Mixed Water Temperature Calculator
Blend two water streams and find the resulting temperature — or work backwards from the temperature you want to the hot and cold flows a mixing valve must deliver.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Mixed water temperature
- 85.6°F
- Mixed flow (approx.)
- 5gpm
2 gpm at 140 °F + 3 gpm at 50 °F mixes to about 85.6 °F.
- Volume-weighted estimate (ignoring density)
- 86°F
Show the working
- Mass flows (gpm × density from IAPWS water properties)hot 2 × 61.377, cold 3 × 62.406 lb/ft³
- Energy balance: T_mix = (ṁ_hot T_hot + ṁ_cold T_cold) ÷ (ṁ_hot + ṁ_cold)85.64 °F
The energy balance
With no heat lost, the heat given up by the hot water equals the heat taken in by the cold water. For one liquid with a constant specific heat, the mixed temperature is the mass-weighted average: T_mix = (m_hot T_hot + m_cold T_cold) ÷ (m_hot + m_cold).
Hot water is slightly less dense than cold, so a gallon of hot water carries a little less mass. The calculator converts each flow to mass with the water density at its temperature, which is why its answer differs slightly from a simple average of gallons.
Setting a mixing valve
Working backwards, the hot-water share by mass is (T_target − T_cold) ÷ (T_hot − T_cold). For 110 °F from 140 °F hot and 50 °F cold water, two-thirds of the mass must be hot.
Thermostatic mixing valves for domestic hot water are set and verified by the installer. Always measure the outlet temperature at the fixture after adjustment.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Calculate
- Mixed temperature from two flows
- Hot water temperature
- 140 °F
- Cold water temperature
- 50 °F
- Hot water flow
- 2 gpm
- Cold water flow
- 3 gpm
Result
- Mixed water temperature
- 85.6 °F
- Mixed flow (approx.)
- 5 gpm
- Volume-weighted estimate (ignoring density)
- 86 °F
2 gpm at 140 °F + 3 gpm at 50 °F mixes to about 85.6 °F.
Assumptions
- Constant specific heat of water across the temperature range; no heat loss to the surroundings.
- Densities of saturated liquid water at each temperature (IAPWS, checked against NIST).
Limitations
- Specific heat varies by under 1 % between 40 °F and 200 °F, which is ignored.
- Water only; glycol mixtures have different properties.
Frequently asked questions
What do 2 gpm of 140 °F and 3 gpm of 50 °F water mix to?
About 85.6 °F; a simple average of the gallons gives 86 °F.
Does this work for glycol?
No. Glycol mixtures have different density and specific heat; this tool is for water.
Why is my real outlet cooler?
Pipes and fittings absorb and lose heat, especially at low flow. The calculator assumes no heat loss.
Sources
- University Physics Volume 2, §1.4 Heat Transfer, Specific Heat, and Calorimetry — OpenStax, Rice University (CC BY 4.0). Q = mcΔT; in calorimetry the heat lost by the hot substance equals the heat gained by the cold one (Example 1.7, final temperature of water in a hot pan, 59.1 °C).
- NIST Chemistry WebBook, SRD 69 — Thermophysical Properties of Fluid Systems (water) — National Institute of Standards and Technology. Saturated-liquid water density and viscosity, e.g. 62.364 lb/ft³ and 1.1211 cP at 60 °F; 61.377 lb/ft³ and 0.46602 cP at 140 °F.
- CoolProp — open-source thermophysical property library (version 8.0.0) — CoolProp developers (MIT licence). Equations of state for pure fluids and mixtures, incompressible fluids and brines, and IAPWS-IF97 water/steam; free for commercial use under its licence. Used here to generate the refrigerant saturation tables.
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