into the evaporation-depth input.

Measuring Actual Water Loss

A falling water level by itself cannot identify evaporation. To estimate evaporation from observations, track the pond level consistently and record rainfall, deliberate additions, and withdrawals over the same interval. A simple nearby container of water may offer a rough comparison, but it does not necessarily evaporate at the pond’s rate: its exposure, depth, temperature, and wind conditions may differ.

For a more useful measurement:

• Mark or measure the water level at the same location and under similar conditions.
• Record the elapsed time and the pond’s surface area at the observed level.
• Track rainfall and any water added or removed; use consistent units.
• Compare measurements over more than one interval to see how weather affects the result.

If the pond has sloping sides, its surface area may change as the level drops. For a short interval this may be minor, but for a large change, estimate the average surface area during the interval or calculate the change in pond volume from its shape. A depth change multiplied by the original area can otherwise overstate or understate the actual volume change.

Frequently Asked Questions

How much water does a pond lose to evaporation per day?
There is no universal daily amount. It depends on local weather and pond conditions, and the volume depends on surface area. Use a depth-loss estimate for the relevant day and multiply it by the pond’s surface area.

Does pond depth affect evaporation?
Surface area is the direct size factor in this volume calculation. Pond depth affects how much water is stored and may influence water temperature, but it does not replace surface area when converting an evaporation depth into volume lost.

How do I calculate evaporation for an irregular pond?
Estimate its horizontal water-surface area by dividing the outline into simple shapes, adding their areas, or using a scaled map. Multiply the resulting area in square metres by evaporation depth in millimetres to get litres.

Is a drop in pond level always evaporation?
No. Rain, filling, leaks, seepage, overflow, and water use can all change the level. Track these separately before attributing a measured change to evaporation.

How many gallons is one millimetre of evaporation?
For each square metre of water surface, 1 mm of evaporation equals 1 litre, or about 0.264 US gallons. Multiply that figure by the pond’s area in square metres.

Related

• Atmospheric Pressure vs Elevation Calculator
• Boiling Water Evaporation Rate Calculator for Food
• Newton’s Law of Cooling Calculator: Find k

in a | Pond surface area | m² | 10 | 0
in e | Evaporation depth | mm | 5 | 0
out l | Water lost | L | a * e | 2
out g | Water lost | US gal | a * e * 0.264172 | 2