Useful Unit Conversions
Conversion factors are from the Bureau of Reclamation's Metric Manual, 1978, by L.D. Pedde, W.E. Foote, L.F. Scott, D.L. King, and D.L. McGalliard, U.S. Government Printing Office, Washington DC.
English weights and volumes are avoirdupois units, and English volumes are based on fluid ounces.
Click on the topic to go directly to specific conversions or calculations:
Metric Prefixes Length Weight Time
Temperature Area Volume Flow Force & Energy
Gas Pressure Concentrations Major Ions Molecular Weights
Water Quality Data Checks Gas Partial Pressures
Metric Prefixes |
pico = p = 10^{-12} |
nano = n = 10^{-9} |
micro = µ = 10^{-6} = 0.000001 |
milli = m = 10^{-3} = 0.001 |
centi = c = 10^{-2} = 0.010 |
hecto = h = 10^{+2} = 100 |
kilo = k = 10^{+3} = 1,000 |
mega = M = 10^{+6} = 1,000,000 |
giga = G = 10^{+9} |
tera = T = 10^{+12} |
Length |
1.0 inch = 0.0254 m = 2.54 cm = 25.4 mm |
1.0 foot (ft) = 0.30480 m = 30.480 cm = 304.8 mm |
1.0 yard (yd) = 0.91440 m = 91.440 cm = 914.4 mm |
1.0 mile (mi) = 1,760 yd = 5,280 ft = 63,360 in = 1.6093 km = 1,609.3 m = 160,934 cm |
1.0 cm = 0.010 m = 10 mm = 0.03281 ft = 0.3937 in |
1.0 meter (m) = 100 cm = 1,000 mm = 1.0936 yd = 3.2808 ft = 39.370 in |
1.0 km = 1,000 m = 100,000 cm = 0.62137 mi = 1,093.61 yd = 3,280.83 ft = 39,370 in |
Weight |
1.0 gram (g) = 1,000 mg = 1,000,000 µg = 1.000 cm^{3} deionized H_{2}O at STP |
1.0 kg = 1,000 g = 2.204622 lb = 35.27396 oz |
1.0 ounce (oz) = 0.06250 lb = 28.34953 g = 0.0283495 kg |
1.0 pound (lb) = 16 oz = 0.45359 kg = 453.59 g |
Time |
1.0 hour = 3,600 s |
1.0 day = 1,440 min = 86,400 s |
1.0 week = 168 hr = 10,080 min = 604,800 s |
Temperature |
Celsius to Fahrenheit: °F = (°C x 1.80) + 32 |
Fahrenheit to Celsius: °C = (°F - 32) x 0.5556 |
Kelvin to Celsius: °C = °K - 273.15 |
Celsius to Kelvin: °K = °C + 273.15 |
STP - standard temperature and pressure = 273 °K at 1 atm |
Area |
1.0 in^{2} = 0.00064516 m^{2} = 6.4516 cm^{2} = 645.16 mm^{2} |
1.0 ft^{2} = 0.1111 yd^{2} = 144 in^{2} = 0.092903 m^{2} = 929.03 cm^{2} = 92,903 mm^{2} |
1.0 yd^{2} = 9 ft^{2} = 1,296 in^{2} = 0.836127 m^{2} = 8,361.27 cm^{2} = 836,127 mm^{2} |
1.0 acre = 0.0015625 mi^{2} = 4,840 yd^{2} = 43,560 ft^{2} = 4,046.87 m^{2}= 0.404687 ha |
1.0 mi^{2} = 640 acres = 27.878 x 10^{6} ft^{2} = 2,589,988 m^{2} = 258.99 ha |
1.0 cm^{2} = 100 mm^{2} = 0.1550 in^{2} |
1.0 m^{2} = 10,000 cm^{2} = 1.1959 yd^{2} = 10.7369 ft^{2} = 1,550.0 in^{2} |
1.0 hectare (ha) = 100 m x 100 m = 10,000 m^{2} = 0.00385901 mi^{2} = 2.47104 acres = 11,959.9 yd^{2} |
1.0 km^{2} = 100 ha = 1,000,000 m^{2} = 0.3860 mi^{2} = 247.104 acres |
Volume |
1.0 fluid oz = 1.8047 in^{3} = 0.029574 L = 29.574 mL |
1.0 in^{3} = 0.5541 oz = 0.016387 L = 16.387 mL |
1.0 pint = 16.0 fluid oz = 0.47318 L = 473.18 mL |
1.0 quart = 2.0 pt = 32.0 fluid oz = 0.94635 L = 946.35 mL |
1.0 gallon = 4.0 qt = 8.0 pt = 128 fluid oz = 3.7854 L |
1.0 ft^{3} = 7.4805 gal = 0.028317 m^{3} = 28.317 L |
1.0 acre-ft = 1233.489 m^{3} = 1.233 X 10^{6} L = 325,851 gal |
