Dissolved Oxygen Saturation Calculator

Values below are the FAO freshwater reference series. A dissolved oxygen reading only means something next to the saturation value for that water at that temperature and pressure. 8.2 mg/L means something different at 15°C than at 30°C, and at sea level than at 1500 m. This calculator gives you the ceiling so the reading can be read as a percentage — which is the number that actually tracks biological demand.

Why the same mg/L reading means different things

Oxygen solubility falls as water warms and as altitude rises. At sea level and 20°C, freshwater saturates near 9.1 mg/L. At 30°C the same water saturates near 7.6 mg/L. A plant reading 7.8 mg/L is at 100% saturation in late summer and at 86% in midwinter — same concentration, very different biological state.

Reporting saturation as a percentage removes that ambiguity, and it is what running an aeration basin on DO control actually requires.

Saturation values by altitude (freshwater)

Saturation deficit and what it means for aeration

The saturation deficit is the gap between what the water could hold and what it currently holds, expressed in mg/L. It is the oxygen mass that aeration equipment has to transfer to reach the target setpoint. Multiplying the deficit by the flow rate gives the oxygen load in kg/day, which is the number that sizes a blower.

Because deficit shrinks as temperature rises, the same aeration basin needs more air in summer than in winter for the same saturation target. Fixed-rate blowers that are adequate in winter under-aerate in summer, and the resulting low DO shows up as filamentous bulking.

Accuracy of the altitude correction

The calculator applies the barometric pressure ratio (P − Pᵥ) / (Pₛₑᵇ − Pᵥ), with Pᵥ the water vapour pressure from the Tetens equation, and interpolates the sea-level saturation from standard reference points. Against the FAO freshwater series this matches within 0.02 mg/L up to about 1500 m and within 0.15 mg/L at 2100 m — roughly 2% at extreme inland altitude.

Where a discharge permit or a design document specifies its own table, use that table. This page is a working aid, not the reference of record.

Why aeration tables disagree

Reference tables are published for specific conditions: freshwater at sea level, or 20°C standard atmosphere, or clean water. Salinity reduces solubility — seawater at 35 PSU holds roughly 20% less oxygen than freshwater at the same temperature. Effluent with dissolved solids at 2000 mg/L is measurably between the two. When a permit or a design document specifies a table, match its assumptions or you will be out by a few percent.

Instruments for this measurement

Models shown are representative families, not a complete catalogue. Send us your application, water analysis and installation constraints and we will match instruments to it.

Frequently asked

This page provides calculation guidance and a screening cross-check. It is not a substitute for the measurement method named in your permit, and values shown are typical ranges rather than guarantees for a particular instrument.