A standardized drop size as used for laboratory and medical purposes is defined as 1/20th of a milliliter (also know as a cubic centimeter, 1 ml = 1 cc). One drop is exactly 0.05 ml.

• 200cm² rain gauge resolution rain equivalent:

  • 0.1 mm resolution equals 2 ml (40 standard drops) of water

  • 0.2 mm resolution equals 4 ml (80 standard drops) of water

• 8” rain gauge resolution rain equivalent:

  • 0.004” inch resolution equals 3.25 ml (65 standard drops) of water

  • 0.01” inch resolution equals 8.25 ml (165 standard drops) of water

• 400cm² rain gauge resolution rain equivalent:

  • 0.1 mm resolution equals 4 ml (80 standard drops) of water

  • 0.2 mm resolution equals 8 ml (160 standard drops) of water

• 500 cm² rain gauge resolution rain equivalent:

  • 0.1 mm resolution equals 5 ml (100 standard drops) of water

  • 0.2 mm resolution equals 10 ml (200 standard drops) of water

The following table shows how rain gauge accuracy in percent (%) is related to measurable amounts of rain. In this sense, one can quickly judge whether a manufacturer is providing incorrect values on their datasheets, as seems to be the case with many meteorological instruments from even the “most reputable” manufacturers since the WMO and NWS or NOAA do not have clear standards under which conditions and how to calibrate most meteorological instruments to reflect real attainable accuracy in real world outdoor conditions.

Calculate the uncertainty of your rain measurements

The basic formula to determine the minimum amount of rain required to reach a given accuracy follows:

• Divide the rain gauge resolution by the accuracy desired: 0.2 mm / 2% = 10 mm or more of accumulated rain is required to reduce the rain gauge resolution uncertainty to less than 2%.

• Adding in the datasheet accuracy of 1% results in a ± 3% minimum uncertainty for the above calculated accumulated rain amount. Less rain will produce a higher error and more rain will improve the uncertainty even more.

• Since there are other uncertainties that we did not consider, like wind blown rain uncertainty and others, the total uncertainty and measurement error will be larger.

• For more details on the complex and complete calculation of rain gauge measurement uncertainty, please see the WMO CIMO/TECO Guide to Instruments and Methods of Observation, 2018 edition - Volume V, chapter 6 on the Measurement of precipitation.

Determining rainfall rate and practical rain intensity error

For practical advice on determining accurate precipitation rates and how to choose a rain gauge for accurately determining rain rates, please see the following guide:

• Practical Guide to Determining Rainfall Rate and Rain Intensity Error