Wastewater Calculator
Choose BOD loading, COD loading, F/M ratio, or hydraulic retention time, enter the flow, concentrations, and tank volume with their units, and read the result with the units shown. These are calculation aids, not plant operating recommendations.
Last updated: September 26, 2026
Frequently asked questions
How is this different from the MLVSS Calculator?
The MLVSS Calculator converts MLSS into volatile suspended solids. This calculator works with loadings, F/M ratio, and retention time, and the F/M mode uses an MLVSS value as an input, which you can get from that calculator.
How are MLVSS and F/M related?
F/M divides the daily BOD mass entering the tank by the mass of volatile biological solids (MLVSS) in it. A different MLVSS concentration or tank volume changes the denominator and therefore the ratio.
Why divide by 1000 in the BOD loading formula?
A concentration in mg/L is the same as g/m³. Multiplying by flow in m³/day gives grams per day, and dividing by 1000 converts it to kilograms per day.
What is hydraulic retention time?
It is the tank volume divided by the flow rate, the average time water spends in the tank. This tool converts everything to m³ and m³/day first and shows the result in hours and days.
Can I use these results to set treatment operations?
No. The results are arithmetic on the numbers you enter. Operating decisions depend on site-specific data, permits, and qualified operator judgment.
Which units does it accept?
Flow in m³/day, m³/hour, L/day, or US million gallons per day, and tank volume in m³, liters, or US gallons. Concentrations are in mg/L.
What is the difference between COD and BOD loading?
Both use the same flow × concentration / 1000 calculation, but COD measures the total oxygen demand from chemically oxidizable material (usually via a dichromate or similar test), while BOD measures oxygen consumed by microorganisms over an incubation period, typically 5 days. COD is normally the larger number for the same sample, and the ratio between them varies by wastewater source.
Is detention time the same as hydraulic retention time?
Yes, on this page they're the same calculation: tank volume divided by flow rate, the nominal average time liquid spends in a tank. 'Detention time' and 'hydraulic retention time (HRT)' are two common names for that same idealized quantity. It assumes ideal plug flow and does not capture short-circuiting, dead zones, or the actual residence-time distribution of a real tank, so treat it as a nominal design figure rather than a guarantee of contact time for every parcel of liquid.