Water flow power calculator
Solve for heat/cooling capacity (kW), water flow rate or temperature difference from the other two — using real water density and specific heat at your system's mean temperature, not a fixed rule-of-thumb constant.
How to use this calculator
- Choose what to solve for — flow rate, capacity or ΔT.
- Enter the mean water temperature.
- Enter the two known values — supply/return temperature, or ΔT directly, plus flow or capacity.
- Read the result, computed from real water density and specific heat at that temperature.
What this calculates
The heat or cooling capacity carried by a water flow follows directly from the sensible-heat relationship — no approximation in the formula itself, only in the water property values used:
Q̇ = V̇ × ρ × cp × ΔT
Q̇ = capacity, kW
V̇ = volumetric flow rate, m³/s
ρ = water density, kg/m³
cp = water specific heat, kJ/(kg·K)
ΔT = temperature difference between supply and return, K
Any one of capacity, flow rate or ΔT follows from the other two — pick which one to solve for and enter the remaining values.
Why density and specific heat aren't fixed constants
The common shorthand "kW = 4.18 × L/s × ΔT" bakes in water's density and specific heat at around 20-25°C. Both properties genuinely shift across the range HVAC water systems actually run at — chilled water near 6-7°C, heating water up to 80-90°C — density in particular drops by roughly 4% from 0°C to 90°C. This calculator looks up density and specific heat from a standard water-property reference table (0-100°C in 10°C steps, atmospheric pressure) and linearly interpolates at your system's mean water temperature, rather than assuming one fixed number regardless of which part of the system you're sizing.
Enter the mean temperature and ΔT directly, or — usually more natural — enter supply and return temperature and let the tool derive both. Pure water only: a glycol/brine mixture has meaningfully different density and specific heat at any given concentration, and isn't covered by this table.