# 6.1: Units Related to Water

The following unit conversions are directly related to sizing and analyzing rainwater harvesting systems in terms of length, area, volume, mass, pressure, and general water conversions.

## Length

• $$1\; m = 3.28\; ft = 100 \;cm$$
• $$1 \;ft = 12\; in = 30.48\; cm$$
• $$1\; mi = 5,280\; ft = 1.61 \;km$$

## Area

• $$Rectangle: length * width$$
• $$Circle: π * r^2$$
• $$1\; m^2 = 10.765 \;ft^2$$
• $$1\; km^2 = 0.386\; mi^2 = 1,000,000\; m^2$$
• $$1\; hectare = 10,000\; m^2 = .01\; km^2 = 2.47\; acres$$
• $$1\; mi^2 = 2.6 \;km^2 = 640 \;acres$$
• $$1\; acre = 4,047 \;m^2 = 43,560 \;ft^2 \;(66\; ft * 660 \;ft) = 1/640\; mi^2 ≈ 40% \;of\; a \;hectare$$

## Volume

• $$Area * depth$$
• $$Rectangular\;Prism: Length * Width * Depth$$
• $$Cylinder: π * r^2 * depth$$
• $$1 \;L = 0.264\; gal = 1000\; cm^3 = 1000 \;ml$$
• $$1 \;m^3 = 1000 \;L = 35.3\; ft^3 = 264 \;gal$$
• $$1\; ft^3 (cf) = 28.32\; L = 7.482\; gal$$
• $$1\; gal = 3.785\; L = 4 \;qt = 16\; cups = 128\; fl. oz.$$
• $$1 \;acre-foot = 43,560\; ft^3 = 325,851 \;gal$$

## Mass

• $$1 \;kg = 1,000 \;g = 2.205\; lb$$
• $$1\; lb = 453.6\; g = 16\;oz$$
• $$1\; US \;short \;ton = 907\; kg = 2,000\; lb$$
• $$1 \;metric\; tonne = 1,000\; kg = 2,205\; lb$$

## Pressure

• $$1MPa = 10\; bar = 9.87 \;atm = 145 \;psi$$
• See water pressure for more

## Water (at 4° C and 1 atm)

• $$Density: 1 \;\frac{g}{ml} = 1 \;\frac{g}{cm^3} = 1\; \frac{kg}{L} = 1,000\; \frac{kg}{m^3}$$
• $$Density: 62.4\; \frac{lb}{ft^3} = 8.34\; \frac{lb}{gal}$$
• $$Pressure: 0.4335 \;psi\; for\; every\; vertical\; foot \;of \;water$$