1.0 cm^{3} = 1.0 mL deionized H_{2}O at STP = 0.001 L |
1.0 liter (L) = 0.001 m^{3} = 1,000 mL = 0.264172 gal = 1.0567 qt = 2.1134 pt |
1.0 m^{3} = 1,000 L = 8.1071 x 10^{-4} acre-ft = 35.315 ft^{3} = 264.17 gal |
Flow |
1.0 gal/min (gpm) = 0.0044191 acre-ft/d = 0.0022280 cfs = 192.5 ft^{3}/d = 3.7854 L/min |
1.0 ft^{3}/s (cfs) = 1.98347 acre-ft/d = 448.831 gal/min = 646,317 gal/d = 0.0283169 m^{3}/s |
1.0 acre-ft/d = 0.504167 cfs = 325,851 gal/d = 14.2764 L/s = 856.584 L/min |
1.0 m^{3}/s = 1,000 L/s = 35.315 ft^{3}/s = 264.17 gal/s |
1.0 L/s = 1,000 mL/s = 0.0010 m^{3}/s = 0.035315 ft^{3}/s = 0.264172 gal/s |
Atmospheric Pressure |
1 atmosphere (atm) = 760 mm Hg (torr) = 29.9213 in Hg = 14.7 lb/in^{2} = 101.325 kPa |
1.0 pascal (Pa) = 1.0 N/m^{2} = 1.45038 x 10^{-4} lb/in^{2} = 0.00750064 mm Hg |
1.0 bar = 100.0 kPa = 100,000 N/m |
1.0 in of Hg = 25.4 mm of Hg = 3,386.38 Pa (at 0 °C) = 3,376.85 Pa (at 16 °C) |
1.0 mm of Hg = 0.0393701 in of Hg = 0.0013158 atm =133.322 Pa = 0.019339 lb/in^{2} |
1.0 in of water = 249.0817 Pa (at 4 °C) = 248.8400 Pa (at 16 °C) |
1.0 lb/in^{2} = 0.068046 atm = 51.71509 mm of Hg = 6.89476 kPa |
Force, Energy, and Radiation |
Force: 1.0 newton (N) = 1.0 kg-m/s^{2} = 10^{5} dynes |
Energy: 1.0 joule (J) = 1.0 N-m = 1.0 W-s = 0.239006 cal = 10^{7} erg = 0.73756 ft-lb |
Radiation: 1.0 becquerel (Bq) = 1 disintegration per second |
Radiation Exposure: 1.0 gray (Gy) = 1.0 J/kg = 0.010 rad (radiation absorbed dose) |
Chemical Concentrations |
1.0 mg/L = 0.001 g/L = 1,000 µg/L = 1,000,000 ng/L |
1.0 µg/L = 0.001 mg/L = 1,000 ng/L |
1.0 ng/L = 0.001 µg/L = 0.000001 mg/L |
1.0 percent = 1.0 g/100g = 10 ^{o}/oo (parts per thousand) = 10 g/kg = 10,000 mg/kg |
1.0 g/kg = 0.10 percent = 1,000 mg/kg |
1.0 mg/kg = 0.0010 g/kg = 0.00010 percent = 1,000 µg/kg |
1.0 µg/kg = 0.001 mg/kg = 1,000 ng/kg |
Molar and Equivalent Weights for Major Ions |
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Compound Name | Chemical Formula | Molecular Weight, mg/mMole^{1} | Equivalent Weight, mg/meq^{1} | Approximate Conductivity Factor, µS/cm per mg/L^{2} |
Carbonate | CO_{3}^{2-} | 60.0094 | 30.0047 | 2.82 |
Bicarbonate | HCO_{3}^{-} | 61.0171 | 61.0171 | 0.715 |
Hydroxide | OH^{-} | 17.0073 | 17.0073 | 5.56 |
Calcium Carbonate | CaCO_{3} | 100.0874 | 50.0437 | N/A |
Sulfate | SO_{4}^{2-} | 96.0636 | 48.0318 | 1.54 |
Chloride | Cl^{-} | 35.4527 | 35.4527 | 2.14 |
Calcium | Ca^{2+} | 40.078 | 20.039 | 2.60 |
Magnesium | Mg^{2+} | 24.3050 | 12.1525 | 3.82 |
Sodium | Na^{+} | 22.9898 | 22.9898 | 2.13 |
Potassium | K^{+} | 39.0983 | 39.0983 | 1.84 |
^{1} Data from 76th edition of the CRC Handbook of Chemistry and Physics ^{2} Data from 20th edition of APHA-AWWA-WEF Standard Methods for The Examination of Water and Wastewater |
Calculations to Check Water Analyses^{3} |
