Convert the two unit categories you reach for most in water engineering — volumetric flow rate and constituent concentration — between SI and US customary units. Pick a category, enter a value, and choose the units to convert from and to.
Water and wastewater engineering straddles two unit systems: SI is standard in design and research, while US plant operations still run on million gallons per day, gallons per minute, and grains per gallon. Mixing them is a common — and sometimes costly — source of error. This converter handles the two categories you meet daily, flow rate and concentration, by scaling every unit to a single SI base and back out.
Each category defines one SI base unit — cubic metres per second for flow, milligrams per litre for concentration — and a conversion factor for every other unit relative to that base. To convert, the tool multiplies your value by the factor of the source unit (giving the SI base value) and divides by the factor of the target unit: result = value × f[from] / f[to]. Switching category resets the from and to selections to valid units so you never compare flow against concentration.
Volumetric flow appears in many guises. Relative to 1 m³/s: one million gallons per day (MGD) = 0.0438126 m³/s; one m³/day = 1/86400 m³/s; one litre per second = 0.001 m³/s; one US gallon per minute (GPM) = 6.30902×10⁻⁵ m³/s; one cubic foot per second (cfs) = 0.0283168 m³/s. A useful sanity check: 1 MGD ≈ 3785 m³/day ≈ 43.8 L/s ≈ 1.547 cfs ≈ 694 GPM.
Relative to 1 mg/L: ppm = 1 (mg/L equals ppm by mass in dilute water, where 1 litre weighs ~1 kg); µg/L = 0.001 mg/L; and one grain per US gallon = 17.118 mg/L, the traditional unit for water hardness. So a hardness of 10 grains/gal is about 171 mg/L as CaCO₃ — moderately hard water. The mg/L ≈ ppm equivalence holds only in dilute aqueous solutions near unit density; in brines or sludge it no longer applies.
A great deal of treatment-plant arithmetic — chemical dosing with the 8.34 factor, mass loadings, surface and weir loading rates — depends on consistent units. Reporting a flow in MGD when a formula expects m³/s, or treating grains/gal as if it were mg/L, produces errors of three to seventeen times. Always convert to one coherent system before plugging numbers into a design equation, and re-check the units of the answer.
In dilute aqueous solutions, yes — to a very good approximation. Because one litre of dilute water weighs about one kilogram, one milligram of solute per litre is one milligram per kilogram, which is one part per million by mass. The equivalence breaks down in concentrated solutions, brines, or sludges where the density departs from 1 kg/L, and it never applies to gases.
One million US gallons per day equals about 3785.41 m³/day (since 1 US gallon = 3.785411 litres). It is also roughly 43.8 L/s, 1.547 cfs, or 694 US GPM. The exact SI base value used here is 1 MGD = 0.0438126 m³/s.
A grain per US gallon is a traditional unit for water hardness and other constituents. One grain per gallon equals 17.118 mg/L. Water softeners are often rated in grains of hardness removed, so converting between grains/gal and mg/L as CaCO₃ is a routine step in sizing softening equipment.
Routing every conversion through a single base unit per category means you only need one factor per unit, not a full matrix of pairwise conversions. Multiply by the source factor to reach the base, divide by the target factor to leave it. It is less error-prone and trivially extensible to new units.
Mixing flow units inside a formula — for instance, using MGD where the equation expects m³/s, or m³/day where it expects L/s — and treating grains/gal as mg/L. Both produce large multiplicative errors. The fix is to convert all inputs to one coherent unit system before calculating and to verify the units of the result.