To determine the accuracy of a digital rain gauge, you need to know the surface area of the funnel.

If the rain gauge has a rectangular inlet:

Area = length x width

If the rain gauge is circular:

Area = ( π x d^{2} ) / 4, where d is the diameter of the rain gauge and π = 3.14159

The calculation is as follows:

Total Rain = Water Volume / Area

**Example 1:**

**Rectangular Rain Gauge:**

Area = 11 x 5 cm or 55 cm^{2}

If you pour in 100 ml of water (or 100 cm^{3}) SLOWLY (over a 10 minute period), one would expect the rainfall to equal:

Total Rain = 100 cm^{3}/ 55cm^{2} = 1.81 cm or 18 mm of rain (+/- the accuracy specification of the rain gauge).

Since 1 mm = 0.03937 inches, the total rain (inches) = 18 x 0.03937 inches/mm = 0.71 inches.

Take into account the resolution of the rain gauge.

If the resolution is 0.04 inches for example, the measurement would be 0.04 inches/tip x 17 tips = 0.68 inches.

**Example 2:**

**Circular Rain Gauge (Osprey):**

Diameter = 11.28 cm

Area = ( π x 11.28^{2} ) / 4 = 100 cm^{2}

If you pour in 100 ml of water (or 100 cm^{3}) SLOWLY (over a 10 minute period), one would expect the rainfall to equal:

Total Rain = 100 cm^{3}/ 100cm^{2} = 1 cm or 10 mm of rain (+/- the accuracy specification of the rain gauge).

Since 10 mm = 0.3937 inches, the total rain (inches) = 0.3937 inches.

Take into account the resolution of the rain gauge.

If the resolution is 0.01 inches for example, the measurement would be 0.01 inches/tip x 39 tips = 0.39 inches.

**Important Note:** Make sure the weather station display is set to total rain and not rain rate.

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