Anion-Cation Balance Because most natural waters are electroneutral, the sum of cations (positive ions) should equal the sum of anions (negative ions) when the concentrations are calculated in milliequivalents per liter, meq/L. This means that existing water quality data for major ions may be independently checked for accuracy. 1. Calculate sum of anions, Σ_{anions} in meq/L, using the conversions in the above table:Σ_{anions} = [OH^{-}] + [CO_{3}^{2-}] + [HCO_{3}^{-}] + [SO_{4}^{2-}] + [Cl^{-}], meq/L 2. Calculate sum of cations, Σ_{cations} in meq/L, using the conversions in the above table: Σ_{cations}= [Ca^{2+}] + [Mg^{2+}] + [Na^{+}] + [K^{+}], meq/L 3. Calculate ion balance percent: Ion Balance Percent = {(Σ_{cations} - Σ_{anions}) ÷ (Σ_{cations} + Σ_{anions})} x 100 4. Ion balance percent should be near zero depending on the concentration of the water: If (Σ_{cations} + Σ_{anions}) is 0 to 6.0 meq/L, ion balance may be: ± 0.2 meq/L If (Σ_{cations} + Σ_{anions}) is 6.0 to 20.0 meq/L, ion balance may be: ± 2.0 percent If (Σ_{cations} + Σ_{anions}) is 20 to 600 meq/L, ion balance may be: ± 5.0 percent |
Measured TDS and Calculated TDS Total dissolved solids (TDS) is determined by weighing a sample after evaporation and then drying at 180 °C (EPA Method 160.1). The measured TDS, TDS_{m}, should be close to calculated TDS, TDS_{c}, determined by adding all major ions concentrations in mg/L. 1. Calculate TDS_{c} from sum of major ions, Σ(ions) in mg/L, using analysis results: |
Measured EC and Calculated EC The measured conductivity, EC_{M}, should be close to calculated conductivity, EC_{C}, determined by summing the product of each major ion concentration in mg/L times the approximate conductivity factors for each ion seen in the table above. 1. Calculate EC_{c} from individual ion concentrations in mg/L, denoted as C_{i}, multiplied by the "Approximate Conductivity Factor, µS/cm per mg/L", denoted as F_{i}, for each of the major ions: |
^{3} Method 1030E, from 20th edition of APHA-AWWA-WEF Standard Methods for The Examination of Water and Wastewater |
Atmospheric Partial Pressures for O_{2} and CO_{2} |
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Elevation, ft | Elevation, m | Example Location | Partial Pressure, pO_{2}, atm | Partial Pressure, pCO_{2}, atm |
0 | 0 | sea level | 0.2070 | 0.000326 |
5,280 | 1,609 | Denver CO | 0.1741 | 0.000274 |
10,400 | 3,170 | Leadville CO | 0.1421 | 0.000224 |
Empirical equations^{4} for estimating pO_{2} and pCO_{2} |
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pO_{2} (atm) = 0.20704 - (6.239 x 10^{-6}) x (elevation, ft) |
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pCO_{2} (atm) = 3.262 x 10^{-4} - (9.9297 x 10^{-9}) x (elevation, ft) |
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^{4} These equations are based on simple linear regression of U.S. standard atmosphere data from 76th edition of the CRC Handbook of Chemistry and Physics. These equations do not take into account local changes in atmospheric pressure from weather systems. More accurate estimates may be obtained by measuring local atmospheric pressure (absolute, in mm Hg) and then adjusting sea level values for pO_{2} and pCO_{2} |