What is the density of water?

Fresh water has a density of 998.2 kg/m³ at 20°C (1.938 slug/ft³ at 60°F). Density peaks at 1000.0 kg/m³ near 4°C and falls as temperature rises, reaching 958.4 kg/m³ at 100°C. For most civil and environmental engineering work at ambient temperature, 1000 kg/m³ or 62.4 lb/ft³ specific weight is accurate to well within 1 percent.

What is the specific weight of water?

The specific weight of water is 9.789 kN/m³ at 20°C and 62.37 lb/ft³ at 60°F. It is the product of density and gravitational acceleration, γ = ρg. The commonly quoted 62.4 lb/ft³ corresponds to roughly 50–60°F and is the value used in most hydrostatic pressure and buoyancy calculations.

What is the kinematic viscosity of water at 20 degrees C?

The kinematic viscosity of water at 20°C is 1.003×10⁻⁶ m²/s, commonly rounded to 1.00×10⁻⁶ m²/s. In US customary units it is 1.05×10⁻⁵ ft²/s at 70°F. Kinematic viscosity ν equals dynamic viscosity μ divided by density ρ, and it is the property that enters the Reynolds number.

What is the vapor pressure of water?

The vapor pressure of water is 2.34 kPa absolute at 20°C (0.256 psia at 60°F), rising to 101.33 kPa (14.70 psia) at 100°C, which is the definition of the normal boiling point. Vapor pressure sets the cavitation limit for pumps and is the term that makes warm water far more prone to cavitation than cold.

Why does water temperature matter in hydraulic calculations?

Temperature changes viscosity far more than it changes density. Between 5°C and 30°C the density of water moves by about 0.5 percent, but kinematic viscosity nearly halves. Because the Reynolds number is inversely proportional to viscosity, the same pipe and velocity can shift regime or friction factor with season. Vapor pressure roughly quadruples over the same range, which is why available NPSH must be checked at the warmest expected temperature.

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Water Properties by Temperature — Reference

By Michael Flynn, PE · water resources & dam safety engineer

Density, specific weight, dynamic and kinematic viscosity, and vapor pressure of fresh water at atmospheric pressure. These four properties feed nearly every hydraulic calculation — Reynolds number, friction loss, settling velocity, and pump NPSH.

The Four Values You Actually Look Up

Property	SI (20°C)	US (60°F)	Where it is used
Density ρ	998.2 kg/m³	1.938 slug/ft³	Momentum, drag, Reynolds number
Specific weight γ	9.789 kN/m³	62.37 lb/ft³	Hydrostatic pressure, buoyancy, uplift
Kinematic viscosity ν	1.003×10⁻⁶ m²/s	1.21×10⁻⁵ ft²/s	Reynolds number, friction factor
Vapor pressure p_v	2.34 kPa abs	0.256 psia	Cavitation, NPSH available

The engineer's shortcut. At ordinary ambient temperatures, γ = 62.4 lb/ft³ (9.81 kN/m³) and ν = 1.0×10⁻⁶ m²/s are accurate to better than 1–2% and are what most design manuals assume. Reach for the full table when you are checking cavitation, cold-weather friction loss, or a settling / thickening process — those are the three places temperature genuinely bites.

SI Units — Fresh Water, 0 to 100°C

T (°C)	ρ (kg/m³)	γ (kN/m³)	μ (×10⁻³ Pa·s)	ν (×10⁻⁶ m²/s)	p_v (kPa abs)
0	999.8	9.806	1.781	1.785	0.61
5	1000.0	9.807	1.518	1.519	0.87
10	999.7	9.804	1.307	1.306	1.23
15	999.1	9.798	1.139	1.139	1.70
20	998.2	9.789	1.002	1.003	2.34
25	997.0	9.777	0.890	0.893	3.17
30	995.7	9.764	0.798	0.800	4.24
40	992.2	9.730	0.653	0.658	7.38
50	988.0	9.689	0.547	0.553	12.33
60	983.2	9.642	0.466	0.474	19.92
70	977.8	9.589	0.404	0.413	31.16
80	971.8	9.530	0.354	0.364	47.34
90	965.3	9.466	0.315	0.326	70.10
100	958.4	9.399	0.282	0.294	101.33

US Customary — Fresh Water, 32 to 212°F

T (°F)	ρ (slug/ft³)	γ (lb/ft³)	μ (×10⁻⁵ lb·s/ft²)	ν (×10⁻⁵ ft²/s)	p_v (psia)
32	1.940	62.42	3.732	1.924	0.0885
40	1.940	62.43	3.228	1.664	0.122
50	1.940	62.41	2.730	1.407	0.178
60	1.938	62.37	2.344	1.210	0.256
70	1.936	62.30	2.034	1.051	0.363
80	1.934	62.22	1.791	0.926	0.507
100	1.927	62.00	1.423	0.739	0.949
120	1.918	61.71	1.164	0.607	1.69
140	1.908	61.38	0.974	0.511	2.89
160	1.896	61.00	0.832	0.439	4.74
180	1.883	60.58	0.721	0.383	7.51
200	1.869	60.12	0.634	0.339	11.52
212	1.860	59.83	0.589	0.317	14.70

Relationships

γ = ρg · ν = μ/ρ · Re = VL/ν

Use g = 9.807 m/s² (32.17 ft/s²). Note the unit trap in US customary work: density is in slug/ft³, not lb/ft³. The familiar 62.4 lb/ft³ is specific weight γ, not density ρ. Mixing the two is the single most common error in US-unit fluid calculations — they differ by the factor g = 32.17.

Where Temperature Actually Changes the Answer

Calculation	Property	Sensitivity
Hydrostatic pressure, uplift, buoyancy	γ	Negligible — under 1% across 0–30°C
Reynolds number, friction factor	ν	Strong — ν nearly halves from 5°C to 30°C
Pump NPSH available / cavitation	p_v	Strong — p_v rises ~5× from 5°C to 30°C
Particle settling (Stokes range)	μ	Strong — settling velocity scales as 1/μ

Cold water is not the conservative assumption. Cold water is more viscous, so it gives a lower Reynolds number and higher friction loss — conservative for pipe sizing. But warm water has a much higher vapor pressure, so it is the governing case for cavitation and NPSH. Check pump suction at the warmest expected water temperature and pipe friction at the coldest.

Sources: White, F.M., Fluid Mechanics, Table A.1 (water properties). Munson, Young & Okiishi, Fundamentals of Fluid Mechanics, Tables B.1 and B.2. Vapor pressures follow the IAPWS-IF97 / NIST steam-table formulation. Values are for fresh water at atmospheric pressure; dissolved solids, and salinity in particular, shift density and vapor pressure and warrant source-specific data.

Checking a flow regime? Open the Reynolds calculator → · Pipe friction? Darcy-Weisbach · Settling? Stokes settling card.

Related cheat sheets and tools

Kinematic viscosity from this card feeds the Reynolds number calculator and the flow-regime thresholds. Pair it with absolute roughness for Darcy-Weisbach head loss, or with the Stokes settling card for clarifier and grit-chamber design. Vapor pressure is the term you need for pump power and NPSH checks. For full network modeling, see HydroComplete.

Water Properties by Temperature — Reference

The Four Values You Actually Look Up

SI Units — Fresh Water, 0 to 100°C

US Customary — Fresh Water, 32 to 212°F

Relationships

Where Temperature Actually Changes the Answer

Related cheat sheets and tools

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