How to Read a Steam Table (With Worked Examples)

Water refuses to behave like an ideal gas anywhere near the conditions a steam system runs at, so its properties can't be computed from a one-line formula. Instead, engineers look them up: steam tables list the measured-and-correlated thermodynamic properties of water and steam — temperature, pressure, specific volume, enthalpy, entropy — over the whole operating range. Reading them fluently is a core skill for anyone sizing boilers, steam mains, condensate lines, or heat exchangers.

This guide walks through what the columns mean, how the saturated and superheated tables differ, how to handle wet steam and interpolation, and the one mistake that ruins more steam calculations than any other.

The three states a steam table covers

At any given pressure there is exactly one temperature — the saturation temperature — at which water boils. At atmospheric pressure that's 100 °C; at 10 bar absolute it's about 180 °C. The saturation line splits the property space into three regions:

• Subcooled (compressed) liquid — water below its saturation temperature. A boiler feedline at 10 bar and 105 °C is subcooled, because saturation at 10 bar is ~180 °C.
• Saturated (two-phase) — liquid and vapour coexisting at the saturation temperature. Most steam distribution happens here; this is what the main "saturated steam table" describes.
• Superheated vapour — steam heated beyond its saturation temperature at the same pressure. Turbine inlets and long transmission lines use superheat to guarantee dry steam.

Each region gets its own table, and the biggest reading error is using the wrong one — more on that below.

Anatomy of the saturated table

A saturated steam table has one row per pressure (or per temperature — the two forms carry identical information). A typical row at 5 bar absolute looks like this:

The subscripts are the key:

• f (from the German Flüssigkeit, liquid) — the property of saturated liquid: water right at boiling point, not yet evaporated.
• g (Gas) — the property of saturated vapour: 100% dry steam at the same temperature and pressure.
• fg — the difference between the two. For enthalpy, h_fg is the latent heat of evaporation: the energy needed to turn 1 kg of boiling water into 1 kg of dry steam without changing its temperature.

So the row above reads: at 5 bar absolute, water boils at 151.8 °C; the boiling water carries 640 kJ/kg, evaporating it takes a further 2108 kJ/kg, and the resulting dry steam carries 640 + 2108 = 2748 kJ/kg. The same f/g logic applies to specific volume (v) and entropy (s).

Two practical readings fall straight out of the table:

• Heat to raise steam: producing 1 kg of dry steam at 5 bar from feedwater at 80 °C (h ≈ 335 kJ/kg) takes roughly 2748 − 335 ≈ 2413 kJ.
• Heat released in a heat exchanger: steam condensing at 5 bar gives up h_fg = 2108 kJ for every kilogram — which is why condensing steam transfers so much more heat than cooling it by a few degrees ever could.

Wet steam and dryness fraction (quality)

Between "all liquid" and "all vapour" the table gives no rows, because the state depends on how much of the mixture has evaporated. That's described by the dryness fraction (or quality) $x$ — the mass fraction of vapour:

Any property of wet steam is the liquid value plus x times the fg difference:

Worked example. Steam leaves a boiler at 5 bar absolute with x = 0.95 (a realistic figure — boilers rarely deliver perfectly dry steam):

That's about 105 kJ/kg less than the dry-saturated value — roughly 4% less useful heat, plus 5% of the mass arriving as water that the steam traps must remove. Whenever a calculation assumes h_g, it is quietly assuming x = 1.

Reading the superheated table

Once steam is superheated, pressure and temperature become independent — so the superheated table needs two lookup values, and is laid out as a grid: one block per pressure, one row (or column) per temperature.

Worked example. Steam at 10 bar absolute and 300 °C: saturation at 10 bar is 179.9 °C, so this steam carries about 120 °C of superheat. The table gives roughly:

• h ≈ 3052 kJ/kg (compare h_g = 2778 kJ/kg for saturated steam at the same pressure)
• v ≈ 0.258 m³/kg
• s ≈ 7.12 kJ/kg·K

A quick self-check when reading a superheated grid: the temperature you look up must be above T_sat for that pressure block. If it isn't, the state is actually wet or subcooled and you're in the wrong table.

Interpolating between rows

Tables can't list every condition, so intermediate values are found by linear interpolation — properties vary smoothly enough between adjacent rows that a straight line is accurate to well within engineering tolerance:

Worked example. Enthalpy of superheated steam at 10 bar and 320 °C, from the 300 °C (h ≈ 3052) and 350 °C (h ≈ 3158) rows:

One caution: never interpolate across the saturation line. Between a saturated row and a superheated row the properties jump (that's the latent heat); a straight line through the jump is meaningless. Interpolate only within one region.

The gauge-vs-absolute trap

Steam tables are tabulated in absolute pressure. Plant gauges read gauge pressure — the amount above atmospheric. The difference is about 1 bar, and forgetting it is the single most common steam-table error:

• A gauge reading 5 bar g means ≈ 6 bar absolute → T_sat ≈ 158.8 °C
• Looking up 5 bar in the table by mistake gives T_sat = 151.8 °C — a 7 °C error

Seven degrees is the difference between a correctly sized heat exchanger and one that mysteriously underperforms, so make the g→absolute conversion a reflex: add atmospheric pressure (≈1.013 bar) to every gauge reading before opening the table.

Where the numbers come from

Modern tables aren't measured point-by-point — they're computed from the IAPWS-IF97 industrial formulation, the international standard equation of state for water and steam used by boiler codes, turbine vendors, and simulation software alike. Printed tables from different publishers round differently, so don't worry when two books disagree by a digit in the last place; they're sampling the same formulation.

That also means you rarely need the printed book: SimuPipe's free steam tables calculator evaluates IAPWS-IF97 directly — saturated properties by temperature or pressure, plus subcooled and superheated states — with no interpolation needed, in SI or imperial units.

Common mistakes to avoid

• Using gauge pressure in an absolute-pressure table. Add ~1.013 bar first. Always.
• Assuming dry steam. If quality is 0.95, using h_g overstates the heat content by ~4% and ignores the condensate load on your traps.
• Wrong table for the state. Check T against T_sat at your pressure: below = subcooled, equal = saturated, above = superheated.
• Interpolating across the saturation boundary. Properties jump there; interpolate within one region only.
• Mixing unit systems. kJ/kg vs kcal/kg, bar vs psia vs MPa — confirm the table's units line before reading a single number.

Put it to work

The lookups in this guide feed directly into everyday steam-system calculations: h_fg drives heat-exchanger duty and condensate load, the liquid enthalpies h_f at two pressures determine flash steam when condensate drops to a lower pressure, and steam enthalpies anchor boiler efficiency by the indirect method. For the property lookups themselves, the steam tables calculator gives IAPWS-IF97 values for any state — no interpolation, no gauge-pressure surprises.