Dissolved Oxygen Saturation Calculator

Understanding the dissolved oxygen saturation calculator — concept and principle

The amount of oxygen water can hold in equilibrium with air depends mainly on temperature and air pressure. This calculator finds the saturation concentration for fresh water at a given temperature and altitude, then compares your measured dissolved oxygen (DO) with it to give percent saturation. Field technicians, aquaculturists and wastewater operators use it to judge whether water is oxygen-depleted or supersaturated.

At 20 °C water can hold 9.09 mg/L, so 7.5 mg/L is 82.49% saturation.

How it is calculated

Saturation at sea level: ln C* = −139.34411 + 1.575701 × 10⁵/T − 6.642308 × 10⁷/T² + 1.2438 × 10¹⁰/T³ − 8.621949 × 10¹¹/T⁴, with T in kelvin (°C + 273.15) and C* in mg/L.
Altitude correction: C*ₕ = C* × (1 − 2.25577 × 10⁻⁵ × h)^5.25588, where h is altitude in metres; the factor is the standard-atmosphere pressure ratio.
Percent saturation = measured DO ÷ C*ₕ × 100.

The principle

Henry's law says the amount of a gas dissolved in a liquid is proportional to that gas's partial pressure above it. Air is about 21% oxygen, so lower air pressure at altitude means less oxygen can dissolve. Warmer water holds less gas because dissolved molecules have more energy to escape. Benson and Krause measured oxygen solubility precisely and fitted the polynomial above, which is the basis of the tables in Standard Methods and USGS procedures.

Worked example

Inputs: T = 20 °C, measured DO = 7.5 mg/L, altitude = 0 m.
T = 293.15 K; the polynomial gives ln C* = 2.2074, so C* = e^2.2074 = 9.09 mg/L.
Altitude factor at 0 m = 1, so C*ₕ = 9.09 mg/L.
Percent saturation = 7.5 ÷ 9.0924 × 100 = 82.49%.

Each point is the calculator re-run with only “Water temperature (°C)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Water temperature (°C)	Saturation DO (mg/L)
5	12.77100036
10	11.28794737
15	10.08385834
20	9.092426043
25	8.263456698
30	7.558796048
40	6.412721786

How to read the result

For reference, saturation at sea level is about 14.6 mg/L at 0 °C, 11.3 mg/L at 10 °C, 8.26 mg/L at 25 °C and 7.56 mg/L at 30 °C. Many fish species are stressed when DO falls below about 5 mg/L. Values well above 100% occur when algae photosynthesise strongly on sunny afternoons, or below dams and waterfalls. A large daily swing between morning lows and afternoon highs is a sign of nutrient enrichment.

Common mistakes

Using this freshwater equation for seawater or brackish water, where salinity lowers solubility.
Entering water temperature in °F.
Using altitude when the actual barometric pressure is known and differs because of weather.

Keep in mind: The altitude term uses a standard atmosphere and ignores weather-driven pressure changes and the small water-vapour correction. For meter calibration or regulatory work, use the local barometric pressure and the official tables.

Frequently asked questions

Why is percent saturation more useful than mg/L sometimes?

It removes the temperature effect, showing whether the water is gaining or losing oxygen relative to equilibrium.

What happens at 1,000 m altitude?

The pressure factor is about 0.887, so at 20 °C saturation drops to about 8.06 mg/L.

Can DO exceed 100%?

Yes. Photosynthesis or entrained air can supersaturate water, and very high levels can cause gas bubble disease in fish.

Sources: B. B. Benson and D. Krause Jr., Limnology and Oceanography 29, 620–632 (1984) · APHA, AWWA and WEF, Standard Methods for the Examination of Water and Wastewater, Method 4500-O · US Geological Survey, National Field Manual for the Collection of Water-Quality Data, chapter on dissolved oxygen

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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