Units and Conversions for Rainwater Collection

UNITS and CONVERSIONS

US Gallons to Imperial

US gallons (USG) are referenced in many rainwater manuals; most tanks and pumps manufactured in the United States refer only to US gallons.

One USG = 4/5ths (0.83) of an Imperial gallon (iGal)

One iGal = 1.2 USG

 For example, a 1,000 USG tank contains only 830 iGal

 A U.S. pump with a pumping rate of 20 US gal per minute is equal to a rate of 16 iGal per minute.

US Conventional to Metric Conversion

One inch = 2.54 cm

1 USG = 3.875 litres

1 litre = 0.22 imperial gallons

1 foot = 0.305 metres

1 square foot = 0.093 square metres

1 cubic metre = 264 USG

Area to Volume Measurements

One millimeter of water over one square metre = 1L

2.54 cm of water over one square metre = 25.4 L

One cubic metre = 1,000 litres

One cubic foot = 6.23 imperial Gallons 

One inch of water over one square foot = 0.62 USG

Volume to Weight

•   One imperial gallon of water weighs 10 pounds

•   One litre of water weighs one kilogram

Water Pressure Created by Gravity

•   One pound per square inch (psi) created for every 2.31 vertical feet of drop

•   One foot of vertical drop produces 0.435 psi

Flow Rate

 ½-inch of rain in one hour on 1,000 square feet = 4.3 gpm (gallons per minute)