Rain Gauge Accuracy Tables

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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.

200 cm2 Automatic rain gauge. Required accuracy per tip to meet WMO, NWS & NOAA requirements
Rain Gauge Accuracy Level± drops of rain± mm of rain± in of rain± drops of rain± mm of rain± in of rain
0.1 mm resolution rain gauge. 0.2 mm resolution rain gauge.
±1% =0.4 drops0.001 mm0.00004 in0.8 drops0.002 mm0.00008 in
±2% =0.8 drops0.002 mm0.00008 in1.6 drops0.004 mm0.00016 in
±3% =1.2 drops0.003 mm0.00012 in2.4 drops0.006 mm0.00024 in
±4% =1.6 drops0.004 mm0.00016 in3.2 drops0.008 mm0.00031 in
±5% =2.0 drops0.005 mm0.00020 in4.0 drops0.010 mm0.00039 in
8" Automatic rain gauge. Required accuracy to meet WMO, NWS & NOAA requirements
Rain Gauge Accuracy Level± drops of rain± mm of rain± in of rain± drops of rain± mm of rain± in of rain
0.004" resolution rain gauge. 0.01" resolution rain gauge.
±1% =0.65 drops0.001 mm0.00004 in1.65 drops0.00254 mm0.0001 in
±2% =1.30 drops0.002 mm0.00008 in3.3 drops0.00508 mm0.0002 in
±3% =1.95 drops0.003 mm0.00012 in4.95 drops0.00762 mm0.0003 in
±4% =2.60 drops0.004 mm0.00016 in6.60 drops0.01016 mm0.0004 in
±5% =3.25 drops0.005 mm0.00020 in8.25 drops0.01270 mm0.0005 in
400 cm2 Automatic rain gauge. Required accuracy per tip to meet WMO, NWS & NOAA requirements
Rain Gauge Accuracy Level± drops of rain± mm of rain± in of rain± drops of rain± mm of rain± in of rain
0.1 mm resolution rain gauge. 0.2 mm resolution rain gauge.
±1% =0.8 drops0.001 mm0.00004 in1.6 drops0.002 mm0.00008 in
±2% =1.6 drops0.002 mm0.00008 in3.2 drops0.004 mm0.00016 in
±3% =2.4 drops0.003 mm0.00012 in4.8 drops0.006 mm0.00024 in
±4% =3.2 drops0.004 mm0.00016 in6.4 drops0.008 mm0.00031 in
±5% =4.0 drops0.005 mm0.00020 in8.0 drops0.010 mm0.00039 in
500 cm2 Automatic rain gauge. Required accuracy per tip to meet WMO, NWS & NOAA requirements
Rain Gauge Accuracy Level± drops of rain± mm of rain± in of rain± drops of rain± mm of rain± in of rain
0.1 mm resolution rain gauge. 0.2 mm resolution rain gauge.
±1% =1.0 drops0.001 mm0.00004 in2.0 drops0.002 mm0.00008 in
±2% =2.0 drops0.002 mm0.00008 in4.0 drops0.004 mm0.00016 in
±3% =3.0 drops0.003 mm0.00012 in6.0 drops0.006 mm0.00024 in
±4% =4.0 drops0.004 mm0.00016 in8.0 drops0.008 mm0.00031 in
±5% =5.0 drops0.005 mm0.00020 in10.0 drops0.010 mm0.00039 in

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:

 
BARANI DESIGN TECHNOLOGIES IS A MANUFACTURER OF PROFESSIONAL RAIN GAUGES, WEATHER STATIONS AND METEOHELIX PERSONAL WEATHER STATIONS

BARANI DESIGN TECHNOLOGIES IS A MANUFACTURER OF PROFESSIONAL RAIN GAUGES, WEATHER STATIONS AND METEOHELIX PERSONAL WEATHER STATIONS