In short: fresh water at sea level can hold about 9.1 mg/L of dissolved oxygen at 20 °C, rising to roughly 14.6 mg/L near freezing and falling to about 7.6 mg/L at 30 °C. Salt and altitude both lower that ceiling further — use the calculator above for your exact water.
These figures are a planning aid for aquarists, pond and stream monitoring, and fieldwork estimates, based on published solubility data. They are not a substitute for a calibrated dissolved-oxygen meter or a water-quality lab.
How to calculate dissolved oxygen saturation
Dissolved oxygen (DO) saturation is the maximum amount of oxygen gas that a volume of water can hold while in equilibrium with the atmosphere above it. It is not a simple formula like volume times density — oxygen's solubility in water is a nonlinear function of temperature, so scientists fit a high-order polynomial to decades of lab measurements. This calculator uses the Benson–Krause equation, the reference method behind the tables used by the USGS and most state water agencies:
Worked example. Take fresh water at 20 °C and sea level. In kelvin, TK = 293.15. Plugging that into the equation above gives ln DOsat ≈ 2.207, and e2.207 ≈ 9.09 mg/L — the same figure the calculator returns for those inputs. That is the ceiling: water at 20 °C simply cannot hold more oxygen than that while sitting under normal air.
Saturation vs. your meter reading. A real water body is rarely at exactly the theoretical saturation value — algae add oxygen by day, respiration removes it at night, and aeration or a waterfall can push a reading briefly above 100%. Saturation is the reference point your meter reading is compared against, not a guarantee of what you will measure.
Why temperature, salinity and altitude change the answer
Temperature has the largest effect. Gas solubility falls as water warms, because faster-moving water molecules hold gas molecules less tightly. The relationship curves sharply: DO saturation drops fast between 0 °C and 20 °C, then more gently above that, which is why cold mountain streams naturally run oxygen-rich and why warm summer ponds are most at risk of low oxygen.
Salinity lowers oxygen solubility through the "salting-out" effect — dissolved ions compete for space with water molecules and reduce the room available for gas. At 35 g/kg (typical seawater), saturation runs roughly 18–20% below fresh water at the same temperature.
Altitude works through atmospheric pressure. Air is thinner at elevation, so less oxygen is pushed into the water from above. The calculator corrects for this using the standard barometric formula for pressure versus altitude (see boiling point at altitude for the same relationship applied to boiling), adjusted for the water's own vapor pressure at that temperature.
Dissolved oxygen saturation by temperature
Fresh water (0 g/kg), sea level, the values this calculator reads from.
| Temperature | DO sat. (mg/L) | DO sat. (µmol/L) |
|---|---|---|
| 0 °C / 32 °F (ice-cold) | 14.62 | 456.9 |
| 5 °C / 41 °F | 12.77 | 399.1 |
| 10 °C / 50 °F (cold stream) | 11.29 | 352.8 |
| 15 °C / 59 °F | 10.08 | 315.0 |
| 20 °C / 68 °F (pond) | 9.09 | 284.1 |
| 25 °C / 77 °F | 8.26 | 258.1 |
| 30 °C / 86 °F (tropical) | 7.56 | 236.3 |
| 35 °C / 95 °F | 6.95 | 217.2 |
| 40 °C / 104 °F | 6.41 | 200.3 |
Quick reference: salinity and altitude effects
Both held at 20 °C, so you can see each effect in isolation.
| Condition | DO sat. (mg/L) | vs. fresh, sea level |
|---|---|---|
| Fresh water, sea level | 9.09 | baseline |
| Brackish, 10 g/kg, sea level | 8.57 | −6% |
| Seawater, 35 g/kg, sea level | 7.40 | −19% |
| Fresh water, 1,000 m elevation | 8.04 | −12% |
| Fresh water, 2,000 m elevation | 7.09 | −22% |
| Fresh water, 3,000 m elevation | 6.23 | −31% |
Worked examples
Trout stream at 10 °C
11.29 mg/L
A cold headwater stream at 10 °C and sea level saturates at about 11.3 mg/L — comfortably above the 6–7 mg/L trout generally need.
Farm pond in late summer
7.83 mg/L
At 28 °C, the same fresh pond can hold only about 7.8 mg/L even at full saturation — one reason warm, still ponds are more prone to oxygen crashes overnight.
Reservoir at 2,000 m elevation
7.09 mg/L
Fresh water at 20 °C would saturate at 9.09 mg/L at sea level, but at 2,000 m the thinner air only supports about 7.09 mg/L — a 22% reduction from pressure alone.
What else affects a real reading
Photosynthesis and respiration. Algae and aquatic plants release oxygen during the day and consume it at night through respiration, so a healthy pond's DO can swing several mg/L between afternoon and dawn — this daily cycle is normal and separate from the saturation ceiling calculated here.
Aeration and turbulence. Waterfalls, fountains, riffles and mechanical aerators speed up the exchange of oxygen at the surface, which can push a reading close to saturation faster than still water would reach it on its own — and briefly above 100% just downstream of an aerator.
Biological and chemical oxygen demand. Decomposing organic matter, excess nutrients and some industrial or agricultural inputs consume dissolved oxygen, which is why a measured reading is so often below the saturation value this calculator returns, even in cool water.
Frequently asked questions
What is a good dissolved oxygen level for fish?
Most warm-water fish do well above 5 mg/L, and cold-water species such as trout generally need 6–7 mg/L or more. Below about 4 mg/L, many fish become stressed, and levels under 2 mg/L can be lethal to sensitive species. These are general ecological guidelines, not a substitute for species-specific advice or a calibrated meter reading.
Why does cold water hold more dissolved oxygen than warm water?
Gas solubility in water falls as temperature rises, because warmer water molecules move faster and hold gas molecules less tightly. At 0 °C, fresh water at sea level can hold about 14.6 mg/L of oxygen at saturation; at 30 °C that maximum drops to about 7.6 mg/L, even though both samples are fully saturated.
How does altitude affect dissolved oxygen?
Higher altitude means lower atmospheric pressure, so the atmosphere pushes less oxygen into the water. At 2,000 m elevation, the saturation concentration is roughly 78% of the sea-level value at the same temperature. This calculator applies that pressure correction when you enter an altitude.
Does salt water hold less oxygen than fresh water?
Yes. Dissolved salts reduce oxygen solubility through the salting-out effect. At 20 °C, fresh water saturates at about 9.1 mg/L, while full-strength seawater (35 g/kg) saturates at about 7.4 mg/L, roughly 19% less.
What is the difference between DO saturation and percent saturation?
DO saturation is the maximum concentration water can hold at equilibrium with air, in mg/L, for a given temperature, salinity and pressure. Percent saturation compares a measured reading to that maximum. A reading of 7.2 mg/L at 25 °C, where saturation is 8.26 mg/L, is about 87% saturated.
Sources & references
- Benson, B. B., & Krause, D. (1984). The concentration and isotopic fractionation of oxygen dissolved in freshwater and seawater in equilibrium with the atmosphere. Limnology and Oceanography, 29(3), 620–632.
- American Public Health Association (APHA). Standard Methods for the Examination of Water and Wastewater, Method 4500-O (Oxygen, Dissolved).
- U.S. Geological Survey. DOTABLES: Dissolved Oxygen Solubility Tables.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)